For accuracy, all temperature sensors must be calibrated to known standards. Only short term stability is checked during calibration. Long term stability should be monitored and determined by users.
Temperature is the most commonly used measurement parameter in the world. Temperature sensors are used in instruments designed to measure temperature. For accuracy, all temperature sensors must be calibrated to known standards.
Only short term stability is checked during calibration. Long term stability should be monitored and determined by users.
Sometimes, the temperature sensor may fail during calibration. This can happen even if the temperature sensor appears to be working properly before it is sent into calibration.
Type of temperature sensor
Thermistors, platinum resistance thermometers (PRT) and thermocouples are the preferred instruments for most temperature measurement applications. Each has its own characteristics and limitations.
Usually, these instruments are reliable and can provide years of trouble free service. However, random application will greatly affect its accuracy and service life. Therefore, they must be handled and used correctly. To do this, you have to understand how they work and their limitations.
5640 series thermistor probe
Thermistors are the most robust of all temperature sensors. They consist of solid-state devices like varistors.
As the temperature changes, the resistance of the thermistor also changes, and has excellent sensitivity and accuracy, as well as a wide range of resistance values. They also have excellent long-term drift characteristics and are insensitive to vibration and will not suffer from other problems that other thermometer types may have.
Since they are insensitive to vibration, their calibration is usually not affected by slight vibration, impact or falling. However, their temperature range is usually limited to 100oC
Platinum resistance thermometer (PRT)
PRT is probably the most widely used of all temperature sensors because of their wide temperature range and high accuracy. Most of them can be used in the temperature range of – 196oc to 420oc, with a few exceptions, up to 500oC or higher. Of course, it depends on the specifications of each model and its respective calibration.
Even if PRT has high accuracy and covers a wide temperature range, they have limitations. Unlike thermistors, the calibration of the PRT changes if the platinum wire is contaminated, exposed to vibration, impact, or drop. Calibration changes made through these processes are cumulative. Therefore, great care must be taken when handling and using PRT.
The advantage of metal thermocouples is that they have a very wide temperature range and low cost. Their disadvantages include relatively low accuracy, and they are prone to nonuniformity at very high temperatures.
Nobel metal thermocouple has a very wide temperature range, higher accuracy, but higher price. Like metal thermocouples, they are prone to nonuniformity.
Eight reasons for failure during calibration
1. Thermistor and self heating in PRT
When calibrating the thermistor and PRT, the nominal excitation current is applied. The amount of current required is usually stated in the calibration report or in the manufacturer’s specifications.
We know from Ohm’s law that when current flows through a resistance, it consumes power (I2R). The power causes the sensor to heat; This is called “self heating”. After calibrating the temperature sensor, its self heating has been considered.
When using any sensor, make sure that the reading is set to the appropriate excitation current. Too little or too much current will lead to measurement error. If too much current is applied, these sensors may even be damaged.
When selecting “thermistor” or “PRT”, some readings will automatically select the appropriate current. Others may need to be set manually. These settings are usually in the probe setup menu. If you select the current manually, always refer to the thermometer specification or calibration report for the correct current.
2. Low insulation resistance and leakage current
Low insulation resistance is sometimes called shunt resistance because it allows current to flow outside the measuring circuit. Electrically, it’s like putting another resistor in parallel with the sensor. When low insulation resistance occurs, the transition junction temperature often becomes too hot（ The hub should not be too hot to touch.)
In addition, if the sheath is bent or the seal is damaged, the insulation resistance may be low, allowing moisture to enter the sensor and wire. This problem can usually be avoided by proper use and handling.
3. Transition connection point
Thermistors and PRTs usually have transition points. The transition point is where the cable lead is connected to the sensor lead. The leads will be soldered or spot welded. If they are welded and the junction becomes too hot, the solder will melt, resulting in an open or intermittent state.
Generally, the junction is sealed with epoxy to prevent moisture and other contaminants. If the seal is exposed to temperatures higher than the epoxy resin cannot, the seal may break. This allows moisture and other contaminants to penetrate the seal and reach the wires and sensors. When the temperature sensor is immersed at a temperature lower than the ambient temperature or the ambient humidity is high, the moisture accumulation is the most obvious.
PRT is usually packaged with powdered insulating material. This material makes PRT less susceptible to mechanical shock induced stresses. Unless there is good sealing, the insulation will absorb moisture in the air at low temperature. Moisture or other contaminants can cause measurement errors and cause the temperature sensor not to be calibrated. Water retention can also bring security risks. If the insulation absorbs a lot of moisture and the temperature sensor is placed in a high temperature heat source, the moisture will turn into steam, which may cause the seal to inflate or rupture the sheath.
4. Broken or intermittent conductor
If pulled, overworked or under pressure, the cable may break, resulting in open circuit or interruption. Sometimes, the sensor or sensor lead may be disconnected or intermittent. Some intermittent events did not attract attention until the temperature sensor was heated, causing the wire to expand and separate.
Even though great care has been taken to prevent disconnection or intermittent connections, they may still occur as long as there is enough time and use time. Repeated expansion and contraction of wires and sensor wires may eventually cause casualties and lead to wire fracture.
Pollution may be caused by chemicals, metal ions or oxidation.
If the liquid reaches the wire or sensor wire, chemical contamination may occur in the PRT. This can change the purity of platinum and thus its electrical properties. Any change in purity is permanent.
The metal ion contamination of platinum wire usually occurs at 600oC or higher. Because PRT sensors are made of high purity platinum wire, they are most vulnerable to this type of contamination. The pollution of metal ions is irreversible, which will lead to the rising temperature of PRT. This is especially obvious in the three water level battery with extremely stable reference temperature. When PRT is manufactured for extremely high temperatures, its construction should protect the sensor from ion contamination.
The temperature sensor sheath is usually sealed to prevent contamination. The industrial temperature sensor and the auxiliary temperature sensor are not drained before sealing. So, usually, they will have some dry air inside. When they are exposed to various temperatures, oxidation forms on the wire surface. Oxidation mainly affects the temperature sensor, whose sensing element contains platinum wire. Oxidation will lead to the increase of rtpw (resistance at the three-phase point of water) in metal RTD. Fortunately, the oxidation can be removed by annealing the RTD using the temperature and procedure recommended by the manufacturer. Before and after annealing, the temperature sensor is compared with the three-point accuracy standard of water tank. This allows you to determine if the process was successful,
6. Hysteresis and nonrepeatability
Hysteresis phenomenon refers to that when the thermometer moves in a continuous temperature range, the reading of temperature sensor will lag or appear “memory” effect. The measurement depends on the previous temperature to which the sensor or wire was exposed. If the temperature sensor is passing through a range of temperatures for the first time (for example, from cold to hot), it will follow a specific curve. If the measurements are repeated in reverse order (cold to hot in our example), the thermometer with hysteresis will deviate from the previous set of measurements. If repeated, the offset may not always be the same.
Standard platinum resistance thermometers (sprts) that are intact do not exhibit hysteresis because sprts are designed to be strain free. However, the rugged PRT is not a strain free design and has at least some hysteresis. Water entering or penetrating into the temperature sensor will cause hysteresis in any type of RTD.
When thermocouple is used at high temperature, its wire may be polluted. This causes the local Seebeck coefficient of the conductor to change from its initial state. In other words, this changes the sensitivity of the wire to temperature changes. However, the temperature and contamination exposed along the length of the thermocouple may not be uniform. The Seebeck coefficient then becomes a function of the position of the thermocouple. This leads to measurement error, which depends on the temperature curve of the thermocouple in the whole length of the thermocouple, not only the temperature at the measurement node.
8. Short term stability
Measurement repeatability is a term that can be used in many different ways. It should be defined by the person who uses the term. It usually refers to the repeatability of rtpw during thermal cycling or calibration.
When the temperature sensor can not meet its short-term stability index, it means that the deviation between measurements at a specific temperature exceeds its index. This may be due to large standard deviations or readings that drift continuously in one direction. The potential causes of short-term stability problems include:
In order to prevent temperature sensor failure and avoid pollution, appropriate preventive measures should be taken when using temperature sensor in harsh environment. Do not make the transition junction bear the temperature higher or lower than that of epoxy seal or transition junction. Please refer to the temperature sensor specifications or contact the temperature sensor manufacturer for the transition junction temperature specifications. If the transition point may be exposed to high temperature or even slight high temperature, it is recommended to use heat shield or radiator.
Other ways to prevent failure:
Do not drop, impact or vibrate PRT
Do not bend sheaths that are not designed to bend. Even slight bending can adversely affect the service life of calibration or temperature sensors.
Do not immerse the adapter in liquid.
Do not exceed the temperature specification of the temperature sensor.
Do not soak the temperature sensor for a long time, especially at the temperature where oxidation may occur.
Do not pull or over tighten the temperature sensor cable.
If the temperature sensor needs to be annealed, use the recommended temperature and technology. Then, the accuracy of the temperature sensor is always verified by comparing it with the main standards.
Regularly compare the accuracy of the temperature sensor with major standards, such as water three-point battery or calibrated SPRT (standard platinum resistance thermometer).
Editor in charge: CC