Maximum differential. Maxima mathematical system

A differential amplifier is a well-known circuit used to amplify the voltage difference of two input signals. Ideally, the output signal does not depend on the level of each of the input signals, but is determined only by their difference. When the signal levels at both inputs change simultaneously, then such a change in the input signal is called common-mode. The differential or difference input signal is also called normal or useful. A good differential amplifier has a high common mode rejection ratio(CMRR), which is the ratio of the desired output signal to the common-mode output signal, provided that the desired and common-mode input signals have the same amplitude. CMRR is usually measured in decibels. The range of variation of the common mode input signal specifies the permissible voltage levels relative to which the input signal must vary.

Differential amplifiers are used in cases where weak signals can be lost in the background noise. Examples of such signals are digital signals transmitted over long cables (a cable usually consists of two twisted wires), audio signals (in radio engineering, the concept of “balanced” impedance is usually associated with a differential impedance of 600 ohms), radio frequency signals (a two-core cable is differential), voltage electrocardiograms, signals for reading information from magnetic memory and many others. A differential amplifier at the receiving end restores the original signal if the common mode interference is not very large. Differential stages are widely used in the construction of operational amplifiers, which we discuss below. They play an important role in amplifier design DC(which amplify frequencies up to DC, i.e. do not use capacitors for interstage coupling): their symmetrical circuit is inherently designed to compensate for temperature drift.

In Fig. Figure 2.67 shows the basic circuit of a differential amplifier. The output voltage is measured at one of the collectors relative to ground potential; such an amplifier is called circuit with single-pole output or difference amplifier and it is the most widespread. This amplifier can be thought of as a device that amplifies a differential signal and converts it into a single-ended signal that can be handled by conventional circuits (voltage followers, current sources, etc.). If a differential signal is needed, then it is removed between the collectors.

Rice. 2.67. Classic transistor differential amplifier.

What is the gain of this circuit? It is not difficult to calculate: let’s say a differential signal is applied to the input, and the voltage at input 1 increases by the amount uin (voltage change for a small signal relative to the input).

As long as both transistors are in active mode, the potential of point A is fixed. The gain can be determined as in the case of an amplifier with one transistor, if you notice that the input signal is applied twice to the base-emitter junction of any transistor: K diff = R k /2(r e + R e). The resistance of the resistor R e is usually small (100 Ohms or less), and sometimes this resistor is absent altogether. The differential voltage is usually amplified several hundred times.

In order to determine the common-mode signal gain, the same I/O signals must be applied to both inputs of the amplifier. If you carefully consider this case (and remember that both emitter currents flow through resistor R 1), you will get K sinf = - R k / (2R 1 + R e). We neglect resistance r e, since resistor R 1 is usually chosen large - its resistance is at least several thousand ohms. In fact, the resistance R e can also be neglected. CMOS is approximately equal to R 1 (r e + R e). A typical example of a differential amplifier is the circuit shown in Fig. 2.68. Let's look at how it works.

Rice. 2.68. Calculation of differential amplifier characteristics.
K diff = U out /(U 1 - U 2) = R to /2(R e + r e):
K diff = R k /(2R 1 + R e + r e);
KOSS ≈ R 1 /(R e + r e).

The resistance of the resistor R k is chosen as follows. so that the quiescent collector current can be taken equal to 100 μA. As always, to get the maximum dynamic range The collector potential is set to 0.5 U kk. Transistor T 1 does not have a collector resistor, since its output signal is removed from the collector of another transistor. The resistance of resistor R 1 is chosen such that the total current is 200 μA and is equally distributed between the transistors when the input (differential) signal is zero. According to the formulas just derived, the differential signal gain is 30, and the common mode gain is 0.5. If we exclude 1.0 kOhm resistors from the circuit, then the gain of the differential signal will become equal to 150, but at the same time the input (differential) resistance will decrease from 250 to 50 kOhm (if it is necessary for the value of this resistance to be of the order of megaohms, then transistors can be used in the input stage Darlington).

Let us recall that in an asymmetrical amplifier with a grounded emitter with an output quiescent voltage of 0.5 U kk, the maximum gain is 20 U kk, where U kk is expressed in volts. In a differential amplifier, the maximum differential gain (at R e = 0) is half as much, i.e. numerically equal to twenty times the voltage drop across the collector resistor with a similar choice of operating point. The corresponding maximum CMRR (provided that R e = 0) is also numerically 20 times greater than the voltage drop across R 1 .

Exercise 2.13. Make sure the given ratios are correct. Design differential amplifiers to suit your own requirements.

A differential amplifier can be figuratively called a “long-tailed pair”, since if the length of the resistor is symbol is proportional to the value of its resistance, the circuit can be depicted as shown in Fig. 2.69. The “long tail” determines the common-mode signal rejection, and the small inter-emitter coupling resistances (including the emitters’ own resistances) determine the differential signal amplification.

Biasing using a current source. The common-mode gain in a differential amplifier can be significantly reduced if resistor R 1 is replaced by a current source. In this case, the effective value of resistance R 1 will become very large, and the common-mode signal gain will be weakened almost to zero. Let's imagine that there is a common-mode signal at the input; The current source in the emitter circuit maintains the total emitter current constant, and it (due to the symmetry of the circuit) is evenly distributed between the two collector circuits. Therefore, the output signal of the circuit does not change. An example of such a scheme is shown in Fig. 2.70. For this circuit, which uses a monolithic transistor pair of type LM394 (transistors T 1 and T 2) and a current source of type 2N5963, the CMRR value is determined by the ratio of 100,000:1 (100 dB). The range of the input common-mode signal is limited to -12 and + 7 V: the lower limit is determined by the operating range of the current source in the emitter circuit, and the upper limit is determined by the quiescent collector voltage.

Rice. 2.70. Increasing the CMRR of a differential amplifier using a current source.

Do not forget that this amplifier, like all transistor amplifiers, must have DC mixing circuits. If, for example, a capacitor is used at the input for interstage coupling, then grounded base resistors must be included. Another caveat applies especially to differential amplifiers without emitter resistors: bipolar transistors can withstand no more than 6 V of reverse bias at the base-emitter junction. Then breakdown occurs; This means that if a higher differential input voltage is applied to the input, the input stage will be destroyed (provided that there are no emitter resistors). The emitter resistor limits the breakdown current and prevents destruction of the circuit, but the characteristics of the transistors can degrade in this case (coefficient h 21e, noise, etc.). In either case, the input impedance drops significantly if reverse conduction occurs.

Applications of differential circuits in DC amplifiers with single-pole output. A differential amplifier can work perfectly as a DC amplifier even with single-ended (single-ended) input signals. To do this, you need to ground one of its inputs and send a signal to the other (Fig. 2.71). Is it possible to eliminate the "unused" transistor from the circuit? No. The differential circuit provides compensation for temperature drift, and, even when one input is grounded, the transistor performs some functions: when the temperature changes, the voltage U be changes by the same amount, while no changes occur at the output and the balancing of the circuit is not disrupted. This means that the change in voltage U be is not amplified by the coefficient K diff (its amplification is determined by the coefficient K sinf, which can be reduced to almost zero). In addition, mutual compensation of voltages U be leads to the fact that at the input there is no need to take into account a voltage drop of 0.6 V. The quality of such a DC amplifier deteriorates only due to inconsistency of voltages U be or their temperature coefficients. The industry produces transistor pairs and integrated differential amplifiers with very high degree coordination (for example, for a standard consistent monolithic n-p-n pairs- for transistors of the MAT-01 type, the voltage drift U be is determined by the value of 0.15 μV/°C or 0.2 μV per month).

Rice. 2.71. The differential amplifier can operate as a precision DC amplifier with single-pole output.

In the previous circuit, you can ground any of the inputs. Depending on which input is grounded, the amplifier will or will not invert the signal. (However, due to the presence of the Miller effect, which will be discussed in Section 2.19, the circuit presented here is preferable for the range high frequencies). The presented circuit is non-inverting, which means that the inverting input is grounded. The terminology associated with differential amplifiers also applies to operational amplifiers, which are the same high-gain differential amplifiers.

Using a current mirror as an active load. Sometimes it is desirable for a single stage differential amplifier, like a simple grounded emitter amplifier, to have high gain. A beautiful solution is provided by using a current mirror as an active load of an amplifier (Fig. 2.72). Transistors T 1 and T 2 form a differential pair with a current source in the emitter circuit. Transistors T 3 and T 4, forming a current mirror, act as a collector load. This ensures a high value of collector load resistance, thanks to which the voltage gain reaches 5000 or higher, provided that there is no load at the amplifier output. Such an amplifier is usually used only in circuits covered by a loop feedback, or in comparators (we will look at them in the next section). Remember that the load for such an amplifier must have a high impedance, otherwise the gain will be significantly weakened.

Rice. 2.72. Differential amplifier with current mirror as active load.

Differential amplifiers as phase splitting circuits. On the collectors of a symmetrical differential amplifier, signals appear that are identical in amplitude, but with opposite phases. If we take the output signals from two collectors, we get a phase splitting circuit. Of course, you can use a differential amplifier with differential inputs and outputs. The differential output signal can then be used to drive another differential amplifier stage, thereby increasing the CMRR value of the entire circuit significantly.

Differential amplifiers as comparators. Due to its high gain and stable performance, the differential amplifier is the main integral part comparator- a circuit that compares input signals and evaluates which one is larger. Comparators are used in a wide variety of areas: to turn on lighting and heating, to obtain rectangular signals from triangular ones, to compare the signal level with a threshold value, in class D amplifiers and pulse code modulation, to switch power supplies, etc. The main idea when constructing a comparator is that. that the transistor should turn on or off depending on the levels of the input signals. The linear gain region is not considered - the operation of the circuit is based on the fact that one of the two input transistors is in cutoff mode at any time. A typical signal capture application is discussed in the next section using a temperature control circuit that uses temperature-dependent resistors (thermistors).

The appearance of fires is characterized by an increase in temperature environment. Therefore, in systems fire alarm most often used heat detectors.

They are able to identify fires at the initial stage, which allows timely measures to be taken to eliminate them. However, such sensors are presented in various modifications on the market.

To choose the right one for a particular room, you should learn as much as possible about them.

Design features of the device

What is a detector? This is a heat-sensitive element enclosed in a plastic case. The operating principle of the most simple models based on closing/opening contacts, leading to the formation of a signal.

For the device to operate, the ambient temperature must rise above the device’s threshold value.

When operating, such heat detectors do not consume current. They are called passive. They use a specific alloy as a thermoelement. Previously, these sensors were disposable and could not be restored, but today reusable models have appeared. In them, under the influence of temperature, the bimetallic element, changing its shape, affects the contact.

There are magnetically controlled samples. The permanent magnet located in them changes its properties as a result of heating, which leads to the operation of the device.

When selecting a heat detector for a room, it is necessary that the threshold temperature value for them be higher than the average for the building by at least 10 ° C. This allows you to avoid false alarms.

Types of devices and their features

Each device is designed for a specific controlled area. By the nature of its detection on:

Spot
Linear

Point thermal fire detectors, in turn, are available in two types:

Maximum
Differential

The operation of the former is based on a change in the state of the thermoelement when the temperature rises to a threshold value. It is worth noting that for operation it is necessary that before the specified technical specifications value, the detector itself has become hot. And this will take some time.

This is an obvious disadvantage of the device, since it does not allow detecting a fire at the initial stage. This can be eliminated by increasing the number of sensors located in one room, as well as using other types of sensors.

Differential heat detectors are designed to monitor the rate of temperature rise. This made it possible to reduce the inertia of the device. The design of such sensors includes electronic elements, which affected the cost.

In practice, most often, these two types are used in combination. Such a maximum differential fire detector is triggered not only by the rate of temperature rise, but also by its threshold value.

Linear devices or thermal cables are twisted pairs, where each wire is coated with a thermally resistive material. When the temperature rises, it loses its properties, which leads to a short circuit in the circuit and the formation of a fire signal.

The thermal cable is connected instead of the system cable. But it has one drawback - a short circuit can be caused not only by fire.

To eliminate such moments linear sensors connect via interface modules, ensuring its connection with the alarm device. Many of them are used in technological elevator shafts and other similar structures.

Manufacturers - choosing the best model

Most widespread in domestic market fire fighting equipment find thermal sensors from Russian companies. This is due to both the features of alarm systems, regulatory requirements, and reasonable prices for them.

The most popular are thermal fire alarm detectors:

Aurora TN (IP 101-78-A1) – Argusspectr
IP 101-3A-A3R – Siberian Arsenal

The Aurora detector is a maximum differential non-addressable detector. It is used to detect fires in a room and transmit a signal to the control panel.

Watch a video about the product:

The advantages of this model include:

High sensitivity
Reliability
Using a microprocessor as part of the device
Easy to maintain

Its cost is more than 400 rubles, but it fully corresponds to the quality of the device.

Explosion-proof thermal detectors IP 101-3A-A3R are also classified as maximum differential. They are designed for use in heated rooms and can work with DC and AC loops.

The advantages of this model include:

Electronic control circuit
Availability LED indicator, allowing you to control the operation of the device
Modern design

The cost of this model is significantly lower and amounts to 126 rubles, which makes them accessible to a wide range of users.

Watch a video about IP 101-7 explosion-proof products:

There are many more various types. This is a thermal explosion-proof detector and many others. Which one to choose for a particular room depends on various factors, which will be discussed below.

What to focus on when choosing?

Each thermal sensor has certain classification characteristics. They are usually reflected in technical documentation. We list those that you should pay attention to:

Response temperature
Operating principle
Design features
Inertia
Type of control zone

For example, for premises with large areas It is recommended to install thermal fire detectors with a linear detection zone. When choosing a device, be sure to pay attention to the response temperature; it should not differ from the average by more than 20 ° C. Sudden changes in the control zone are unacceptable, they can lead to false alarms

Is it possible to use sensors everywhere?

There is a list of documents regulating the use of fire fighting equipment. They indicate that heat detectors are acceptable for use in most industrial and residential facilities. But at the same time, there is a list of premises where their work is impractical:

computing centers
rooms with suspended ceilings

Op amps are characterized by amplification, input, output, energy, drift, frequency and speed characteristics.

Gain characteristics

Gain (K U) is equal to the ratio of the output voltage increment to the differential input voltage that caused this increment in the absence of feedback (FE). It varies from 10 3 to 10 6.

The most important characteristics of the op amp are amplitude (transfer) characteristics (Fig. 8.4). They are represented in the form of two curves, corresponding respectively to the inverting and non-inverting inputs. The characteristics are taken when a signal is applied to one of the inputs with a zero signal at the other. Each curve consists of a horizontal and an inclined section.

The horizontal sections of the curves correspond to the fully open (saturated) or closed mode of the output stage transistors. When the input voltage changes in these sections, the output voltage of the amplifier remains constant and is determined by the voltages +U out max) -U out max. These voltages are close to the voltage of the power supplies.

The sloping (linear) section of the curves corresponds to the proportional dependence of the output voltage on the input. This range is called the gain region. The angle of inclination of the section is determined by the gain of the op-amp:

K U = U out / U in.

Large values of the op-amp gain make it possible, when such amplifiers are covered by deep negative feedback, to obtain circuits with properties that depend only on the parameters of the negative feedback circuit.

The amplitude characteristics (see Fig. 8.4) pass through zero. The state when U out = 0 at U in = 0 is called op-amp balance. However, for real op-amps the balance condition is usually not satisfied. When Uin = 0, the output voltage of the op-amp can be greater or less than zero:

U out = + U out or U out = - U out).

Drift characteristics

The voltage (U cmo) at which U out = 0 is called input offset voltage zero (Fig. 8.5). It is determined by the voltage value that must be applied to the input of the op-amp to obtain zero at the output of the op-amp. Usually amounts to no more than a few millivolts. The voltages U cm and ∆U out (∆U out = U shift - shear stress) are related by the relation:

U cm = ∆U out / K U .

The main reason for the appearance of bias voltage is a significant spread in the parameters of the elements of the differential amplifier stage.

The dependence of the op amp parameters on temperature causes temperature drift input offset voltage. Input offset voltage drift is the ratio of the change in input offset voltage to the change in ambient temperature:

E smo = U smo / T.

Typically E cmo is 1…5 µV / °C.

Transfer characteristic of an op-amp for a common-mode signal shown in (Fig. 8.6). It shows that at sufficiently large values of U sf (comparable with the voltage of the power source), the common-mode signal amplification factor (K sf) increases sharply.

The range of input voltage used is called the common mode rejection region. Operational amplifiers are characterized by common mode rejection ratio (To oss) – differential signal gain ratio (K u d) to the common-mode signal gain factor (K u sf).

K oss = K u d / K u sf.

Common mode gain is defined as the ratio of the change in output voltage to the change in common mode that caused it.
o input signal). Common mode rejection ratio is usually expressed in decibels.

Input characteristics

Input resistance, input bias currents, difference and drift of input bias currents, as well as the maximum input differential voltage characterize the main parameters of the op-amp input circuits, which depend on the circuit of the differential input stage used.

Input bias current (I cm) – current at the amplifier inputs. Input bias currents are determined by the base currents of the input bipolar transistors and gate leakage currents for op-amps with field-effect transistors at the input. In other words, I cm is the currents consumed by the inputs of the op-amp. They are determined by the finite value of the input resistance of the differential stage. The input bias current (I cm), given in the reference data for the op-amp, is defined as the average bias current:

I cm = (I cm1 – I cm2) / 2.

Input shift current is the difference in displacement currents. It appears due to inaccurate matching of the current gains of the input transistors. The shift current is variable, ranging from several units to several hundred nanoamperes.

Due to the presence of input bias voltage and input bias currents, op-amp circuits must be supplemented with elements designed to initially balance them. Balancing is carried out by applying some additional voltage to one of the inputs of the op-amp and introducing resistors into its input circuits.

Input current temperature drift – a coefficient equal to the ratio of the maximum change in the input current of the op-amp to the change in ambient temperature that caused it.

Temperature drift of input currents leads to additional error. Temperature drifts are important for precision amplifiers because, unlike offset voltage and input currents, they are very difficult to compensate for.

Maximum differential input voltage the voltage supplied between the inputs of the op-amp in the circuit is limited to prevent damage to the transistors of the differential stage

Input impedance depends on the type of input signal. There are:

· differential input resistance (R input differential) – (resistance between the amplifier inputs);

· common-mode input resistance (Rin sf) – resistance between the combined input terminals and the common point.

The values of Rin diff range from several tens of kilo-ohms to hundreds of mega-ohms. The input common-mode resistance Rin sf is several orders of magnitude greater than Rin diff.

Output characteristics

The output parameters of the op-amp are the output resistance, as well as the maximum output voltage and current.

The operational amplifier must have a small output impedance (R out) to ensure high values output voltage at low load resistance. Low output resistance is achieved by using an emitter follower at the op-amp output. Real Rout is units and hundreds of ohms.

Maximum output voltage (positive or negative) close to the supply voltage. Maximum output current limited by the permissible collector current of the op-amp output stage.

Energy characteristics

The energy parameters of the op-amp are assessed maximum current consumption from both power sources and, accordingly, the total power consumption .

Frequency characteristics

The amplification of harmonic signals is characterized by the frequency parameters of the op-amp, and the amplification of pulsed signals by its speed or dynamic parameters.

The frequency dependence of the op-amp gain without feedback is called amplitude-frequency response (AFC).

The frequency (f 1) at which the op-amp gain is equal to unity is called unity gain frequency .

Due to the phase shift of the output signal relative to the input created by the amplifier in the high-frequency region phase-frequency response The op-amp at the inverting input acquires an additional (over 180°) phase shift (Fig. 8.8).

To ensure stable operation of the op-amp, it is necessary to reduce the phase lag, i.e. adjust the amplitude-frequency response of the op-amp.

Speed characteristics

The dynamic parameters of the op-amp are output slew rate voltage (response speed) and output voltage settling time . They are determined by the reaction of the op-amp to the impact of a voltage surge at the input (Fig. 8.9).

Output voltage slew rate is the ratio of the increment ( U out) to the time interval ( t) during which this increment occurs when a rectangular pulse is applied to the input. That is

V U out = U out / t

The higher the cutoff frequency, the faster the slew rate of the output voltage. Typical values V U out – units of volts per microsecond.

Output voltage settling time (t set) – the time during which U out of the operational amplifier changes from level 0.1 to level 0.9 of the steady-state value of U out when the op-amp input is exposed to rectangular pulses. The settling time is inversely proportional to the cutoff frequency.

(differential pressure): The difference between the inlet and outlet pressure of the component under test under specified conditions.

11 gaslift differential pressure

12 bottomhole differential pressure

13 differential pressure switch

14 differential pressure gage

Rice. 2.23

a - arrow drive diagram;
1 - “positive” bellows;
2 - “negative” bellows;
3 - rod;
4 - lever;
5 - torsion terminal;
7 - compensator;
8 - plane valve;
9 - base;
10 and 11 - covers;
12 - inlet fitting;
13 - cuff;
14 - throttling channel;
15 - valve;
16 - lever system;
18 - arrow;
19 - adjusting screw;
20 - tension spring;
21 - plug;

Rice. 2.24

1 - membrane box;

4 - body;
5 - transmission mechanism;
6 - arrow;
7 - dial

Rice. 2.25

1 - “plus” camera;
2 - “minus” camera;
4 - transmitting rod;
5 - transmission mechanism;

Rice. 2.26

1 - “plus” camera;
2 - “minus” camera;
3 - input block;
5 - pusher;
6 - sector;
7 - trib;
8 - arrow;
9 - dial;
10 - separating bellows

Rice. 2.27

1 - “plus” camera;
2 - “minus” camera;
3 - transmitting rod;
4 - sector;
5 - trib;
6 - rocker

Rice. 2.28.

1 - rotating magnet;
2 - arrow;
3 - body;
4 - magnetic piston;
6 - working channel;
7 - plug;
8 - range spring;
9 - electrical contact block

1 and 2 - holders;
3 and 4 - tubular springs;
5 and 8 - tribs;

Topics

Synonyms

EN

DE

FR

15 differential pressure indicator

Small values of differential pressure can be measured with devices based on membranes and bellows.
Differential bellows indicating pressure gauges type chipboard-160 found wide application on the territory of the CIS. The principle of their operation is based on the deformation of two autonomous bellows blocks under the influence of “plus” and “minus” pressure. These deformations are converted into movement of the instrument's pointer. The pointer moves until equilibrium is established between the “positive” bellows, on the one hand, and the “minus” and coil spring, on the other.

Rice. 2.23

Differential bellows pressure gauge:

a - arrow drive diagram;
b - primary conversion block;
1 - “positive” bellows;
2 - “negative” bellows;
3 - rod;
4 - lever;
5 - torsion terminal;
6 - cylindrical spring;
7 - compensator;
8 - plane valve;
9 - base;
10 and 11 - covers;
12 - inlet fitting;
13 - cuff;
14 - throttling channel;
15 - valve;
16 - lever system;
17 - trib-sector mechanism;
18 - arrow;
19 - adjusting screw;
20 - tension spring;
21 - plug;
22 - rubber sealing ring

“Plus” 1 and “minus” 2 bellows (Fig. Fig. 2.23, b) are connected to each other by a rod 3, functionally connected to a lever 4, which, in turn, is fixedly fixed on the axis of the torsion bar 5. To the end of the rod at the outlet of the “negative” bellows, a cylindrical spring 6 is attached, secured bottom base on compensator 7 and working in tension. Each nominal pressure drop is associated with a specific spring.

The “plus” bellows consists of two parts. Its first part (compensator 7, consisting of three additional corrugations and plane valves 8) is designed to reduce the temperature error of the device due to changes in the volume of the filler liquid caused by variations in the ambient temperature. When the ambient temperature changes and accordingly working fluid its increasing volume flows through the plane valve into the internal cavity of the bellows. The second part of the “positive” bellows is working and is identical in design to the “minus” bellows.

The “plus” and “minus” bellows are attached to the base 9, on which covers 10 and 11 are installed, which together with the bellows form the “plus” and “minus” chambers with the corresponding pressure inlets 12 p + and p

The internal volumes of the bellows, as well as the internal cavity of the base 9, are filled with: PMS-5 liquid for normal and corrosion-resistant versions; composition PEF-703110 - in oxygen version; distilled water - in the option for food industry and liquid PMS-20 - for gas version.

In the designs of differential pressure gauges intended for measuring gas pressure, a cuff 13 is placed on the rod, the movement of the medium is organized through the throttling channel 14. By adjusting the size of the passage channel using valve 15, the degree of damping of the measured parameter is ensured.

The differential pressure meter works as follows. Media of “plus” and “minus” pressure enter through the supply fittings into the “plus” and “minus” chambers, respectively. “Plus” pressure has a greater effect on bellows 1, compressing it. This leads to the flow of liquid inside into the “negative” bellows, which stretches and decompresses the coil spring. Such dynamics occur before the interaction forces between the “plus” bellows and the pair - “minus” bellows - cylindrical spring are balanced. The measure of the deformation of the bellows and their elastic interaction is the movement of the rod, which is transmitted to the lever and, accordingly, to the axis of the torsion bar. A lever system 16 is fixed on this axis (Fig. 2.23, a), which ensures the transmission of rotation of the torsion bar axis to the tribular-sector mechanism 17 and the arrow 18. Thus, the impact on one of the bellows leads to angular movement of the torsion bar axis and then to rotation indicator arrow of the device.
Adjusting screw 19 with the help of tension spring 20 adjusts the zero point of the device.

Plugs 21 are intended for purging impulse lines, washing the measuring cavities of the bellows block, draining the working medium, and filling the measuring cavities with separating liquid when putting the device into operation.
When one of the chambers is overloaded on one side, the bellows is compressed and the rod moves. The valve in the form of a rubber sealing ring 22 fits into the seat of the base, blocks the flow of liquid from the internal cavity of the bellows, and thus prevents its irreversible deformation. During short-term overloads, the difference between “plus” and “minus” pressure on the bellows block can reach 25 MPa, and in certain types of devices it should not exceed 32 MPa.
The device can be produced in both general and ammonia (A), oxygen (K), corrosion-resistant food grade (PP) versions.

Rice. 2.24

Indicating differential pressure gauge based on membrane box:

1 - membrane box;
2 - positive pressure holder;
3 - “minus” pressure holder;
4 - body;
5 - transmission mechanism;
6 - arrow;
7 - dial

Have become quite widespread devices based on membranes and membrane boxes. In one of the options (Fig. 2.24), the membrane box 1, into which “positive” pressure enters through the inlet fitting of the holder 2, is the sensitive element of the differential pressure gauge. Under the influence of this pressure, the movable center of the membrane box shifts.
“Minus” pressure is supplied through the supply fitting of the holder 3 inside the sealed housing 4 of the device and acts on the membrane box from the outside, creating resistance to the movement of its moving center. Thus, the “plus” and “minus” pressures balance each other, and the movement of the moving center of the membrane box indicates the magnitude of the difference - differential pressure. This shift is transmitted through a transmission mechanism to the index hand 6, which on the dial scale 7 shows the measured differential pressure.
The range of measured pressure is determined by the properties of the membranes and is limited, as a rule, from 0 to 0.4...40 kPa. In this case, the accuracy class can be 1.5; 1.0; 0.6; 0.4, and in some devices 0.25.

Mandatory structural tightness of the housing determines high protection against external influences and is determined mainly by the IP66 level.

Beryllium and other bronzes, as well as stainless steel are used as materials for sensitive elements of devices; copper alloys are used for fittings and transmission mechanisms. corrosion-resistant alloys, including stainless steel.
Devices can be manufactured in cases of small (63 mm), medium (100 mm), and large (160 mm) diameters.

Diaphragm indicating differential pressure gauges, like instruments with diaphragm boxes, are used to measure small values of differential pressure. A distinctive feature is stable operation at high static pressure.

Rice. 2.25

Diaphragm indicating differential pressure gauges with vertical diaphragm:

1 - “plus” camera;
2 - “minus” camera;
3 - sensitive corrugated membrane;
4 - transmitting rod;
5 - transmission mechanism;
6 - safety valve

A differential pressure gauge with a vertical membrane (Fig. 2.25) consists of “plus” 1 and “minus” 2 working chambers, separated by a sensitive corrugated membrane 3. Under the influence of pressure, the membrane is deformed, as a result of which its center moves along with the transmitting rod 4 attached to it. The linear displacement of the rod in the transmission mechanism 5 is converted into axial rotation of the tube, and, accordingly, the index arrow, which reads the measured pressure on the instrument scale.

To maintain the functionality of the sensitive corrugated membrane when the maximum permissible static pressure is exceeded, an opening safety valve 6 is provided. Moreover, the designs of these valves may be different. Accordingly, such devices cannot be used when contact of media from the “plus” and “minus” chambers is not allowed.

Rice. 2.26

Diaphragm indicating differential pressure gauge with horizontal diaphragm:

1 - “plus” camera;
2 - “minus” camera;
3 - input block;
4 - sensitive corrugated membrane;
5 - pusher;
6 - sector;
7 - trib;
8 - arrow;
9 - dial;
10 - separating bellows

A differential pressure gauge with a horizontal sensitive membrane is shown in Fig. 2.26. Input block 3 consists of two parts, between which a corrugated membrane 4 is installed. A pusher 5 is fixed in its center, transmitting movement from the membrane, through sector 6, tribka 7 to arrow 8. In this transmission link, the linear movement of the pusher is converted into axial rotation of arrow 8 , tracking the measured pressure on the dial scale 9. This design uses a bellows system for removing the pusher from the working pressure zone. The separating bellows 10 with its base is hermetically attached to the center of the sensitive membrane, and with its upper part it is also hermetically attached to the inlet block. This design eliminates contact between the measured and the environment.
The design of the input block provides the possibility of washing or purging the “plus” and “minus” chambers and ensures the use of such devices for operation even in contaminated working environments.

Rice. 2.27

Diaphragm two-chamber indicating differential pressure gauge:

1 - “plus” camera;
2 - “minus” camera;
3 - transmitting rod;
4 - sector;
5 - trib;
6 - rocker

A two-chamber differential pressure measurement system is used in the design of the device shown in Fig. 2.27. The measured flows of the medium are directed to the “plus” 1 and “minus” 2 working chambers, the main functional elements which are autonomous sensitive membranes. The predominance of one pressure over another leads to linear movement transmitting rod 3, which is transmitted through the rocker arm 6, respectively, to sector 4, tribka 5 and the dial indicator system of the measured parameter.
Differential pressure gauges with a two-chamber measurement system are used to measure small differential pressures under high static loads, viscous media and media with solid inclusions.

Rice. 2.28.

Differential pressure gauge with magnetic transducer:

1 - rotating magnet;
2 - arrow;
3 - body;
4 - magnetic piston;
5 - fluoroplastic seal;
6 - working channel;
7 - plug;
8 - range spring;
9 - electrical contact block

A fundamentally different indicating differential pressure gauge is shown in Fig. 2.28. The rotating magnet 1, at the end of which an arrow 2 is installed, is placed in a housing 3 made of non-magnetic metal. The magnetic piston, sealed with a fluoroplastic seal 5, can move in the working channel 6. The magnetic piston 4, on the “minus” pressure side, is supported by plug 7, which in turn is pressed by range spring 8.
The “positive” pressure medium, through the corresponding supply fitting, acts on the magnetic piston and moves it along with plug 7 along channel 6 until such displacement is balanced by opposing forces - “minus” pressure and the range spring. The movement of the magnetic piston leads to axial rotation of the rotating magnet and, accordingly, the index arrow. This shift is proportional to the movement of the arrow. Full coordination is achieved by selecting the elastic characteristics of the range spring.
The differential pressure gauge with a magnetic transducer has a block 9 that closes and opens the corresponding contacts when passing near its magnetic piston.

Devices with a magnetic transducer are resistant to static pressure (up to 10 MPa). They provide a relatively low error (approximately 2%) in the operating range up to 0.4 MPa and are used for measuring the pressure of air, gases, and various liquids.

Indicating differential pressure gauge based on a tubular spring

1 and 2 - holders;
3 and 4 - tubular springs;
5 and 8 - tribs;
6 - arrow of “positive” pressure;
7 and 9 - scales overpressure;
10 - “minus” pressure arrow

In devices of this type, tubular springs are installed on autonomous holders 1 and 2, connected together. Each holder, together with a tubular sensing element, forms autonomous measuring channels. The “positive” pressure medium enters the tube 4 through the inlet fitting of the holder 2, deforms its oval, as a result of which the tip of the tube moves and this movement is transmitted through the corresponding toothed sector to the tube 5. This tube accordingly leads to the deflection of the index arrow 6, which points to scale 7 is the value of “positive” excess pressure.

“Minus” pressure through holder 1, tubular spring 3, tube 8 leads to movement of dial 9, combined with arrow 10, which on scale 7 tracks the value of the measured parameter.

Differential pressure gauges (hereinafter referred to as differential pressure gauges), as noted in paragraph 1.3, are the name classified in our country as indicating devices. (Devices that provide an electrical output signal proportional to the measured differential pressure are called differential pressure transducers.) Although some manufacturers, as well as some operating specialists, pressure difference measuring transducers are also called differential pressure gauges.

Differential pressure gauges have found their main application in technological processes for measuring, monitoring, recording and regulating the following parameters:

· consumption of various liquid, gaseous and vaporous media by pressure drop across various kinds narrowing devices (standard diaphragms, nozzles, including Venturi nozzles) and additional hydro- and aerodynamic resistances introduced into the flow, for example, on Annubar-type converters or on non-standard hydro- and aerodynamic obstacles;

· differential - pressure difference, vacuum, excess, at two points in the technological cycle, including losses on filters of ventilation and air conditioning systems;

· level of liquid media according to the size of the hydrostatic column.

Topics

Synonyms

EN

DE

FR

16 differential-pressure gage

Small values of differential pressure can be measured with devices based on membranes and bellows.
Differential bellows indicating pressure gauges type DSP-160 are widely used in the CIS. The principle of their operation is based on the deformation of two autonomous bellows blocks under the influence of “plus” and “minus” pressure. These deformations are converted into movement of the instrument's pointer. The pointer moves until equilibrium is established between the “positive” bellows, on the one hand, and the “minus” and coil spring, on the other.

Rice. 2.23

Differential bellows pressure gauge:

“Plus” 1 and “minus” 2 bellows (Fig. Fig. 2.23, b) are connected to each other by a rod 3, functionally connected to a lever 4, which, in turn, is fixedly fixed on the axis of the torsion bar 5. To the end of the rod at the outlet The “minus” bellows is connected to a cylindrical spring 6, fixed by the lower base to the compensator 7 and working in tension. Each nominal pressure drop is associated with a specific spring.

The “plus” bellows consists of two parts. Its first part (compensator 7, consisting of three additional corrugations and plane valves 8) is designed to reduce the temperature error of the device due to changes in the volume of the filler liquid caused by variations in the ambient temperature. When the ambient temperature and, accordingly, the working fluid changes, its increasing volume flows through the plane valve into the internal cavity of the bellows. The second part of the “positive” bellows is working and is identical in design to the “minus” bellows.

The internal volumes of the bellows, as well as the internal cavity of the base 9, are filled with: PMS-5 liquid for normal and corrosion-resistant versions; composition PEF-703110 - in oxygen version; distilled water - in the version for the food industry and PMS-20 liquid - for the gas version.

Rice. 2.24

Indicating differential pressure gauge based on membrane box:

1 - membrane box;
2 - positive pressure holder;
3 - “minus” pressure holder;
4 - body;
5 - transmission mechanism;
6 - arrow;
7 - dial

Mandatory structural tightness of the housing determines high protection from external influences and is determined mainly by the IP66 level.

Beryllium and other bronzes, as well as stainless steel are used as materials for sensitive elements of devices; for fittings and transmission mechanisms - copper alloys, corrosion-resistant alloys, including stainless steel.
Devices can be manufactured in cases of small (63 mm), medium (100 mm), and large (160 mm) diameters.

Rice. 2.25

Diaphragm indicating differential pressure gauges with vertical diaphragm:

1 - “plus” camera;
2 - “minus” camera;
3 - sensitive corrugated membrane;
4 - transmitting rod;
5 - transmission mechanism;
6 - safety valve

Rice. 2.26

Diaphragm indicating differential pressure gauge with horizontal diaphragm:

1 - “plus” camera;
2 - “minus” camera;
3 - input block;
4 - sensitive corrugated membrane;
5 - pusher;
6 - sector;
7 - trib;
8 - arrow;
9 - dial;
10 - separating bellows

Rice. 2.27

Diaphragm two-chamber indicating differential pressure gauge:

1 - “plus” camera;
2 - “minus” camera;
3 - transmitting rod;
4 - sector;
5 - trib;
6 - rocker

A two-chamber differential pressure measurement system is used in the design of the device shown in Fig. 2.27. The measured flows of the medium are directed into the “plus” 1 and “minus” 2 working chambers, the main functional elements of which are autonomous sensitive membranes. The predominance of one pressure over the other leads to linear movement of the transmitting rod 3, which is transmitted through the rocker arm 6, respectively, to sector 4, tribka 5 and the dial indicator system of the measured parameter.
Differential pressure gauges with a two-chamber measurement system are used to measure small differential pressures under high static loads, viscous media and media with solid inclusions.

Rice. 2.28.

Differential pressure gauge with magnetic transducer:

1 - rotating magnet;
2 - arrow;
3 - body;
4 - magnetic piston;
5 - fluoroplastic seal;
6 - working channel;
7 - plug;
8 - range spring;
9 - electrical contact block

Indicating differential pressure gauge based on a tubular spring

1 and 2 - holders;
3 and 4 - tubular springs;
5 and 8 - tribs;
6 - arrow of “positive” pressure;
7 and 9 - excess pressure scales;
10 - “minus” pressure arrow

“Minus” pressure through holder 1, tubular spring 3, tube 8 leads to movement of dial 9, combined with arrow 10, which on scale 7 tracks the value of the measured parameter.

Differential pressure gauges have found their main application in technological processes for measuring, monitoring, recording and regulating the following parameters:

· flow rate of various liquid, gaseous and vaporous media by pressure drop on various types of restriction devices (standard diaphragms, nozzles, including Venturi nozzles) and additional hydro- and aerodynamic resistances introduced into the flow, for example, on Annubar-type converters or on non-standard hydro- and aerodynamic obstacles;

· differential - pressure difference, vacuum, excess, at two points in the technological cycle, including losses on filters of ventilation and air conditioning systems;

· level of liquid media according to the size of the hydrostatic column.

Topics

pressure measuring instruments differential pressure measurement Wikipedia
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