【Daily Question 20090331】What are the main sources of error in temperature measurement using thermoresistors?

1: Error inherent in the thermal resistor itself. 2: Error in the resistance value of the wires; a three-wire or four-wire system is generally used. 3: Error of the secondary meter. Other sailors, please add more details

Also pay attention to the installation location and method

1. Heat transfer error: When using a thermal resistor for temperature measurement, heat exchange necessarily occurs through the thermal resistor, regardless of whether the temperature of the medium being measured is higher or lower than the ambient temperature. When heat transfer occurs, the temperature sensed by the working end of the thermoresistor cannot accurately reflect the temperature of the medium being measured, thereby causing measurement errors. The greater the difference between the temperature of the medium under test and the ambient temperature, the larger this error becomes. The layout of the measurement points and the temperature measurement methods are also important causes of heat transfer errors. To reduce these errors, appropriate measures must be taken based on the specific measurement requirements in order to minimize heat transfer between the thermal resistor and its surrounding environment. 2-degree error: There is a difference between the resistance-temperature relationship of the thermoresistor and the standard calibration table. The magnitude of this difference must not exceed the specified range; therefore, it is necessary to conduct inspections before using the thermoresistor, and regular calibrations should also be carried out after it is put into use. Error caused by 3 joules of heat: When measuring the resistance value of a thermistor, a certain current must flow through it; as a result, heat is generated in the resistor. The increase in temperature of the thermistor itself leads to additional errors in temperature measurement, and the magnitude of these errors varies depending on the size of the current flowing through it. This error cannot be eliminated. Only the value of the temperature rise can be limited. For metal thermoresistors used in industry, the limiting current is set at less than 6 mA (usually 3 mA); this error is very small, around 0.1°C. For semiconductor resistors and carbon resistor thermometers with poor thermal conductivity, the current is limited to the microampere range. The lower the measurement temperature, the greater the error caused by Joule heating, and the working current should be made smaller. 4. Errors caused by line resistance: When a thermal resistor is used in conjunction with a display instrument or transmitter for temperature measurement, there are specified values for the resistance of the wires connecting the two. If the line resistance does not meet these specified values, it can lead to errors in temperature measurement. Since the ambient temperature cannot be kept constant, this error is also inevitable. Some measures can also be taken for the actual measurement circuit to reduce or eliminate this effect, such as adding adjustment resistors or using three-wire or four-wire connections. 5 shows the basic error of the instrument; the magnitude of this error is determined by the instrument’s accuracy class.

What was said on the 2nd floor is completely correct; I’ll add another point: the error caused by the heat generated when the thermistor is powered. In actual measurements, the resistor itself always has to carry a certain current, which causes the thermistor to heat up and leads to errors due to changes in the resistance value.

As mentioned above, there is remote transmission with a cable; now let me talk about local measurement: The main errors in using thermal resistance thermometers for temperature measurement arise from whether the insertion depth and angle are in accordance with the specifications, followed by selecting the wrong range. For example: a large range was selected for a small area.

I studied it seriously! :victory:

During the measurement process, many factors can cause measurement errors, but the main ones are as follows. 1. Error of measuring instruments refers to the error inherent in the measuring instruments themselves. It includes the sum of all errors that occur during the design, manufacturing, and calibration of measuring instruments; this total of errors is reflected in the indication error and the repeatability of the measurements. 2. Method error refers to the errors caused by the imperfections in the measurement method. 3. Environmental condition errors refer to the errors caused by the environmental conditions during measurement, including temperature, humidity, air pressure, lighting, vibration, dust, etc., when these conditions do not meet the standard requirements. 4. Human error refers to the errors caused by the measurer themselves.

One more source of error: the error caused by the protective sleeve of the thermal resistor.

There are also errors that can arise from the installation location, as well as errors caused by strong electrical interference

Everyone upstairs has already said everything; I’ll add that installation errors can also be significant. Additionally, the transmission of thermal resistance values to the control room, as well as the range settings of the system, also have an impact.

To address the errors associated with the thermal resistance itself, appropriate choices should be made regarding the selection of the thermal resistance as well as its precision, which can help reduce such errors. Errors caused by environmental conditions can generally be effectively addressed; for instance, disturbances such as strong magnetic fields or vibrations can be mitigated by selecting another suitable measurement point to stay away from these disturbances, and shock absorbers can be used to deal with vibrations. Human-induced measurement errors should be solvable; it is necessary to start with the attitude of the instrument technician himself, who needs to have a sense of responsibility for working diligently. Carefulness during measurement readings or operations can help reduce such errors. In short, errors can be reduced, but not eliminated. A humble opinion

Fault symptoms: 1. The display reading is lower than the actual value or the reading is unstable； 2. The gauge indicates infinity ； 3. The relationship between resistance and temperature changes ； 4. The gauge displays a negative value. Possible causes: 1. Metal shavings and dust inside the protective tube, dirt between the terminals, and short circuit of the thermal resistor ； 2. Open circuit in the thermal resistor or leads, as well as loose connection terminals ； 3. The material of the thermal resistance wire is corroded and deteriorated ； 4. There is an error in the wiring between the display instrument and the thermal resistor, or the thermal resistor is short-circuited. Solution: 1. Remove metal shavings and dust, locate the short circuit point, and enhance insulation ； 2. Replace the thermal resistor, and weld or tighten the terminal screws ； 3. Replace the thermal resistor ； 4. Correct the wiring, locate the short circuit point, and improve insulation.

The inherent error, the error caused by the protective sleeve, the error resulting from the installation location, and the error due to environmental conditions

The main factors affecting measurement error are as follows: 1. The influence of insertion depth； Due to its good thermal conductivity, the insertion depth of the metal protective tube should be greater, at about 15 to 20 times the diameter ； Ceramic materials have good thermal insulation properties, allowing them to be inserted at a shallower depth, around 10–15 times their diameter. 2. Impact of response time: It is best to choose sensors with fast response times. For thermal resistors, apart from the influence of the protective tube, the diameter of the measurement end is also a key factor. The finer the wire, the smaller the diameter of the measurement end, and thus the shorter its thermal response time. 3. Effect of thermal radiation: To reduce thermal radiation errors, heat conduction should be increased, and the temperature at the measurement point should be made as close as possible to the temperature of the thermocouple. Additionally, during installation it should also be noted that the thermocouple should be placed as far away as possible from the thermal radiation emitted by solids, so that this radiation does not reach the surface of the thermocouple ； Thermocouples should preferably be equipped with a thermal radiation shield. 4. Effect of increased thermal impedance: If the medium being measured is in gaseous state, dust and other deposits on the surface of the protective tube will melt there, thereby increasing the thermal impedance of the protective tube ； If the medium being measured is a melt, slag will deposit during use, which not only increases the response time of the thermocouple but also results in a lower indicated temperature. Therefore, in addition to regular calibration, it is also necessary to conduct frequent spot checks in order to reduce errors.

For wire resistance error, a three-wire system is generally used

Let me say a few things. I’m new here, so everyone please follow the guidelines. Additionally, due to the corrosive gases present at the site, the wiring terminals in the thermistor junction box become oxidized, which leads to contact errors. The solution is to remove it, grind it, and reattach it. Haha, simple and effective.