Calibration of temperature sensors. Measuring temperature differences and calibrating sensors How to calibrate a temperature sensor
The built-in temperature sensor in most modern hard drives may give incorrect results. The difference between the measured and actual temperature can be 7-9 degrees Celsius, and in some cases even more.
To solve this problem, it is recommended to measure the actual temperature of the hard drive using an external infrared thermometer or bezel temperature sensor. And then set the difference between the measured value and the temperature that the Hard Disk Sentinel displays (as reported by the drive itself) as a temperature offset. This is called calibration.
After measuring the actual temperature (with a thermometer or other external sensor), the offset can be calculated by subtracting the value specified by the program from the measured value. The offset can be positive (the program shows a lower temperature than the real one) or negative (otherwise).
This offset can be specified in the S.M.A.R.T tab. hard drive by selecting attribute No. 194 (hard drive temperature) and using the + / – buttons (by clicking on the number between these characters, you can directly enter the offset value Celsius).
Hard Disk Sentinel automatically increases (or decreases) all reported hard disk temperatures according to configured offsets. Thus, the correct (real) temperature will be displayed in any case (for example, when comparing the hard drive temperature with a threshold value, when saving reports, etc.)
Note: if calibration is not possible ( computer unit cannot be opened), the estimated offset value can be determined by comparing the first displayed temperature value immediately after starting the computer with the temperature value environment(room, office). At this time, the central processor, video card or other components are not too hot and do not affect the temperature of the hard drive. Of course, this is only true if the computer has been given enough time to cool down to ambient temperature (it hasn't been turned on for about 8 hours).
For example, if the hard drive temperature is 17 degrees Celsius (immediately after starting the computer) and the room temperature is 22 degrees, then this difference (5) can be configured as an offset value (because the hard drive cannot be cooler than ambient temperature) . This offset is better than nothing, but an external thermometer is still needed to determine the proper temperature offset.
Note : The temperature offset must be determined by Celsius , regardless of the selected temperature unit (Celsius or Fahrenheit).
Note: the unregistered version of the program automatically resets all offset values to 0 if the user reboots Hard Disk Sentinel.
Temperature sensors are often installed on objects in such a way that their dismantling is almost impossible or causes great difficulties. At the same time, it is necessary to have confidence in the accuracy of their testimony. For such cases, methods are being developed to monitor the performance of sensors during their operation without dismantling. In addition, periodic verification sometimes has to be abandoned due to the high cost of the verification itself compared to the cost of the sensor. Publications on this topic and brochures from manufacturers describe several approaches to solving the problem of reliability of temperature sensors.
1) A statistical analysis of the drift of the characteristics of sensors of a particular type at operating temperatures is carried out, and their service life is established, during which the accuracy is within the specified tolerances with a high probability. After this period has expired, all sensors must be replaced.
2) An excessive number of sensors are installed on the object. The result is determined either by the arithmetic mean of their readings or is developed more complex circuit analysis, including comparing sensor drifts and identifying sensors exhibiting above-average drift. A common model are sensors with two and three sensitive elements in one housing.
3) Sensors are installed on the object different types(eg resistance thermometers and thermocouples). This avoids errors associated with the same influence temperature conditions and conditions for sensors of the same type. A self-checking thermometer was patented in the USA, combining the properties of a sensitive resistance element and a thermocouple.
4) Sometimes the channels for placing sensors are designed in such a way that it is possible to insert a standard thermometer next to the working sensor during verification and remove it at the end of verification. Non-dismantling verification methods are important at hazardous facilities, such as, for example, a reactor core. Unfortunately, there are no standards for methods of non-dismantling testing and monitoring the performance of sensors. However, the problem is very often raised at international seminars and conferences.
One solution to the problem of checking thermocouples during operation without dismantling them from the facility is the method of using thermocouples with an additional channel into which a reference thermocouple is installed during verification. This thermocouple design and its verification method were patented in 2007 by PC TESEI LLC (invention patent 2299408). A thin cable thermocouple of the TNN type (nichrosil-nisil) 3 digits is used as a reference measuring instrument.
The TNV thermocouple is inserted into the additional channel of the main thermocouple only for a short time - the verification time, therefore the formation of thermoelectric inhomogeneity in the thermoelectrodes is unlikely. You can read more about this method in the “Publications” section.
Similar designs of thermometers and thermocouples for non-dismantling verification under nuclear power plant conditions with additional channels for reference sensors are produced at ZAO NPK Etalon (Volgodonsk).
We found the following material in the section at the TEMPMECO 2010 conference. There, an interesting report was presented from the German company Electrotherm on thermocouples with a built-in reference point for metal melting, which allows accurate periodic verification of thermocouples. With permission of the company publish brief information about the design of a thermocouple. ( Russian companies, who manufacture similar installations, we invite you to send your material for publication on the website)
Thermocouple with built-in reference point
Thermocouple with built-in reference point(developed and produced by Electrotherm, Germany) company website www.electrotherm.de
The main element of this measuring system is a thermocouple with a built-in reference point cell and a miniature heating element. The reference point cell contains a high-purity substance (pure metal or eutectic alloy). When the temperature of the medium slowly rises to a value above the melting point of the metal, a reproducible “platform” with a constant thermal emf, the so-called “melting plateau,” is observed on the curve tracking the thermocouple signal. During this plateau, a phase transition occurs, i.e. heat coming from outside is used to destroy the crystal lattice of the metal, and the temperature rise stops. The recorded TEMF value can be used to calibrate the thermocouple at a known phase transition temperature. As the temperature decreases, a “solidification platform” can be observed.
Heating of the thermocouple for calibration can also be carried out without heating the object, using a miniature built-in heater.
The table provides data on reference points for thermocouple calibration.
Each thermocouple with a built-in reference point is equipped with a transmitter, the signal from which is sent to a computer and processed using a special software. The computer controls the entire heating, calibration and data analysis cycle. It can connect to 8 measuring modules at once and also communicate via network cards with a central control computer.
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 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 “ “ set the required number of calibration points and press the button “ENTER”. In this case, a window will appear on the display with the value of the solution temperature in the top line, the conditional calibration number and the number of the calibration point 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, repaired and in service analyzers 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. Conditions for repair
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.
Appendix No. 4: Temperature sensor calibration.
When released from production, the temperature sensor built into the amperometric sensor is calibrated using a method, the execution algorithm of which is recorded in the service menu of the analyzer. You should resort to calibrating the temperature sensor only when replacing the sensor with a new one. In this case, connect the new sensor to the measuring device and turn on the analyzer. To calibrate the temperature sensor, you need to assemble the installation shown in the figure. Using this installation, it is necessary to provide three marks on the temperature scale in the range of 5 -50 o C. If your laboratory does not have a thermostat, you can provide three marks on the temperature scale to provide more in a simple way. For this you need a thermos, a glass of distilled water room temperature And plastic cup with ice. Pour distilled water heated to 50 +5 o C into a thermos. Make a hole with a diameter of 10 mm in a glass with ice. To increase the diameter of this hole to 16 mm, pour warm water into it. After 5-10 minutes, the water in the hole will have an ice melting temperature of ~ 0 o C.
To calibrate the temperature sensor, you must go to the service calibration menu. To do this, enter the Calibration menu and, while holding the “DOWN” key, press the “ENTER” key. In the service menu that appears, select the “TEMPERATURE” option, press “ENTER”. 
In the window that opens, select the “Bottom Point” option and press “ENTER.”
Immerse the sensor and the reference thermometer in a thermostatic glass with a temperature of the lower mark of the scale: 5+1 o C or in a well in a glass with ice. 
In the window that opens, enter the temperature of the bottom point using the cursor keys and press “ENTER”.
After the message about successful calibration of the low point, the temperature sensor calibration menu will reappear on the screen. Select the Top Point option and press ENTER.
Immerse the sensor and the reference thermometer in a thermostatic glass or thermos with the temperature at the top of the scale and, after waiting for the thermometer readings to set, press “ENTER”. 
Read the reading of the reference thermometer and use the cursor keys to enter this value.
message about successful calibration of the upper point, the temperature sensor calibration menu will reappear on the screen. Select the T Correction option and press ENTER.
Follow the instructions shown on the analyzer display and press “ENTER”.
Wait for the thermometer readings to settle and press “ENTER”. 
Read the temperature reading from the reference thermometer and enter this value using the keyboard. Press ENTER.
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. 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 in its pure form, relatively high temperature coefficient and linear dependence temperature resistance:
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 high initial resistance can be made from them, which makes it possible to ignore the resistance of 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, changes in resistance depending on the ambient (air) temperature 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 measuring circuit 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 therefore the 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 a fitting operation 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 the greater consumption of material used for the manufacture of connecting wires.
The order of work.
4.1. Familiarize yourself with the principle of operation and design of resistance thermometers and electrical devices of the 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 automatic control and regulation systems.
4. What is the purpose of automatic electronic balancing bridges?
5. Operating principle of balanced bridges.
