Temperature Instrument Calibration Overview
Most of the time, it is just not enough to measure temperature. We have to measure temperature accurately. The only way to achieve the required accuracy is to have our temperature measuring instruments calibrated. There are so many different types of temperature instruments and some of them (like simple commercial thermometers) are used in everyday applications by all kinds of users. Others, (like dry block calibrators, infrared thermometers and chart temperature recorders) are used in specific applications, such as industrial processes or calibration activities.
There are two methods which, depending on the application, can be used when performing temperature calibrations:
Thermal (Temperature) Calibration
This method involves direct temperature measurements and can also be separated into two sub-methods:
– The Comparison Method: This method, which is the most commonly used, is based on the comparison of the thermometer under test to a higher accuracy standard thermometer. For example, a Liquid in Glass thermometer is inserted into a known, controlled temperature source (i.e. a dry block calibrator) and its reading is compared to the reading of a reference thermometer.
– The Fixed Points Method: Under this method, absolute temperatures are provided by fixed point cells and comparison measurements are not needed. Fixed point temperatures are considered to be the freezing/melting points, triple points or vapour pressure points of pure materials. Two of the most commonly used fixed points are the Triple Point of Water (0.01 oC) and the Melting Point of Gallium (29.7646 oC).
This method uses electrical signals (voltage, resistance) to simulate temperature probes. For example, by supplying precisely generated voltage signals (corresponding to the ones produced by the appropriate thermocouple at several temperatures) to a temperature indicator, we can calibrate and/or adjust the indicator. In order to easily perform simulation temperature calibrations, a wide range of instruments, called simulators, are available. They vary depending on their type (voltage and/or resistance simulators), their accuracy and their package (laboratory simulators, outdoor simulators, explosive area simulators). They are “easy to use instruments” and in most of the cases, there is no need to perform manual calculations. The user only has to set the desired temperature and the electrical signal is generated automatically.
Calibrations must be performed by calibration laboratories, which have the appropriate reference equipment as well as properly established and documented measurement procedures. The calibration results are presented in the calibration certificate which is issued by the calibration laboratory and which must at least contain any calibration error at the various test temperatures and the measurement uncertainties. Calibration laboratories which are accredited to ISO 17025 for temperature measurements can easily fulfil the requirements and prove their capability on measurements, their measurement traceability to national standards and the validity of their measurement procedures, results and uncertainty budget.
Temperature Sensors & Instrument Calibration Methods
Both of the calibration methods, described above, are considered to be equally reliable. In order to decide which one shall be used, we must take into account the operational properties of the temperature instruments under test. So let’s study some of the temperature instruments widely used and their calibration.
They are devices which are normally used in conjunction with a temperature sensing device (resistance thermometers or thermocouples) to measure temperature. These instruments indicate in temperature units, a value that is derived from the measurement of a temperature dependent upon the input parameter such as a resistance or thermal EMF. The conversion from this into temperature units is normally based on standard reference tables.
The calibration of temperature indicators is performed by using the simulation method. A calibrated electrical source is required in order to simulate the temperature sensor. Using reference tables, the electrical output of the temperature sensor (resistance or voltage) at a specific calibration temperature point is determined and the output of the electrical source is set to this level. This electrical signal is applied to the temperature indicator and the indicator’s reading is compared with the simulated input temperature. The error of the temperature indicator is determined.
As described above, these instruments generate electrical signals which correspond to the output of temperature sensors or thermocouples. The calibration principal for these instruments is based on the verification of its conversion process by the direct measurement of the electrical signal produced. During the calibration procedure the simulator is set to the required temperature calibration point. The electrical output, produced by the simulator, is measured using a calibrated electrical measuring instrument (i.e. a high accuracy digital multimeter). The measured value is converted into the equivalent temperature using reference tables and the deviation of the simulator is determined.
Dry Block Calibrators
These devices are very convenient for checking various types of thermometers and temperature probes because they generate a wide range of temperatures (depending on their model) and they can achieve rapid heating and cooling. A dry block calibrator consists of a specially designed heated block within which is located an insert containing thermowells for the probes. Their operation is controlled either by heating elements (for units used only for temperatures above ambient conditions), or by peltier elements (for units which are able to provide also temperatures below ambient conditions and below 0 oC as well). Most of the times, dry block calibrators are portable units and they are commonly used for on site temperature calibrations. The calibration of these devices is performed by using the comparison method. The dry block calibrator is set to the required temperature calibration point and left to stabilize. A reference resistance thermometer is inserted into the calibrator’s thermowell and its reading is compared to the calibrator’s built-in temperature indicator. When calibrating dry block calibrators there are also other parameters that need to be checked: possible temperature differences between various inserts and thermowells, axial inhomogeneity of temperature in the inserts, temperature deviation due to heat conduction, etc.
Written by Sofia