Sensor calibration necessary for reliability of measured values
Temperature calibration sensors should be calibrated once a year

In field measurement of physical values such as temperature, pressure, and electrical conductivity, the most critical aspect of measuring instruments is the reliability of measured values. Measured values can vary depending on environmental changes, but they also vary depending on the condition of the measuring instrument.

In measurement technology, calibration refers to placing a test unit and reference equipment that measures the same physical quantities under identical conditions and comparing their values with each other. If the test unit value deviates from the required accuracy, calibration of the unit itself is necessary.

When calibrating temperature measuring instruments, dry well, liquid constant temperature bath, and electric furnace equipment are used. Among these, a dry well is a temperature generation device used to calibrate platinum temperature sensors. Without using liquid medium, it has the advantage of quickly implementing and maintaining desired temperatures by controlling solid heat conduction and air flow through heating and cooling. Its compact size makes it suitable for field use.

When using a dry well, errors in the heat source can occur depending on the usage method. Due to the structural design of the dry well where the temperature sensor is located at the bottom, the sensor to be calibrated must be inserted to the bottom to obtain accurate values. It is also important to align the sensing areas of the standard sensor and the sensor under calibration.

If the temperature sensor to be calibrated is long, the portion not immersed in the calibrator is affected by atmospheric conditions, causing errors. The thicker the diameter of the temperature sensor, the more thermal conductivity can affect measurements. To reduce this error, the immersion depth should be approximately 20 times the diameter of the temperature sensor.

As the set temperature of the dry well increases, the deviation from atmospheric conditions widens, increasing the error rate in axial temperature. While it is ideal to immerse the sensor close to the dry well's control sensor, if the sensor to be calibrated is short, uncertainty regarding uniformity must be considered.

So what factors should be considered when selecting a temperature calibrator? The most important factor when selecting a calibrator is the measurement range of the temperature sensor. Due to the characteristics of the medium, liquid constant temperature baths can measure from -100°C to 500°C. Electric furnaces are mainly used to calibrate high-temperature sensors exceeding 1000°C. Dry wells, composed of materials such as aluminum and copper internally, have a range of approximately -40°C to 600°C.

Field temperature calibrators must also consider performance and portability. Generally, larger equipment can provide higher stability and uniformity but is difficult to transport. Conversely, lightweight and portable dry wells have inferior performance.

Fluke's field dry well, the 914X series, is divided into three models depending on temperature range. With a weight of 7kg, it is easy to transport and allows input of resistance thermometer standard temperature coefficient values. It comes equipped with a basic switch test function to measure temperature contacts, and once the desired set temperature is reached, the stabilization status is displayed after internal stabilization is achieved. Dual zone control enables fast temperature control speed, and the insert section can be customized.

Kim Sung-hoon, manager at Korea Fluke, explained: "If temperature sensors are not calibrated, oxidation or impurities in the product itself attach to the sensor, changing its characteristics. Values measured this way are difficult to rely upon," adding "while it varies depending on the temperature sensor accuracy of the calibrator used, the application, and management methods, it is generally recommended to perform calibration once a year."