Additional error and how to deal with it. Additional temperature error Temperature error for pressure sensors

Spring pressure gauges The following instrumental errors are typical.

1. Characteristic errors (scale errors) caused by incomplete mutual compensation of the nonlinearity of the characteristics of the sensing element and the transmission-multiplying mechanism, and in sensors, the electrical converter. These errors are minimized by individually adjusting the mechanism in manufactured samples of instruments and sensors.

Exist special mechanisms, making it possible to reduce errors to zero at many points of the characteristic. An example of such a mechanism is a mechanical scale error corrector, in which a roller slides over a cam made of flexible tape; the curvature of the cam can smoothly change due to local bending of the tape using adjusting screws (Fig. 6.15.). The roller is mounted on a lever, which, when rotated, imparts additional angular movement of one sign or another to the output axis. The sign of the additional movement depends on whether the roller hits the lobe or the recess of the cam.

2. Errors caused by the influence of harmful forces, which include, first of all, frictional forces in the transmission-multiplying mechanism and electrical converter, forces from imbalance of moving parts, electromagnetic or electrostatic forces from mutual attraction or repulsion of moving and stationary parts of the electrical converter. These errors can be reduced in the following ways:

a) reduction of harmful forces by improving the quality of supports, careful balancing of the mechanism, etc. Increasing the accuracy of balancing makes it possible to loosen the tension of the springs that select the backlash, which in turn helps to reduce friction forces;

b) increasing the effective area of ​​the sensitive element;

c) the use of differential electrical converters, in which in the initial position the attractive forces are mutually compensated;

d) the use of tracking systems that relieve the sensitive element from friction forces.

3. Temperature errors of pressure gauges caused by the influence of temperature environment on the physical parameters of materials and geometric dimensions of parts.

Temperature most significantly affects the elastic modulus of the sensing element.

The linearized dependence of the elastic modulus on temperature has the form

n/m 2,

Where E o- initial value E(at 6 = 9o) in n/m 2 ;

- temperature coefficient E;

The characteristic of the sensitive element of a differential pressure gauge is related to the elastic modulus by the relation

Relative value of temperature error

The influence of temperature on the geometric dimensions of the sensitive element and the transmission-multiplying mechanism is expressed by the dependence

m,

where is the geometric size;

Linear expansion coefficient.

This influence has a much weaker effect on the instrument readings due to the fact that the temperature coefficients of linear expansion of metals are an order of magnitude smaller than the temperature coefficients of the elastic modulus.

Temperature also affects the residual pressure height inside aneroids (sensitive evacuated elements) used in absolute pressure gauges. When the temperature changes by an amount, an error occurs

. Finally, as the temperature changes, the output parameter may change R, L, M or WITH electrical converter.

Reducing temperature errors is achieved in the following ways:

a) the manufacture of sensitive elements from an alloy of the Elinvar type, which have a very low temperature coefficient of elasticity modulus;

b) reducing the residual pressure inside the aneroids by more thoroughly vacuuming them;

c) by introducing into the design of the device special bimetallic compensators, which, depending on the temperature, cause an increase in the reading of the device, equal in magnitude and opposite in sign to the temperature error of the device.

There are bimetallic compensators of the 1st and 2nd types.

The operation of compensators of the 1st type (Fig. 6.16, a) is based on the introduction in series with the elastic sensitive element of a kinematic element, made in the form of a cantilever mounted bimetallic plate, the linear movement of the free end of which, proportional to the temperature increase, is added to the deflection s of the elastic sensitive element ( or subtracted from it). The calculation of the value for a plate-type bimetallic compensator (see Fig. 6.19, a) is carried out according to the formula (see Chapter II):

m,

where is the thickness of the bimetallic plate in m;

- coefficients of linear expansion of components

bimetal;

Plate length in m;

- temperature increment °C.

A type 1 compensator compensates only for the additive temperature error.

The action of type 2 compensators (see Fig. 6.16.6) is based on the introduction into the crank of a kinematic link made in the form of a bimetallic plate, the movement of the free end of which, proportional to the temperature increase, causes an increase or decrease in the crank arm by an amount , which is determined in the same way as the value of As for a compensator of the 1st kind, according to formula (6.16). The nature of the influence of the 2nd type compensator on the increment in instrument readings depends on the initial angle of installation of the crank (see Fig. 6.16, a). If this angle is close to zero, that is, if at s = 0 the crank is approximately perpendicular to the connecting rod, then the increment of the crank arm almost does not cause the initial rotation of the crank, but only changes the gear ratio of the mechanism. Therefore, at = 0, the correction introduced by the 2nd type compensator is purely multiplicative in nature.

d) the use of differential electrical converters that produce two variable parameters z 1 And z 2 and connected according to a voltage divider circuit; when operating on a high-resistance load, the differential converter has no temperature error, since the magnitude of the voltage removed depends on the parameter values z 1 And z 2 does not depend, but is determined by the relation z 1 / z 2 it is important to ensure only equality of temperature coefficients of parameters z 1 And z2,

e) the use of electrical compensators, made in the form of wire or semiconductor thermal resistances and connected to the external electrical circuit so as to compensate for temperature errors introduced by all other elements of the sensor. Variants of such schemes are discussed in Chap. VII.

4. Errors from backlash in supports, hinges and guides of the transmission and multiplying mechanism. To eliminate errors from backlash, a spiral spring (hair) is installed on the output axis of the transmission-multiplying mechanism, which is given the initial tension. The amount of tension is selected based on considerations so that, over the entire range of rotation angles of the output axis, the moment created by the spring around its axis slightly exceeds the reduced unbalance moment multiplied by the maximum value of vibration overload or overload from linear accelerations. Too much spring tension is undesirable, as it leads to increased friction errors.

5. Errors from hysteresis and elastic aftereffect. Reducing these errors is achieved by selecting materials with good elastic properties and improving their heat treatment conditions. Sensitive elements made of 47ХНМ type alloys and beryllium bronze have the smallest errors from hysteresis and elastic aftereffect.

6. Errors from the influence of ambient pressure. These errors arise in pressure gauges with dual sensitive elements (see Fig. 3.6 and 6.8) if their effective areas are unequal. To reduce errors, sensitive elements with the closest possible effective areas are selected.

1. Features of the use of pressure sensors

The areas of application of pressure sensors (pressure transducers) are quite wide, but, as a rule, each specific application has its own specifics that must be taken into account in the design of the sensors.

In general, all applications of pressure transducers can be divided into two main groups:

• Measuring the actual pressure (or vacuum) of any medium in a pipeline or technological installation;
• Measuring the level of liquids in containers (tanks) by measuring the pressure of the liquid column (hydrostatic level sensor).

When selecting pressure sensors of both groups, it is necessary to clarify the following application features:

• Hygiene requirements: The food and pharmaceutical industries place high demands on pressure sensors in terms of hygiene both at the point of contact with the product and outside (as a rule, they are made entirely of stainless steel). The assortment of KIP-Service LLC includes pressure sensors KLAY-INSTRUMENTS, which are specially designed for use in dairy, brewing and Food Industry .
• Availability of certificates: often, for various applications, in addition to the usual GOST R certificate of conformity (or declaration of conformity), additional certificates are required. For example, accounting systems require a certificate of approval of the type of measuring instruments; for the use of pressure sensors in the food industry, a conclusion from the SES is required; for applications in hazardous industries, permission from Rostechnadzor is required, etc.
• Explosion protection requirements: In explosive industries (for example, oil and gas, chemical, alcohol industries), explosion-proof pressure sensors are used. The most widely used types of explosion protection for sensors are intrinsically safe Ex ia circuits and explosion-proof enclosure Ex d, the choice of which is determined by the specific application.
• Type of measured medium: if the medium being measured is viscous, aggressive, weakly fluid, or has any other specific properties (for example, the presence of dirt particles), these features must also be taken into account. It is possible that this application requires the use of membrane pressure sensors (equipped with a separating membrane), which protect the sensitive element of the sensor from exposure to aggressive media.
• Presence of external influences: the presence of vibration, electromagnetic fields or other mechanical or electrical influences.

When selecting pressure sensors for Group I applications when measuring pressures greater than 1 bar, you also need to consider:

• Presence of water hammer in the system: if there may be water hammer in the system, the pressure sensor must be selected with a sufficient margin for overload (peak pressure) or measures must be taken to compensate for water hammer (silencers, special sensors, etc.) on site;
• Optional equipment: As a rule, when measuring pressure, sensors are mounted using 3-way valves; in addition, when measuring steam pressure, it is recommended to connect pressure sensors through a special device - Perkins tube, which reduces the temperature of the medium acting on the pressure sensor.

When selecting pressure sensors for use as hydrostatic level sensors, it is necessary to take into account the fact that the pressure value at the same height of the liquid column can change with changes in the density of the measured medium.

2. Measuring range

Pressure sensor measurement range - the range of pressure values, when applied, the sensor will carry out measurements and linear conversion of the measured value into a unified output signal.

The measurement range is determined by the lower and upper measurement limits, which correspond to the minimum and maximum values ​​of the measured pressure. Examples of measuring ranges: 0…1 bar, 0…2.5 MPa, –100…100 KPa.

When selecting pressure sensors, it is necessary to take into account that sensors come with both a fixed measurement range (for example, PD100 pressure transducers) and with an adjustable measurement range (for example, KLAY-INSTRUMENTS pressure sensors). For pressure sensors with a fixed measuring range, the output signal values ​​are strictly tied to the measurement limits. For example, a PTE5000 pressure sensor at a pressure of 0 MPa will output 4 mA, and at a pressure of 0.6 MPa it will output 20 mA, since it is rigidly configured for the range of 0 ... 0.6 MPa. In turn, the KLAY 8000-E-S pressure sensor has an adjustable range of 0-1...4 bar, which means that at a pressure of 0 bar the sensor will similarly output 4 mA, and the sensor will output 20 mA at any value from the range of 1...4 bar, which is adjusted by the user using a special potentiometer “SPAN”.

3. Process temperature

The temperature of the measured medium is very important parameter when choosing pressure sensors. When selecting a sensor, it is necessary that the process temperature does not go beyond the permissible operating temperature range.

In the food industry, short-term (20 to 40 minutes) CIP and SIP cleaning (sanitization) processes occur where ambient temperatures can reach 145 °C. For such applications, sensors should be used that are resistant to such temporary exposure to high temperatures, such as KLAY-INSTRUMENTS SAN pressure sensors - 8000-SAN and 2000-SAN.

The readings of all pressure sensors using the tensor-resistive principle of conversion strongly depend on the temperature of the measured medium, since the resistance of the resistors that make up the measuring circuit of the pressure sensor also changes with temperature changes.

For pressure sensors, the concept of “temperature error” is introduced, which is an additional measurement error for every 10 °C change in the temperature of the measured medium relative to the base temperature (usually 20 °C). Thus, the process temperature must be known to determine the total measurement error of the pressure sensor.

To reduce the influence of temperature, pressure meters use various temperature compensation schemes.

Based on the use of temperature compensation, all pressure sensors can be divided into three groups:

• Budget pressure sensors that do not use thermal compensation circuits;
• Mid-price sensors using passive thermal compensation circuits;
• Pressure Sensors high level, for systems requiring measurement accuracy that use active temperature compensation circuits.

To measure the pressure of media with a constant temperature of more than 100 °C, special high-temperature pressure sensors are used, which make it possible to measure the pressure of media with temperatures up to 250 °C. As a rule, such sensors are equipped with a cooling radiator and/or have a special design that allows the part of the sensor with electronics to be placed in an area with an acceptable operating temperature.

4. Type of connection between the sensor and the process

Type of connection of the sensor to the process - the type of mechanical inclusion of the pressure sensor in the process to carry out measurements.

The most popular connections for pressure transmitters of general industrial design are threaded connections G1/2″ DIN 16288 and M20x1.5.

When selecting a sensor, the type of connection must be specified to ensure ease of installation into an existing system without additional work (welding, cutting other types of threads, etc.)

The most diverse types of process connections used are the food, pulp and paper and chemical industries. For example, KLAY-INSTRUMENTS pressure sensors, which are specially designed for these industries, can be manufactured with more than 50 various options inclusion in the process.

The choice of connection type is most relevant for the food industry, because along with convenience, the connection must first of all ensure “sanitary” and the absence of “dead zones” for the sanitization process. For pressure sensors intended to operate in contact with food products, there are special certificates confirming their “sanitary” properties - the European EHEDG (European Hygienic Equipment Design Group) certificate and the American 3A Sanitary Standards certificate. In Russia, for sensors in contact with food media, availability is required Sanitary and epidemiological conclusions. In the assortment of KIP-Service LLC, the requirements of these certificates are met by sensors of the 8000-SAN and 2000-SAN series from KLAY-INSTRUMENTS.

5. Environmental parameters

When selecting pressure transmitters, the following environmental parameters should be taken into account:

• Ambient temperature;
• Ambient humidity;
• Presence of aggressive environments;

All environmental parameters must be within acceptable limits for the selected pressure sensor.

If there are aggressive substances in the environment, many manufacturers of pressure sensors (including KLAY-INSTRUMENTS BV) offer special versions that are resistant to chemical influences.

When working in conditions high humidity With frequent temperature changes, pressure sensors from many manufacturers are faced with the problem of pressure sensor corrosion. The main cause of sensor corrosion in pressure sensors is the formation of condensation.

To measure relative pressure, excess pressure sensors require communication between the sensor and the atmosphere. In inexpensive sensors, the sensor is connected to the atmosphere due to the non-sealed housing (IP65 connector); wet air, with this design, after getting inside the sensor, it condenses as the temperature drops, thereby gradually causing corrosion of the measuring element.

For applications where conventional pressure sensors fail due to sensor corrosion, KLAY-INSTRUMENTS industrial pressure sensors are ideal. For KLAY pressure transducers, the sensor is connected to the atmosphere through a special “breathable” membrane made of Gore-Tex material, which prevents moisture from penetrating into the sensor.

In addition, the sensor contacts of all KLAY sensors are filled by default with a special synthetic compound for additional protection sensor against corrosion.

6. Pressure sensor output type

The most common analog output signal for pressure sensors is a unified 4...20 mA current signal.

Almost always 4 mA corresponds to the lower value of the measurement range, and 20 mA to the upper value, but sometimes a reverse signal occurs (usually on vacuum ranges). Also in industry there are pressure sensors with other types of analog output signals, for example: 0...1 V, 0...10 V, 0...20 mA, 0...5 mA, 0...5 V.

The range of pressure sensors stocked by KIP-Service LLC includes only sensors with an output signal of 4...20 mA. To obtain another type of output signal from 4...20 mA, you can use the universal signal converter Seneca Z109 REG2, which mutually converts almost all types of unified current and voltage signals, while providing galvanic isolation.

Smart pressure sensors, in addition to the main 4...20 mA signal, can be manufactured with support for the HART protocol, which can be used to configure or obtain information about the status of the sensor and additional information.

In addition to analog output, smart pressure sensors also come with digital output. These are sensors with output via the Profibus PA protocol, which SIEMENS uses in its devices.

7. Required measurement accuracy

When calculating the measurement error of pressure sensors, it is necessary to take into account that in addition to the main error, there is an additional error.

Basic error- the value of the pressure sensor error relative to the measurement range, declared by the manufacturer for normal operating conditions. As a rule, the following conditions are understood as normal operating conditions:

• Ambient and working temperature - 20 °C;
• The pressure of the working medium is within the measuring range of the sensor;
• Normal atmospheric pressure;
• There is no flow turbulence or other phenomena at the location where the sensor is installed that could affect the readings.

Additional error - the error value caused by deviation of operating conditions from normal, due to the characteristics of this particular application. One of the main components of the additional error is the temperature error, which is indicated in technical documentation to pressure sensors and can be calculated for a specific temperature of the working medium.

Also, additional error can be caused by turbulence of the flow of the measured medium, changes in the density of the medium during hydrostatic level measurement, dynamic loads on equipment while moving in space (vessels, vehicles, etc.) and other possible factors.

When calculating the error of the measuring system as a whole, it is also necessary to take into account the accuracy class measuring instrument- indicator.

As an example, let's calculate the total measurement error for the following system:

Given:

• The KLAY-Instruments 8000-SAN-F-M(25) pressure sensor is installed on the product pipeline;
• The maximum product pressure is 4 bar, so the sensor is set to a range of 0…4 bar;
• Maximum product temperature - 60 °C;
• Flow turbulence and other factors do not affect accuracy.

Solution:

• According to the passport data, we find that the main error of the 8000-SAN-F-(M25) sensor is 0.2%
• Temperature error according to the passport it is 0.015%/°C, so the temperature error at 60 °C is 0.015%/°C x (60 °C – 20 °C) = 0.6%
• 0.2% + 0.6% + 0.25% = 1.05% - total relative error;
• 1.05% x 4 bar = 0.042 bar - the absolute measurement error of this system.

When choosing pressure sensors, any consumer sets the goal of measuring pressure with the accuracy stated in the technical documentation. This is one of the sensor selection criteria. In the passport for the sensor, GOST standards require that acceptable values ​​be indicated basic error measurements (+ - from true pressure). These values ​​according to GOST 22520 are selected from the range 0.075; 0.1; 0.15; 0.2; 0.25; 0.4; 0.5%; etc. depending on the technical capabilities products. The main error indicator is normalized for normal (i.e. ideal) conditions measurements. Normal conditions are determined according to GOST 12997. These conditions are also specified in the measuring instrument verification procedure. For example, according to MI1997, to determine the main error, you need to set the following environmental conditions. Wednesday:
- temperature 23+-2оС,
- humidity from 30 to 80%,
- atm. pressure 84-106.7 kPa,
- power supply 36+-0.72V,
- absence of external magnetic fields, etc.
As you can see, the operating conditions for the sensor when determining the main error are almost ideal. Therefore, each calibration laboratory must have the ability to regulate them. For example, to regulate the temperature in a room, microclimate devices (heater, air conditioner, etc.) are used. But what readings from the sensor we will get in real operating conditions at the facility, for example at +80°C or -30°C, is a question. The answer to this question is given by the indicator additional error, which is also standardized in TU and GOST.
Additional error- Deviation of the conversion function caused by one influencing quantity (temperature, pressure, vibration, radio interference, supply voltage, etc.). Calculated as difference(ignoring the sign) between the error value in workers(actual) measurement conditions, and the error value under normal conditions.
Of course, all operating conditions factors affect the output signal. But for pressure sensors (transmitters) the most significant effect is the deviation of the ambient air temperature. In GOST 22520, the additional error is normalized for every 10°C deviation from normal conditions (i.e. from 23°C). Tolerances according to GOST look like this:

If the sensor meets these tolerances during temperature testing, then it “complies with GOST 22520,” which in most cases is written in the documentation for the sensor.
Let's analyze the accuracy of the sensor, which complies with GOST 22520, when exposed to temperature. For example, a sensor with a basic error of 0.5% and an operating temperature range of -30..+80°C at 30°C can err by 0.5+0.45=0.95%, at 40°C (deviation of 2 deci.°C) 1.4% accordingly, and finally at 80°C we get an accuracy of 3.2% - this is the sum of the main and additional errors. Let me remind you that we are dealing with a 0.5% sensor, and when operating at 80°C we get an accuracy of 3.2% (approx. 6 times worse), and such a sensor meets the requirements of GOST 22520.
The results do not look very nice and will certainly not please the buyer of a sensor with a stated accuracy of 0.5%. Therefore, most manufacturers do thermal compensation of the output signal and the requirements for additional sensors are tightened in the specifications for a specific sensor. errors due to temperature. For example, for SENSOR-M sensors, in the technical specifications we set a requirement of less than 0.1% per 10°C.
Purpose of temperature compensation– reduce additional error from temperature to zero. Nature additional We will consider temperature errors and methods of temperature compensation of sensors in detail in the next article. In this article I would like to summarize.
Need to take into account main error and additional depending on the required measurement accuracy within operating temperatures sensor The additional error of each sensor can be found in the passport, operating manual or specifications for the product. If the indicator is additional errors are not specified in those. Documentation for the sensor, then it simply meets the GOST requirements that we analyzed above.
One should also distinguish temperature compensation range And Operating temperature range. In the temperature compensation range, additional the error is minimal; when you go beyond the temperature compensation range, the requirements apply again

It is clear that after 4 years the question is no longer relevant, but as I understand it, at +23C an error was obtained (25.04/25-1)*100%= +0.16% (in% of URL, which is 25MPa), at +55C it was The resulting error is (24.97/25-1)*100% = -0.12%.

And the sensor error at +23C is normalized as 0.2% of URL, and at +55C it should be 0.2%+0.08%*(55C-23C)/10C = 0.456% of URL.

that is, there cannot be any problems with verification (at +23C we have +0.16% with a tolerance of +/-0.2%, at +55C we have -0.12% with a tolerance of +/-0.456%). At +55C the device even turned out to be more accurate than at normal (+23C) temperature.

That is, there can be no problems with verification (at +23C we have +0.16% with a tolerance of +/-0.2%...

Everything seems to be readings taken fit within the basic error , equal in this case to 0.05MPa....

The following question arose: the pressure sensor, which is preparing for type testing for a measuring instrument...

During these tests, the correctness and validity of the MX... proposed by the developer of this sensor must be established, in this case additional sensor error due to temperature changes environment...

The measured values ​​showed that the main error of the tested sensor did not exceed the value of the limits of permissible error proposed by the developer for it - ±0.2% or in absolute values ​​±0.05 MPa, but

the obtained value of the additional error from the temperature change for this sensor exceeded The developer's proposed value for the limits of permissible additional error:

According to the method for calculating the additional temperature error, we obtain:

(24.97-25.04)/(25*0.1*(55-23)) * 100 = -0.0875%, i.e. The sensor does not fit into the additional temperature error!!!

Those. the developer assumed that this type of sensor has additional error from a change in temperature of ±0.08% of URL for every 10°C, and when checking this value on the first sensor it came across, it turned out to be -0.0875%....

Here the question immediately arises as to whether the developer has set the value correctly additional error from a temperature change equal to ±0.08% of URL for every 10°C..., because it is necessary to check not the total error of the sensor at a temperature of +55°C, as you do (imagine what would happen if the obtained value of the main error was at the permissible limit for this sensor...), namely, the parameter which is normalized..., i.e. size changes errors from the corresponding changes temperatures....

Moreover, the measured values ​​make it possible to estimate the additional error from temperature changes only up from the temperature taken as normal +23°C.

It is also necessary to estimate the additional error from temperature changes down from the temperature taken as normal +23°C, i.e. at -40°C, and this change is not 32°C, as up to a temperature of +55°C, but 63°C...., i.e., most likely, the value of the additional error from the temperature change down the result will be even greater than the value obtained for this sensor up (-0.0875%)....

As a rule, the additional error from temperature changes for SI is set to the maximum of the additional errors up And down...., or, in rare cases, two - different...

Therefore, in this case, it is necessary to carry out a series of additional tests on a representative sample of the sensors under consideration in order to establish an adequate additional error for them (for this type of sensor) from temperature changes...