Temperature is a commonly used measurement parameter. Temperature sensors are used in instruments that measure temperature. To ensure accuracy and precision, all temperature sensors must be calibrated according to known standards. However, various problems can arise during routine calibration. Below, Hequan has compiled eight common reasons why temperature sensors cannot be calibrated:
Temperature sensor
1. Self-heating in thermistors and PRTs
When calibrating thermistors and PRTs, a nominal excitation current is applied. The required current amount is usually specified in the calibration report or the manufacturer's specifications.
We know from Ohm's Law that when current flows through a resistor, it consumes power (I²R). This power causes the sensor to heat up; this is known as "self-heating." Self-heating has been accounted for when calibrating the temperature sensor.
When using any type of sensor, ensure that the reading is set to the appropriate excitation current. Too little or too much current will cause measurement errors. Applying too much current may even damage these sensors.
When you select "Thermistor" or "PRT," some readings will automatically select the appropriate current. Others may require manual setting. These settings are usually found in the probe settings menu. If you manually select the current, always refer to the thermometer's specifications 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, this is equivalent to connecting 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 so hot that it is difficult to touch.)
Furthermore, if the sheath is bent or the seal is damaged, it may result in low insulation resistance, allowing moisture to enter the sensor and wiring. This problem can usually be avoided through proper use and handling.
3. Transition connection point
Thermistors and PRTs typically have transition connection points. A transition connection point is where the cable leads connect to the sensor leads. The leads are either soldered or spot-welded. If they are soldered and the junction becomes too hot, the solder will melt, resulting in an open circuit or intermittent state.
Typically, epoxy resin is used to seal the seal against moisture and other contaminants. However, if the seal is subjected to temperatures exceeding the limits of epoxy resin, it may crack. This allows moisture and other contaminants to penetrate the seal and reach the wires and sensor. Moisture accumulates when the temperature sensor is immersed in temperatures below ambient temperature or when the ambient humidity is high.
PRTs are typically packaged with powdered insulation material. This material makes the PRT less susceptible to stress caused by mechanical shock. Unless there is a good seal, the insulation layer will absorb moisture from the air at low temperatures. Moisture or other contaminants can cause measurement errors and prevent the temperature sensor from being calibrated. Retained moisture also poses safety hazards. If the insulation layer absorbs a lot of moisture and the temperature sensor is placed in a high-temperature heat source, the moisture will turn into vapor, which may cause the seal to swell or the sheath to rupture.
4. The wire is broken or discontinuous.
If pulled, overworked, or subjected to stress, the cable may break, resulting in an open circuit or intermittent connection. Sometimes, the sensor or sensor lead may disconnect or become intermittent. Some intermittent events don't come to attention until the temperature sensor is heated, causing the wires to swell and separate.
Even with meticulous care to prevent disconnections or intermittent connections, they can still occur given sufficient time and usage. Repeated expansion and contraction of wires and sensor leads can ultimately cause injury or death, resulting in wire breakage.
5. Pollution
Pollution may be caused by chemicals, metal ions, or oxidation.
If liquid reaches the wires or sensor wires, chemical contamination can occur in the PRT. This can alter the purity of the platinum, thereby changing its electrical properties. Any change in purity is chemical.
Metal ion contamination of platinum wire typically occurs at temperatures of 600°C or higher. Because PRT sensors are made using high-purity platinum wire, they are most susceptible to this type of contamination. Metal ion contamination is irreversible and causes the PRT temperature to rise continuously. This is particularly evident in triple-level cells where the reference temperature is extremely stable. When PRTs are manufactured for extremely high temperatures, their construction should protect the sensor from ion contamination.
Temperature sensor housings are typically sealed to prevent contamination. Industrial and auxiliary temperature sensors are not purged before sealing. Therefore, they usually contain some dry air. When exposed to various temperatures, this air can cause oxidation to form on the wire surface. Oxidation primarily affects temperature sensors, whose sensing elements contain platinum wire. Oxidation causes an increase in the RTPW (resistance at the triple point of water) in the metallic RTD. Fortunately, oxidation can be removed by annealing the RTD using the manufacturer's recommended temperature and steps. Before and after annealing, the temperature sensor is compared to standards such as the three-point accuracy of the water tank. This allows you to determine if the process was successful.
6. Hysteresis and non-repeatability
Hysteresis refers to the phenomenon where a temperature sensor reading lags behind or exhibits a "memory" effect when a thermometer moves across a continuous temperature range. The measured value depends on the previous temperature the sensor or wiring was exposed to. If the temperature sensor is traversing a range of temperatures for the first time (e.g., from cold to hot), it will follow a specific curve. If measurements are repeated in reverse order (cold to hot in our example), a thermometer with hysteresis will deviate from the previous set of measurements. If repeated, the offset may not always be the same.
A properly functioning standard platinum resistance thermometer (SPRT) will not exhibit hysteresis because it is designed to be strain-free. However, robust PRTs are not strain-free and exhibit at least some hysteresis. Moisture ingress or seepage into the temperature sensor will cause hysteresis in any type of RTD.
7. Non-uniformity
When thermocouples are used at high temperatures, their wires can become contaminated. This causes the local Seebeck coefficient of the wires to change from their initial state. In other words, this alters the wires' sensitivity 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 thermocouple's location. This leads to measurement errors that depend on the temperature profile the thermocouple experiences over its entire length, not just at the measurement junction.
8. Short-term stability
Measurement repeatability is a term that can be used in many different ways. It should be defined by the person using the term. It typically refers to the repeatability of the RTPW during thermal cycling or calibration.
