The temperature detection instruments commonly used in industry are divided into two main categories: non-contact pyrometers and contact temperature measurement instruments. This article will analyze and explain how to deal with faults in the actual work of temperature instruments. See below for details.
1. RTD Temperature Gauge
A common fault with industrial RTDs is industrial RTD breakage and short-circuiting. Usually, a break is the most common due to the RTD wire’s fineness.
1.1 Methods for determining breaks and short circuits
Leaves and shorts are easy to decide on. We can use the “×1Q” gear of the multimeter. If the measured resistance is less than R0, there may be a short circuit. If the multimeter indicates infinity, the resistor body can be judged to have been disconnected.
1.2 Methods for handling disconnection and short circuit
Resistor body short circuit is generally easier to deal with, as long as it does not affect the length and thickness of the resistance wire. Find the short course to blow dry, and strengthen insulation can be.
Resistor body break repair must change the length of the electric sun by affecting the value of the electric sun. For this reason to replace the new electric sun body is good. If the proper use of slow connection repair, welding to be calibrated after passing before use.
2. Thermocouple Thermometer
If we use thermocouples correctly, we can get an accurate temperature value and ensure that the product is qualified. It can also save the thermocouple material consumption to save money and ensure product quality.
2.1. Improper Installation of the Error Introduced
If the thermocouple is installed in a position and insertion depth that does not reflect the actual temperature of the furnace chamber, etc. Therefore, the thermocouple should not be installed too close to the heating place.
2.1.1 Depth of Insertion of the Thermometer
The insertion depth should be at least 8 to 10 times the diameter of the protection tube. If the interval between the thermocouple protection tube and the wall is not filled with a thermal insulator, this will lead to heat spillage or cold air intrusion into the furnace. Therefore, the space between the thermocouple protection tube and the hole in the furnace wall should be filled with an insulating material such as refractory clay or asbestos. It will prevent hot and cold air from convecting and affecting the temperature measurement.
2.1.2 Installation of Thermocouples
When the cold end of the thermocouple is too close to the furnace body, the temperature exceeds 100°C. The thermocouple should be installed as far away from strong magnetic and electric fields as possible. Therefore, we should not install thermocouples and power cable lines in the same – conduit to avoid introducing interference and causing errors.
Thermocouples should not be installed in areas where the measured medium rarely flows. For example, when using a thermocouple to measure the temperature of a gas in a tube, the thermocouple must be installed against the direction of flow and in full contact with the gas.
2.2. Insulation Deterioration and the Introduction of the Error
If the thermocouple is insulated, the protection tube and the cable pulling plate are too much dirt or salt slag, which will lead to poor insulation between the thermocouple poles and the furnace wall, more serious under high temperatures. It will not only cause a loss of thermal potential but also introduce interference resulting in errors sometimes up to 100 degrees.
2.3 Errors Introduced by Thermal Inertia
Due to the thermal inertia of thermocouples, the indication value of the instrument lags behind the change in the measured temperature, and this effect is particularly pronounced when carrying out rapid measurements. Therefore, we should use thermocouples with thin thermal electrodes and small protection tube diameters wherever possible.
2.3.1 Temperature Measurement Environment
The protection tube can even be removed when the temperature measurement environment permits. Due to the measurement hysteresis, the amplitude of temperature fluctuations detected with a thermocouple is smaller than the amplitude of the furnace temperature fluctuations.
The larger the measurement lag, the smaller the amplitude of the thermocouple fluctuations and the more significant the difference with the actual furnace temperature. The instrument shows a slight temperature fluctuation when using a thermocouple with a considerable time constant to measure or control the temperature. Still, the actual furnace temperature fluctuations may be significant. To measure the temperature accurately, a thermocouple with a short time constant should be selected.
2.3.2 Time Constants
The time constant is inversely proportional to the transmission coefficient and proportionate to the diameter of the thermocouple’s hot end, the material’s density, and the specific heat. Therefore, if you want to reduce the time constant and increase the heat transfer coefficient, the most effective way is to minimize the size of the hot end.
We usually use materials with good thermal conductivity—protective sleeves with thin walls and small internal diameters. In more precise temperature measurements, bare wire thermocouples without protective sleeves are very easy to damage, and we should correct and replace them in time.
2.4 Thermal Resistance Error
When measuring high temperatures, such as a layer of soot on the protection tube and dust attached to it, the thermal resistance increases, which will prevent the conduction of heat; at this time, the temperature indicated value is lower than the actual value of the measured temperature. Therefore, we should keep the outside of the thermocouple protection tube clean to reduce the error.
3. Common Faults
The temperature control instrument is an instrument that controls the object to be measured using an RTD or thermocouple. The common faults are as follows.
① Improper installation position so the medium can not fully heat exchange with the measuring element, resulting in low indication.
② Poor insulation of the temperature measurement point, resulting in the same part of the rapid heat dissipation, resulting in temperature measurement at lower than the system temperature.
③ Loose wiring, poor contact caused by false indication. The RTD is high, and the thermocouple is low.
④ Short circuit fault. Causes low or minimum RTD and inadequate or faulty thermocouple.
⑤ Broken circuit (open circuit) fault. Causes the RTD to indicate maximum and the thermocouple to be non-indicating and minimum.
In addition, in the temperature control instrumentation system fault analysis, we should note that the vast majority of its system instrumentation is selected for electric instrumentation measurement, indication, and control, and measurement lag is significant.
4. Common Fault Analysis Methods
4.1 Checking the indication values
First, we should check the indication values of the temperature instrumentation system. If the indication value changes to the maximum or minimum, we can determine the faulty instrument system.
It is because the temperature instrument system measurement generally has a large hysteresis, and sudden changes do not occur. One of the faults of the temperature control meter is in the thermocouple, RTD, and the compensation wire break. The second is that its transmitter amplifier fails and causes a fault.
4.2 Check the Parameters of the Indicated Value
Check that the temperature control instrumentation system is not constantly and rapidly oscillating. This phenomenon is generally caused by improper adjustment of the control parameters PID.
4.3 Check the Fluctuation of the Indication Value
Check whether the temperature control instrumentation system indicates a large and slow fluctuation. Changes in the process operation generally cause this phenomenon. If no process operation changes exist, we can determine the faulty instrumentation control system.
4.4 Check the Input Signal of the Control Valve
When the temperature control system is faulty, we have first to check the regulating valve input signal of the instrument to see if there is a change.
Suppose there is no change in the input signal and the regulating valve has acted. In that case, we can determine that the regulating valve diaphragm is leaking: check the valve positioner input signal.
If there is no change in the input signal and the output signal changes, it is decided that the instrument positioner is faulty.
Check the instrument positioner’s input signal and the instrument regulator’s output signal. If the regulator input signal is not changing and the output signal is changing, it is decided that the instrument’s regulator is faulty.