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Reasons Affecting the Calibration of Temperature Sensors

Reasons Affecting the Calibration of Temperature Sensors

1. The wire is broken or interrupted


If pulled, overworked, or subjected to stress, the cable may break, thus resulting in an open circuit or discontinuity. Occasionally, the sensor or the sensor leads may be broken or intermittent. Some intermittent events went unnoticed until the temperature sensor was heated, causing the wires to expand and separate.


Even though great care has been taken to prevent disconnection or intermittent connections, they can still happen given enough time and usage. Repeated expansion and contraction of leads and sensor leads can eventually lead to injury or death, resulting in wire breakage.


2. Pollution


Contamination can be caused by chemicals, metal ions, or oxidation.


Chemical contamination can occur in the PRT if the liquid reaches the leads or the sensor leads. This can change the purity of platinum and thus change its electrical properties. Any change in purity is permanent.


Metal ion contamination of platinum wire usually occurs at 600 degrees Celsius or higher. Because PRT sensors are fabricated by using high-purity platinum wire, they are most susceptible to this type of contamination. The contamination of metal ions is irreversible and will cause the temperature of the PRT to rise continuously. This is especially evident in three-water level cells where the reference temperature is extremely stable. When the PRT is manufactured for extremely high temperatures, it should be constructed to protect the sensor from ionic contamination.


Temperature sensor jackets are usually sealed to prevent contamination. Neither the industrial temperature sensor nor the auxiliary temperature sensor is vented prior to sealing. So usually, they will have some dry air inside them. Oxidation forms on the surface of the wires when they are exposed to various temperatures. Oxidation primarily affects temperature sensors, whose sensing elements contain platinum wires. Oxidation leads to an increase of metallic RTDs in RTPW (resistance at the triple point of water). Fortunately, the oxidation can be removed by annealing the RTD through the manufacturer's recommended temperatures and procedures. Before and after annealing, compare the temperature sensors to a standard such as the three-point accuracy of the tank. This allows you to determine if the process was successful.


3. Hysteresis and non-repeatability


Hysteresis is a condition in which the temperature sensor's readings lag or experience a "memory" effect as the thermometer is moved over a continuous temperature range. The measurement depends on the previous temperature to which the sensor or wire is exposed. If the temperature sensor is passing a range of temperatures for the first time (for example, from cold to hot), it will follow a specific curve. A thermometer with hysteresis issues will deviate from the previous set of measurements if the measurements are repeated in reverse order (cold to hot in our example). If there are duplicates, the offsets may not always be the same.


An intact standard platinum resistance thermometer (SPRT) will not exhibit hysteresis because the SPRT is designed to be strain-free. However, rugged PRTs are not strain-free designs and have at least some hysteresis. The ingress, or infiltration of moisture inside the temperature sensors, can cause hysteresis in any type of RTD.


4. Inhomogeneity


When a thermocouple is used at high temperatures, its wires may be contaminated. This causes the local Seebeck coefficient of the wire to change from its initial state. In other words, this changes the wire's 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 position along with the thermocouple. This results in a measurement error that depends on the temperature profile the thermocouple is exposed to over the entire length of the thermocouple, not just the temperature at the measurement junction.


5. 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 the RTPW during thermal cycling or calibration.

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