For certain control purposes, for example to control a heating system, it may be important to measure the temperature difference. This measurement can be carried out, in particular, by the difference between the external and internal temperatures or the inlet and outlet temperatures.
Rice. 7.37. Measuring bridge to determine the absolute values of temperature and temperature difference at 2 points; U Br – bridge voltage.
The basic design of the measuring circuit is shown in Fig. 7.37. The circuit consists of two Wheatstone bridges, and the middle branch (R3 - R4) of both bridges is used. The voltage between points 1 and 2 indicates the temperature difference between Sensors 1 and 2, while the voltage between points 2 and 3 corresponds to the temperature of Sensor 2, and between points 3 and 1 the temperature of Sensor 1.
Simultaneous measurement of temperature T 1 or T 2 and the temperature difference T 1 - T 2 is important when determining the thermal efficiency of a heat engine (Carnot process). As is known, the efficiency W is obtained from the equation W = (T 1 – T 2)/T 1 = ∆T)/T 1.
Thus, to determine, you only need to find the ratio of the two voltages ∆U D 2 and ∆U D 1 between points 1 and 2 and between points 2 and 3.
To fine-tune the described instruments designed to measure temperature, fairly expensive calibration devices are needed. For the temperature range 0...100°C, the user has quite accessible reference temperatures, since 0°C or 100°C, by definition, are the crystallization or boiling points of pure water, respectively.
Calibration at 0°C (273.15°K) is carried out in water with melting ice. To do this, an insulated vessel (for example, a thermos) is filled with highly crushed pieces of ice and filled with water. After a few minutes, the temperature in this bath reaches exactly 0°C. By immersing the temperature sensor in this bath, sensor readings corresponding to 0°C are obtained.
They act similarly when calibrated at 100°C (373.15 K). A metal vessel (for example, a saucepan) is half filled with water. The vessel, of course, should not have any deposits (scale) on the inner walls. By heating the vessel on a hot plate, bring the water to a boil and thereby reach the 100-degree mark, which serves as the second calibration point for the electronic thermometer.
To check the linearity of a sensor calibrated in this way, at least one more test point is required, which should be located as close as possible to the middle of the measured range (about 50°C).
To do this, the heated water is cooled again to the specified area and its temperature is accurately determined using a calibrated mercury thermometer with an accuracy of 0.1 ° C. At temperatures around 40°C, it is convenient to use a medical thermometer for this purpose. By accurately measuring the water temperature and output voltage, a third reference point is obtained, which can be considered a measure of the linearity of the sensor.
Two different sensors, calibrated by the method described above, give identical readings at points P 1 and P 2, despite their different characteristics (Fig. 7.38). An additional measurement, for example of body temperature, reveals the nonlinearity of the characteristic IN sensor 2 at point P 1. Linear characteristic A sensor 1 at point P 3 corresponds to exactly 36.5% of the total voltage in the measured range, while the nonlinear characteristic B corresponds to a clearly lower voltage.
Rice. 7.38. Determination of the linearity of the sensor characteristics with a range of 0...100ºС. Linear ( A) and nonlinear ( IN) the characteristics of the sensors coincide at the reference points 0 and 100ºС.
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Temperature sensors made of platinum and nickel
Thermocouple
Silicon temperature sensors
Integral temperature sensors
Temperature controller
Thermistors with negative TCS
Thermistors with positive TCR
Level sensor based on a thermistor with positive TCR
Temperature difference measurement and sensor calibration
PRESSURE, FLOW AND SPEED SENSORS
Like temperature sensors, pressure sensors are among the most widely used in technology. However, for non-professionals, pressure measurement is of less interest, since existing pressure sensors are relatively expensive and have only limited applications. Despite this, let's look at some options for using them.
Calibration of an external temperature sensor for measuring ion concentration in automatic temperature compensation mode (type TD-1, TKA-4 etc. with a resistance of the sensitive element of no more than 5 kOhm) is carried out in order to adjust the temperature sensitivity in automatic mode at several points (from 2 to 5). Calibration must be carried out using a thermostat that ensures that the set temperature is maintained with an accuracy of no worse than 0.1 o C.
Connect the temperature sensor to the connector "sensor" or "THAT 2 » measuring transducer. Turn on the analyzer, enter the mode “Additional Mode” and press the button “ENTER”.
Buttons “ “ And “ “ select option “Gradthermometer” and press the button “ENTER”. To enter the thermometer calibration mode, you must enter a password. The display will show
ENTER PASSWORD
Enter the number
You must enter a number from the keyboard "314" and press the button "ENTER".
Enter the number of graduation points. To do this, click the button "N".The following message will appear on the display:
Number of points
Buttons “ “ And “ “ install required number calibration points and press the button “ENTER”. In this case, a window will appear on the display with the solution temperature value in the top line, the conditional calibration number and the calibration point number in the bottom line, for example:
25.00 0С
xxxxx.xxx n1
Set the water temperature in the thermostat at the beginning of the temperature compensation range, for example (5 0.5) 0 C. Go to the first calibration point. To do this, click “ “ select the window with the graduation point number in the bottom line n1. Then click the button “Izm”. The display will show a changing calibration value.
numbers. After establishing its constant value, press the button “ENTER”.After the message:
Entering a change?
YES - ENTER NO - CANCEL
click the button “ENTER”. Then click the button “Number”. A message appears "Enter the number". Enter the temperature measured by the reference thermometer and press the button “ENTER”.After message
Entering a change?
YES - ENTER NO - CANCEL
press the buttons in sequence “ENTER”.
Similarly, calibrate the remaining temperature points, for example at temperatures (20 0.5) 0 C and (35 0.5) 0 C.
This will automatically adjust the temperature sensitivity of the device.
3.6. Verification instructions
3.6.1. All newly produced analyzers, those coming out of repair and those in operation are subject to verification.
3.6.2. Periodic verification of analyzers must be carried out at least once a year by the territorial bodies of the metrological service of Gosstandart.
3.6.3. Verification of analyzers is carried out in accordance with the “Verification Methodology”
3.7. Performer qualification requirements
Persons with higher or secondary specialized education, who have undergone appropriate training, have experience working in a chemical laboratory, and must annually undergo a safety knowledge test are allowed to perform measurements and process results.
3.8. Security measures
3.8.1. In terms of safety requirements, the device meets the requirements of GOST 26104, protection class III.
3.8.2. When carrying out tests and measurements, safety requirements in accordance with GOST 12.1.005, GOST 12.3.019 must be observed.
3.8.3. When working with analyzers, you must perform general rules work with electrical installations up to 1000V and the requirements provided for by the “Basic Rules safe work in the chemical laboratory”, M; Chemistry, 1979-205p.
4. REPAIR
4.1. Repair conditions
Analyzers are complex electronic devices, therefore qualified personnel of the manufacturer or official representatives are allowed to repair them under the terms of service. After repair, it is mandatory to check the main technical characteristics of the device in accordance with the “Verification Methodology”.
When repairing analyzers, safety measures should be taken in accordance with the current rules for the operation of electrical installations up to 1000 V.
4.2. Possible malfunctions and ways to eliminate them
A list of some of the most common or possible malfunctions analyzers, their symptoms and solutions are given in Table 4.
Table 4.1
|
Name of the malfunction and external manifestation |
Probable Causes |
Remedies |
|
After turning on the analyzer, there is no information on the indicator |
1. There are no batteries or they are completely discharged 2. There is no voltage in the network 3. The power supply is faulty 4. Low battery |
1. Install or replace batteries 2. Connect the power supply to a working outlet 3. Replace the power supply 4. Charge the battery by connecting the power supply |
|
After turning on the analyzer, the indicator “Change batteries” appears on the indicator. |
Batteries are low |
Replace batteries |
Other faults are corrected by the manufacturer.
The calibrator can be used as either a dry-block or a liquid thermostat. The calibrator uses unique technology heat pump Stirling with gas coolant(FPSC). Appearance workplace is shown in Figure 4.
Figure 4 - Appearance of the workplace
The calibrator's thermostat has two zones with separate regulation. The regulator of the lower zone maintains the set temperature value, and the upper one maintains a “zero” temperature difference relative to the lower zone. This method ensures high temperature uniformity in work area and low error of its assignment.
The calibrator is equipped with a circuit for measuring the signal from an external reference resistance thermometer. Such a thermometer is installed next to the sensor being verified and connected to a special connector on the calibrator. This greatly simplifies calibration using the comparison method, which has a significantly lower error.
The calibrator is equipped with a DLC circuit - dynamic compensation for the influence of heat loss through the sensors being verified. The DLC thermometer is installed next to the sensor being verified, measures the temperature difference in the working area of the insert tube and controls the regulator upper zone thermostat. This ensures highly uniform temperature distribution in the working area up to 60 mm from the bottom of the tube, regardless of the number and/or diameter of the inserted sensors.
The calibrator allows you to measure the signals of verified thermocouples and resistance thermometers (mV, Ohm, V, mA) according to GOST, IEC and DIN.
Unique Features:
Lowest limit negative temperature-100°C;
Extremely high stability;
High temperature uniformity in the working area up to 60 mm from the bottom of the insert tube;
Low error;
A unique circuit for dynamic compensation of the influence of thermostat loading;
Fast heating, cooling;
Full compensation for the influence of surges and instability of the mains power supply;
Built-in means of measuring the output signals of various temperature sensors;
Built-in circuit for measuring the signal of an external reference smart resistance thermometer, in the memory of which individual calibration coefficients are stored;
Saving calibration/verification results in the internal memory of the calibrator;
Friendly Russified menu-based user interface;
Full automation of verification/calibration of temperature sensors both in standalone mode and when working with a PC under software control, including verification of several sensors simultaneously using ASM-R switches.
In addition to ensuring the setting of temperature settings, the calibrator automatically implements verification/calibration in a stepwise temperature change mode, as well as (in version B) calibration of the thermal relay.
Russified software allows you to:
Check temperature sensors in automatic mode or load verification/calibration tasks into the calibrator and, after performing it in offline mode, transfer the verification results to a PC.
Recalibrate the calibrator for temperature and electrical signals.
The software provides access to control all functions of calibrators and, in addition, allows you to load multiple calibration tasks into the calibrator and, after they are completed in autonomous or automatic modes, transfer the results to a personal computer for processing and storage.
Using the software, you can adjust the internal (“READ”) thermometer of the calibrators, as well as the channels for measuring electrical quantities, including the channel of the external (“TRUE”) thermometer. This software allows you to load a calibration characteristic for an external high-precision resistance thermal converter into the calibrator.
Software structure:
Support for verifiable/calibrated temperature measuring instruments;
Configuring the temperature measuring instrument verification/calibration scheme;
Temperature measuring instrument verification/calibration scheduler;
Verification/calibration of temperature measuring instruments using a PC.
Connectors for connecting to a computer, as well as for connecting external devices are presented in Figure 5.
Figure 5 - Digital connectors.
Nbsp; LABORATORY WORK No. 8 Temperature measurement using resistance thermometers and bridge measuring circuits 1. Purpose of the work. 1.1. Familiarization with the principle of operation and technical device resistance thermometers. 1.2. Familiarization with the structure and operation of automatic electronic bridges. 1.3. Study of two and three wire circuits for connecting resistance thermometers.
General information.
2.1. Design and operation of resistance thermometers.
Resistance thermometers are used to measure temperatures in the range from -200 to +650 0 C.
The operating principle of metal resistance thermometers is based on the property of conductors to increase electrical resistance when heated. The heat-sensitive element of a resistance thermometer is a thin wire (copper or platinum) spirally wound around a frame and enclosed in a sheath.
Electrical resistance wire at a temperature of 0 0 C strictly defined. By measuring the resistance of a resistance thermometer with a device, you can accurately determine its temperature. The sensitivity of a resistance thermometer is determined by the temperature coefficient of resistance of the material from which the thermometer is made, i.e. a relative change in the resistance of the heat-sensitive element of a thermometer when it is heated by 100 0 C. For example, the resistance of a thermometer made of platinum wire changes by approximately 36 percent when the temperature changes by 1 0 C.
Resistance thermometers, for example, have a number of advantages compared to manometric ones: higher measurement accuracy; the ability to transmit readings over long distances; the ability to centralize control by connecting several thermometers to one measuring device (via a switch).
The disadvantage of resistance thermometers is the need for an external power source.
Automatic electronic bridges are usually used as secondary devices complete with a resistance thermometer. For semiconductor thermal resistances, the measuring instruments are usually unbalanced bridges.
For the manufacture of resistance thermometers, as noted above, pure metals (platinum, copper) and semiconductors are used.
Platinum most fully meets the basic requirements for a material for resistance thermometers. In an oxidizing environment, it is chemically inert even at very high temperatures, but performs significantly worse in a recovery environment. In a reducing environment, the sensing element of a platinum thermometer must be sealed.
The change in platinum resistance within the temperature range from 0 to +650 0 C is described by the equation
R t =R o (1+at+bt 2),
where R t, R o is the resistance of the thermometer, respectively, at 0 0 C and temperature t
a, b are constant coefficients, the values of which are determined by calibrating the thermometer according to the boiling points of oxygen and water.
The advantages of copper as a material for resistance thermometers include its low cost, ease of production pure form, relatively high temperature coefficient and linear dependence of resistance on temperature:
R t =R o (1+at),
where R t, R o - resistance of the thermometer material, respectively at 0 0 C and temperature t;
a - temperature coefficient of resistance (a = 4.26*E-3 1/deg.)
The disadvantages of copper thermometers include low resistivity and easy oxidation at temperatures above 100 0 C. Semiconductor thermal resistances. A significant advantage of semiconductors is their large temperature coefficient of resistance. In addition, due to the low conductivity of semiconductors, small-sized thermometers with a large initial resistance can be made from them, which makes it possible to ignore the resistance connecting wires and other elements electrical diagram thermometer. Distinctive feature Semiconductor resistance thermometers have a negative temperature coefficient of resistance. Therefore, as the temperature increases, the resistance of semiconductors decreases.
For the manufacture of semiconductor thermal resistances, oxides of titanium, magnesium, iron, manganese, cobalt, nickel, copper, etc. or crystals of certain metals (for example, germanium) with various impurities are used. Thermal resistance types MMT-1, MMT-4, MMT-5, KMT-1 and KMT-4 are most often used to measure temperature. For all thermal resistances of the MMT and KMT types in the operating temperature ranges, the resistance varies with temperature according to an exponential law.
Platinum resistance thermometers (PRT) for temperatures from -200 to +180 0 C and copper resistance thermometers (RCT) for temperatures from -60 to +180 0 C are commercially produced. Within these temperature ranges, there are several standard scales.
All commercially produced platinum resistance thermometers have symbols: 50P, 100P, which corresponds at 0 0 C to 50 ohms and 100 ohms. Copper resistance thermometers are designated 50M and 100M.
As a rule, the resistance of resistance thermometers is measured using bridge measuring circuits (balanced and unbalanced bridges).
2.2. Construction and operation of automatic electronic balancing bridges.
Automatic electronic bridges are devices that work with various sensors in which the measured process parameter (temperature, pressure, etc.) can be converted into a change in resistance. The most widely used automatic electronic bridges are used as secondary devices when working with resistance thermometers.
Schematic diagram balanced bridge is shown in Fig. 1. Figure 1-a shows a diagram of a balanced bridge with a two-wire connection of the measured resistance Rt, which, together with the connecting wires, is the arm of the bridge. Arms R1 and R2 have constant resistance, and arm R3 is a flux (variable resistance). Diagonal ab includes the power supply of the circuit, and diagonal cd includes null device 2.
Fig.1. Schematic diagram of a balanced bridge.
a) two-wire connection diagram
b) three-wire connection diagram.
The bridge scale is located along the rheochord, the resistance of which, when Rt changes, is changed by moving slider 1 until the zero pointer of instrument 2 is set to zero. At this moment there is no current in the measuring diagonal. Engine 1 is connected to the scale pointer.
When the bridge is in equilibrium, the equality holds
R1*R3=R2*(Rt+2*Rpr)
Rt=(R1/R2)*R3-2*Rpr
The resistance ratio R1/R2, as well as the resistance of the connecting wires Rpr for a given bridge, are constant values. Therefore, each value of Rt corresponds to a certain resistance of the rheochord R3, the scale of which is calibrated either in Ohms or in units of the non-electrical quantity for which the circuit is intended to measure, for example, in degrees Celsius.
If there are long wires connecting the sensor to the bridge in a two-wire circuit, the resistance changes depending on the temperature environment(air) can introduce significant errors in the measurement of resistance Rt. A radical means of eliminating this error is to replace the two-wire circuit with a three-wire one (Fig. 1-b).
In a balanced bridge circuit, changing the power supply voltage does not affect the measurement results.
In automatic balanced electronic bridges the following circuit is used to balance the circuit. The schematic diagram of an electronic bridge of the KSM type is shown in Fig. 2. The operation of the electronic bridge is based on the principle of measuring resistance using the equilibrium bridge method.
The bridge circuit consists of three arms with resistances R1, R2, R3, a rheochord R and a fourth arm containing the measured resistance Rt. A power source is connected to points c and d.
When determining the resistance value, the currents flowing along the arms of the bridge create a voltage at points a and b, which is recorded by null indicator 1 connected to these points. By moving the engine 2 of the rheochord R using the reversible engine 4, it is possible to find an equilibrium position of the circuit at which the voltages at points a and b will be equal. Therefore, by the position of the slider motor 2, you can find the value of the measured resistance Rt.
At the moment of equilibrium of the measured circuit, the position of arrow 3 determines the value of the measured temperature (resistance Rt). The measured temperature is recorded using pen-5 in diagram 6.
Electronic bridges are divided according to the number of measurement and recording points into single-point and multi-point (3-, 6-, 12- and 24-point), with a strip diagram and devices with a disk diagram. Electronic bridges are produced with accuracy classes 0.5 and 0.25.
The recording device of a multi-dot device consists of a printing drum with dots and numbers printed on its surface.
Devices are powered from the mains alternating current voltage 127 and 220V, and the measuring circuit of the bridge is powered by a direct current of 6.3 V from a power transformer device. Devices powered by a dry element are used in cases where the sensor is installed in fire hazardous areas.
Temperature Sensor Calibration
The resistance thermal converter is connected to the measuring device using copper (sometimes aluminum) wires, the cross-section, length, and, consequently, resistance of which is determined by the specific measurement conditions.
Depending on the method of connecting the resistance thermal converter to the measuring device - according to a two-wire or three-wire circuit (Fig. 1., option "a" and "b"), the resistance of the wires is included entirely in one arm of the bridge circuit of the device, or is divided equally between its arms. In both cases, the readings of the device are determined not only by the resistance of the resistance thermal converter, but also by the connecting wires. The degree of influence of the connecting wires on the instrument readings depends on the value of their resistance. So, in each specific measurement condition, i.e. for each specific value of this resistance, the readings of the same device measuring the same temperature (when the thermal converter has the same resistance) will be different. To eliminate such uncertainty measuring instruments are calibrated at a certain standard resistance of the connecting wires, which is necessarily indicated on their scale by writing, for example R in = 5 Ohm. If during operation of the device the connecting line has the same resistance, the readings of the device will be correct. Therefore, measurements must be preceded by the operation of adjusting the connecting line, which consists in bringing its resistance to the specified calibration value R ext.
The resistance of the connecting line, even with careful adjustment, is equal to the calibration value only if the ambient temperature does not differ from that at which the adjustment was made. A change in the line temperature will lead to a change in the resistance of the copper (aluminum) wires, a violation of the correct fit and, ultimately, to the appearance of a temperature error in the device readings. This error is especially noticeable with a 2-wire communication line, when the temperature increase in line resistance occurs in only one arm of the bridge circuit. With a 3-wire line, the temperature increase in line resistance is received by two adjacent arms and the state of the bridge circuit changes less than in the first case. As a result of this, the temperature error is smaller. Therefore, a 3-wire line is more preferable, despite higher consumption material used to make connecting wires.
The order of work.
4.1. Familiarize yourself with the principle of operation and design of resistance thermometers and electrical devices stand. Assemble a two-wire measurement circuit in accordance with Fig. 3a.
4.2. Set the toggle switch to the 2-wire position and the switch to position 0.
4.3. Set the MS bridge, simulating a resistance thermometer, to a resistance in Ohms corresponding to the table data (Table 1), take temperature readings at 0 C on the MPR51 scale and calculate the absolute and relative error of the measurements indicated in Table 1 of the temperatures.
Study of a 2-wire circuit.
4.4. Set the toggle switch to the 2-wire connection diagram position.
4.5. Set the resistance switch of the connecting wires to position 1 (corresponds to R pr = 1.72 Ohm).
4.6. Carry out point 4.3 and enter the measurement results in Table 1 on lines 5-7, corresponding to the 2-wire connection diagram with R pr = 1.72 Ohm.
4.7. Set the resistance switch of the connecting wires to position 2 (corresponds to R pr =5 Ohm).
4.8. Carry out point 4.3 and enter the measurement results in Table 1 on lines 8-10 corresponding to the 2-wire connection diagram with R pr = 5 Ohms.
Study of a 3-wire circuit.
4.9. Set the toggle switch to the 3-wire connection diagram position (Fig. 3 b).
4.10.Fulfill points 4.5-4.8 and enter the results in lines 11-16 of Table 1 corresponding to the resistances of the connecting wires R pr = 1.72 Ohm and R pr = 5 Ohm.
4.11. Provide an analysis of the accuracy of measurements with a two-wire and three-wire measurement circuit.
4.12. The report provides conclusions based on the test protocol (Table 1).
Control questions.
1. Name the types of resistance thermometers and their principle of operation.
2. Name the advantages and disadvantages of resistance thermometers.
3. Give examples of the use of resistance thermometers in systems automatic control and regulation.
4. What is the purpose of automatic electronic balancing bridges?
5. Operating principle of balanced bridges.
December 2012
Sensors are critical to proper process control, something that is often overlooked in modernization existing systems. The accuracy of the sensors must be carefully checked, otherwise any modernization becomes meaningless.
Many OEMs promise as easy as two-twos turn-on of replaceable system modules that do not require replacement of existing networks, wiring, system enclosures or power supplies, while reducing downtime from weeks and months to “a day or less.”
Sensor efficiency
In reality, things are a little different. Updating systems to achieve more high level enterprise management using computers and software, without assessing the effectiveness of the sensors that supply these systems with data is an exercise in futility. To correctly perceive and transmit data from process parameters, sensors must be accurate.
Pressure Sensors
The accuracy of pressure sensors is, as a rule, from 0.25% of the measured pressure range. For less stringent application scenarios, accuracy may be around 1.25% of range.
The accuracy of a pressure sensor depends on how well the sensor is calibrated and how long it can maintain that calibration. Initial calibration of industrial pressure sensors at a calibration station is achieved by applying a constant pressure source, such as a deadweight tester. Once the pressure sensor is installed, its accuracy can be assessed taking into account the influence of environmental influences, static pressure effects, etc. on the initial calibration accuracy.
Automated calibration systems operate by using a programmable pressure source to produce specified pressure signals applied to the sensor to be calibrated. First, the sensor readings are recorded before calibration. The sensor is then tested with increasing and decreasing input signals to account for any occurrence of hysteresis effect. The system then compares the received data to the calibration acceptance criteria for pressure sensors and automatically determines whether the sensor should be calibrated. If so, the system provides the necessary signals to the sensor to calibrate it and keeps the input value constant while adjustments are made, and lowest pressure, on which it must be calibrated. The system then produces a report that includes pre- and post-calibration data and stores it for trend analysis and incipient failure detection.
Temperature sensors
A typical type of industrial temperature sensor, a resistance thermometer (RTM), typically does not achieve an accuracy of more than 0.05 - 0.12 °C at 300 °C, while it is usually required to provide an accuracy of more than 0.1 °C at 400 °C. The installation process of resistance thermometers can also introduce additional accuracy errors. Another common type of temperature sensor, the thermocouple, generally cannot provide an accuracy better than 0.5°C at temperatures up to 400°C. The higher the temperature, the less thermocouple accuracy can usually be achieved.
Calibration of resistance thermometers
The accuracy of a temperature sensor is established by calibration, comparing its readings to a universal calibration chart or a custom calibration in a high-precision environment. RTDs, unlike thermocouples, can be “cleaned” and recalibrated after installation. Industrial temperature sensors are typically calibrated in tanks of ice, water, oil, or sand, and in an oven, or by a combination of these methods. The type of calibration reservoir depends on the selected temperature range, accuracy requirements, and sensor application. The calibration process usually involves measuring the temperature of the calibration reservoir using a standard thermometer. For individually calibrated vehicles, accuracy is ensured by the calibration process, which in turn depends on the accuracy of the equipment used for calibration, as well as errors such as hysteresis, self-heating, interpolation and installation errors.
Thermocouple calibration
While the thermocouple can be recalibrated after installation, the thermocouple cannot. A thermocouple that has lost its calibration should be replaced. Industrial thermocouples are usually not individually calibrated. Instead, their readings are compared to standard reference tables. For calibration, as a rule, one of two methods is used: the comparison method (in which the emf of the thermocouple is compared with a reference sensor) or the method fixed point(The emf of a thermocouple is measured in several established states). When assessing the accuracy of a temperature sensor, it is important to consider not only the calibration of the sensor itself, but also the influence of sensor installation and conditions technological process for this accuracy.
Sensors How to evaluate response time?
To display data at a frequency consistent with plant requirements or industry regulations, sensors must be fast enough to detect sudden changes in process parameter values. Accuracy and response time are largely independent metrics. Since the efficiency of sensors has vital importance For production systems, systems modernization efforts must begin with a thorough assessment of the system, along with an assessment of the accuracy and reliability of the sensors.
While sensor accuracy can be restored through recalibration, response time is an inherent characteristic that generally cannot be changed once the sensor has been manufactured. The two main methods for evaluating the response time of sensors are the immersion test (for temperature sensors) and the linear test (for pressure sensors).
The calibration and response times of sensors, especially temperature sensors, depend heavily on process conditions, including static pressure, process temperature, ambient temperature, and fluid flow rate.
On-the-job inspection
There are some methods that are often referred to as on-site testing or online testing. They were designed to test the calibration and response time of sensors already in use in a process. For temperature sensors, LCSR test ( Loop Current Step Response) will check dynamic characteristics The most common temperature sensors are thermocouples and resistance thermometers - where they are installed in the operating process. The LCSR method shows the actual response time of the RTD (resistance thermometer) “during operation”.
Unlike resistance thermometers and thermocouples, the response times of pressure, level and flow sensors generally do not change after installation. This is because these sensors are electromechanical devices that operate independently of ambient and process temperatures. The difficulty in evaluating pressure sensors comes from the presence of a process-wire-sensor interface system that connects the sensor to the actual process. These measurement lines (wires) add a few milliseconds of delay to the response time of the sensors. Although this delay is negligible, hydraulic delays can add tens of milliseconds to the response time for sensing system pressure.
The noise analysis technique measures the response time of pressure sensors and measuring lines in a single test. As with the LCSR method, the noise analysis technique does not interfere with operation, using existing exits sensors to determine their response time, and can be performed remotely for sensors that are installed in production. The noise analysis technique is based on the principle of monitoring the normal AC output of pressure sensors using a fast data acquisition system (frequency from 1 kHz). The alternating current output from the sensor, called "noise", is produced by random fluctuations in the process associated with turbulence, vibration and other natural phenomena. Since these extraneous noises occur over more high frequencies than the dynamic response of pressure sensors, they can be separated from the signal using low-pass filtering. Once the AC signal or noise is separated from the signal direct current Using signal processing equipment, the AC signal is amplified, passed through antialiasing filtering, digitized, and stored for later analysis. This analysis provides dynamic response times of the pressure sensor and measuring lines.
A number of equipment are available to collect and analyze noise data for pressure sensors. Commercial spectral analysis equipment can collect noise data and perform real-time analysis, but this equipment usually cannot handle the multitude of data analysis algorithms required to produce results with exact time response. This is why PC-based data acquisition systems, consisting of isolated nodes, amplifiers and filters for signal conditioning and smoothing, are often optimal choice to collect noise data and analyze it.
Sensor life
When should sensors be replaced? The answer is simple: sensors should be replaced after the service life established by the manufacturer for the specified product has expired, for example 20 years. However, this can be very expensive and impractical.
An alternative is to continue to use sensors after their service life has passed, but be sure to use sensor performance monitoring systems to determine if and when to replace the sensor. Experience has shown that high quality sensors are very likely to continue to show good results work well beyond the service range outlined by the manufacturer. Consensus between factory recommendations and actual use of the sensors can be achieved by operating the latter as long as the calibration stability is acceptable and its response time is not reduced.
Many people joke that sensors that work correctly should be “left alone”, but high quality “aged” sensors may well be just as good, if not better, than new sensors of the same model and manufacturer.