The temperature detection instrument commonly used in industry is divided into two categories: non-contact temperature measurement instruments (such as radiation, and infrared). Contact temperature measurement instrumentation (such as expansion, pressure, thermocouple, RTD). This article will analyze the failure of the temperature instrument during the actual work and explain the treatment. Please see below for details.
1 RTD Thermometer
The common failures of industrial RTDs are industrial RTD breaks and short circuits. Generally, disconnection is more common, which is due to the finer thermal resistance wire.
Disconnection and short-circuit are very easy to judge, and the multimeter can be used “× 1Ω” file. If the measured resistance is less than R0, there may be a short-circuit place; if the multimeter indicator is infinity, it can be determined that the resistor body has been disconnected. Resistor body short circuit is generally easier to deal with, as long as it does not affect the length and thickness of the resistor wire, to find the short-circuit blow-dry, and strengthen the insulation. Resistor body break repair must change the length of the resistance wire and affect the resistance value, in order to replace the new resistance body is good, if the welding repair, welding to be calibrated after passing before use. RTD temperature measurement system in the operation of common failures and treatment methods is as follows.
| Failure Phenomenon | Possible Causes | Treatment |
| The indicated value of the display instrument is lower than the actual value or the indicated value is unstable. | a. Metal shavings, dust in the protective tube
b. Dirt between terminal blocks and short-circuiting of RTDs (accumulation of water, etc.) |
a. Remove metal shavings.
b. Clean up dust, water droplets, etc., c. Locate short-circuit points. d. Strengthen insulation
|
| Infinity of the display instrument indication value | Industrial RTD or lead wire breakage and loose terminals | a. Replace resistor body
b. Solder and tighten terminal screws. |
| Negative value indicated by the display instrument | Display instrument and RTD wiring is wrong, or RTD has a short circuit phenomenon. | a. Correct the wiring, or find the short circuit
b. Strengthen the insulation
|
| There is a change in the resistance value in relation to the temperature. | Corrosion and deterioration of thermal resistance wire material | Replace resistance body (RTD) |
2 Thermocouple Thermometer
Correct use of thermocouples can not only accurately get the value of the temperature to ensure that the product is qualified, but also save the thermocouple material consumption, both to save money and ensure product quality. In addition to the common errors caused by the compensation wire reverse, wrong and loose wiring (treatment: correct use of the compensation wire, tighten the terminals), incorrect installation, thermal conductivity and time lag and other errors, they are thermocouples in the use of the main errors.
(1). Error Introduced by Improper Installation
Such as thermocouple installation location and insertion depth does not reflect the true temperature of the furnace, in other words, thermocouple should not be installed too close to the door and heating, insertion depth should be at least 8 to 10 times the diameter of the protection tube; thermocouple protection sleeve and the wall of the interval between the insulation material is not filled resulting in the furnace heat overflow or cold air intrusion, so the thermocouple protection tube and the furnace wall holes between the gap between the refractory mud or asbestos Rope and other adiabatic material plugging to avoid hot and cold air convection and affect the accuracy of temperature measurement; thermocouple cold end is too close to the furnace body so that the temperature exceeds 100 ℃; thermocouple installation should be avoided as far as possible strong magnetic and electric fields, so should not be thermocouples and power cables are installed in the same conduit to avoid the introduction of interference caused by error; thermocouples can not be installed in the measured medium rarely flow in the region, when measuring the temperature of gas by thermocouple tube, you must make the thermocouple When using thermocouples to measure the temperature of the gas in the tube, the thermocouple must be installed against the direction of flow, and fully in contact with the gas.
(2). Error Introduced by the Deterioration of Insulation
Such as thermocouple insulation, protection tubes, and cable boards too much dirt or salt slag between the thermocouple poles and furnace wall insulation, is more serious at high temperatures, which will not only cause the loss of thermal potential but also introduce interference, resulting in errors sometimes up to hundreds of degrees.
(3). Error Introduced by Thermal Inertia
Because of the thermal inertia of the thermocouple, the indicator value of the instrument lags behind the measured temperature change, in the rapid measurement of this effect is particularly prominent. So should be used as far as possible, the heat electrode is thin, and the diameter of the protection tube is a small thermocouple. When the temperature measurement environment permits, even the protection tube can be removed. Due to the measurement hysteresis, the amplitude of the temperature fluctuations detected by the thermocouple is smaller than the amplitude of the furnace temperature fluctuations. Â The larger the measurement hysteresis, the smaller the amplitude of the thermocouple fluctuations and the greater the difference with the actual furnace temperature. When using a thermocouple with a large time constant temperature measurement or temperature control, the temperature displayed on the meter fluctuations are small, but the actual furnace temperature fluctuations may be large. To accurately measure the temperature, you should choose a thermocouple with a small time constant. The time constant and heat transfer coefficient is inversely proportional to the diameter of the hot end of the thermocouple, the density of the material, and the specific heat is proportional to, such as to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way to minimize the size of the hot end. In use, usually using materials with good thermal conductivity, thin walls, and small inner diameter of the protective casing. In the more precise temperature measurement, the use of a bare wire thermocouple without the protective sleeve, but the thermocouple is easy to damage, should be promptly calibrated and replaced.
(4). Thermal Resistance Error
At high temperatures, such as a layer of soot on the protective tube, or dust attached to it, the thermal resistance increases, hindering heat conduction, and the temperature is lower than the true value of the measured temperature. Therefore, the thermocouple protection tube is externally clean, to minimize the error.
Industrial thermocouple common faults and treatment methods:
| Failure Phenomenon | Possible Causes | Treatment |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Thermal electrode short-circuit | Find out the cause of the short-circuit, if it is due to moisture, then dry it; if it is due to insulator damage, then replace the insulator; clean up the accumulated dust;
Short circuit between compensation wires: find out the short circuit point, strengthen the insulation or replace the compensation wires. |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Deterioration of industrial thermocouple thermal electrodes | Where length permits, subtract the deteriorated section and solder again, or replace it with a new thermocouple. |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Reverse polarity of compensation leads and industrial thermocouples | Rewired correctly. |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Compensation wire mismatch with industrial thermocouples | Replacement of matching compensating conductors |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Industrial thermocouples installed in the wrong position or inserted at a depth that does not meet requirements | Re-installation to specification |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is lower). | Industrial thermocouple cold-end temperature compensation does not meet the requirements. | Adjustment of cold end compensator |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Industrial thermocouples mismatched with display instruments | Make industrial thermocouples and display instruments matched |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Compensation wire mismatch with industrial thermocouples | Replace it with matched one |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | There are DC interference signals coming in. | Troubleshooting DC interference |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Unstable thermoelectric potential output | If the contact between the industrial thermocouple terminal and thermal electrode does not work, tighten the terminal screws |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Industrial thermocouple measurement line insulation is broken, causing intermittent short circuits or grounding. | Find the point of failure and repair the insulation |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Poorly mounted industrial thermocouples or external vibration | Tighten industrial thermocouples to eliminate vibration or take shock-absorbing measures |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | Thermal electrodes to be disconnected | Repair or replace industrial thermocouples |
| The thermoelectric potential is smaller than the actual value (the indicated value on the display instrument is higher). | External interference (AC leakage, electromagnetic induction, etc.) | Identify sources of interference and implement shielding measures |
| Large error in thermoelectric potential | Thermal electrode deterioration | Replacement of thermal electrode |
| Large error in thermoelectric potential | Industrial thermocouples improperly are installed. | Changing the mounting position |
| Large error in thermoelectric potential | Dust buildup on the surface of the protection pipe | clear accumulated dust |
3 Bimetallic Thermometers
The working principle of the bimetallic thermometer is to use two different temperature expansion coefficients of the metal, one end of the welded in a fixed point, the other end when the temperature changes when the twisted deformation, which will be converted into a pointer deflection angle, indicating the temperature.
If there is a linear error that can be adjusted by adjusting the back of the thermometer pointer knob to adjust the temperature indication is not allowed to adjust the problem, after adjusting the thermometer by the calibration of qualified before use.
4 Pressure Thermometers
Pressure thermometers use liquid thermal expansion and contraction to carry out temperature measurements, temperature packets, capillary tubes, and spring tubes composed of a closed system filled with temperature measurement medium – liquid when the temperature packet feels the temperature change, the pressure within the closed system due to changes in the volume of the liquid changes and changes in the curvature caused by the spring tube curvature changes in the free end of the displacement, and then through the connecting rod and the transmission mechanism to drive the pointer to rotate in the table on the dial Indicates the measured temperature on the dial. This instrument has a linear scale, a small temperature packet volume is small, a fast response speed, high sensitivity, intuitive reading, and other characteristics.
Commonly used pressure thermometer general failure for the pointer does not move, indicating a large deviation, for the pointer card astringent can be used to pull out the pointer needle, re-needle calibration, and then use.
(1). Common Failures
Temperature control instrumentation is through the RTD or thermocouple control of the measured object for control of the instrument, its common failures are mainly the following:
– Not installed in the right position, so that the medium can not be sufficient heat exchange with the measuring element, resulting in an indication of low;
– The temperature measurement point insulation poor, resulting in local heat dissipation fast, resulting in temperature measurement at the lower than the system temperature;
– The wiring loose, poor contact caused by the indication is not allowed. The thermistor is high and the thermocouple is low;
– It is a short-circuit fault. Causes the RTD to be low or minimum, and the thermocouple to be low or malfunction;
– It is a broken circuit (open circuit) fault. This causes the RTD to indicate maximum and the thermocouple to be unindicated or minimum.
In addition, in the temperature control instrumentation system failure analysis, it should be noted that the vast majority of its system instrumentation selection of electric instrumentation measurement, indication, and control, measurement lag is large.
(2). Common Failure Analysis Methods
A. First check the indication value of the temperature instrumentation system.If its indication value changes to the maximum or changes to the minimum, it can be determined that the instrumentation system failed. The reason is that the temperature instrumentation system measurement generally has a large lag, sudden changes will not occur. Temperature control instrumentation failure is in the thermocouple, RTD, and compensation wire breakage, and the second is its transmitter amplifier malfunction and lead to failure.
B. Check the temperature control instrument system indicator value is not non-stop rapid oscillation, this phenomenon is generally a control parameter PID improperly adjusted failing.
C. Check whether the temperature control instrumentation system indicator value is a large slow fluctuation.This phenomenon is generally caused by changes in process operations. If there is no change in the state of process operations, it can be determined that the instrumentation control system itself has a failure.
D. Determine the failure of the temperature control system itself.The first instrument regulator valve input signal check to see if there is a change, if the input signal does not change, and the regulator valve has been acted on, it can be judged to be the regulator valve diaphragm head diaphragm leakage failure; check the regulator valve locator input signal. If the input signal does not change, and the output signal in the change, it is judged to be the instrument of the locator has a failure; Check the input signal of the instrument positioner and the output signal of the instrument regulator. If the input signal of the regulator does not change, and the output signal is changing, it can be determined that the regulator of the instrument itself is out of order.