About Thermometer Calibration

Thermometer & Thermocouple Types

We use temperature measuring instruments every day. If we want to measure a fever we use a thermometer. In a refrigerator, we have to know the temperature inside in order to ensure that our food is stored under the correct conditions. In a fuel station, we have to perform corrections according to the fuel temperature in order to sell the fuel at the correct price. Weather stations, industrial processes, home appliances, clinical processes… Temperature measurements are practically used everywhere!

The only way to be sure our temperature measurements are correct is to use calibrated temperature instruments or thermometer. Before describing the methods of temperature calibration, let’s first see what types of temperature instruments are most commonly used.

Resistance thermometers

Resistance thermometers consist of one or more sensing resistors with wire leads and protective sheath. The resistors are manufactured from Platinum, Copper or Nickel. They have a known value at a temperature of 0 ^oC and by changing the temperature, the resistor’s value changes. Each material has a characteristic and well defined polynomial equation which provides the measuring temperature as output when the resistance is at 0 ^oC and the measured resistance at the temperature under measurement are used as inputs.

Resistance thermometers are characterized by their tolerance class and their measuring range which are defined in the following table:

Resistance thermometers may have two, three or four leads depending on the circuitry intended for the measurement of the resistance. These thermometers must be protected from corrosion, the ingress of moisture and mechanical and thermal stresses. A very commonly used Platinum resistance thermometer is a Pt100 which at 0 ^oC has a reference resistance of 100 Ohms.

Thermocouples

A thermocouple consists of two dissimilar conductors connected together at the measuring junction. The other ends (the reference junctions) are connected, either directly or by some suitable means, to a device for measuring the thermo electromotive force (emf) generated in the circuit. This electromotive force (emf), generated by a thermocouple, is a function of the temperatures of the measuring and reference junctions but, more specifically, it is generated as a result of the temperature gradients that exist along the lengths of the conductors.

Most commonly used types of thermocouples are shown in the table below:

Thermocouples are widely used in industry and science but their limitation is in accuracy since it is difficult to achieve system errors of less than 1 degree (^oC).

Liquid in Glass

A liquid in glass thermometer consists of a bulb (reservoir of the thermometer liquid), a stem (tube containing the capillary in which the thermometric liquid moves with a change of temperature), a thermometric liquid and an inert gas above the liquid column. The working principal of these thermometers is based on the fact that the volume of the liquid changes slightly with temperature, causing the liquid to arise into the tube. The body of the thermometer is scaled, allowing us to read the measured temperature directly. The liquids used for this type of thermometers and their temperature ranges are presented in the following table:

Accuracy classes of liquid in glass thermometers and their Maximum Permissible Errors (MPE) are defined in OIML R 133 as follows:

There are other types of thermometers also used in various applications:

Bimetallic thermometers

They consist of two strips of different metals which expand differently upon increasing or decreasing temperature. Their working principle is based on the mechanical displacement caused by the temperature change. Commonly used metals are steel and copper or in some cases steel and brass.

Infrared thermometers

They measure the temperature from a distance without any contact and for this reason, they are sometimes called non-contact thermometers, temperature guns or laser thermometers (if a laser is used to help aim the thermometer). These thermometers measure temperature from a portion of the thermal radiation (sometimes called blackbody radiation) emitted by the object being measured.

Thermistors

Their working principle is similar to the resistance thermometers since they measure temperature based on resistance change, but they differ from resistance thermometers in that the material used in a thermistor is generally a ceramic or a polymer. Also, thermistors achieve higher precisions within a limited temperature range (typically -90 to +130 ^oC).

Thermometer Calibration Procedure

A thermometer consists of:

• The measuring element (resistor, thermocouple, etc.)
•  The conversion method (resistance to temperature, emf to temperature, etc.)
•  The readout instrument (or temperature indicator)

A thermometer can be calibrated either as a whole system (containing all the three above subsystems) or by calibrating each subsystem separately. In the case of a liquid in glass thermometer, the three components cannot be separated, so it is calibrated as a whole system. On the other hand, in the case of a resistance thermometer, the sensing element (resistor) can be calibrated separately (by measuring directly resistance at various temperature points) and the temperature indicator can be calibrated on its own by applying known resistor values and checking the indication. However, when possible, it is better to calibrate the thermometer as a whole system, since this is how it is used in practice.

Temperature is one of the most widely measured physical quantities. But unlike other quantities, such as mass and time whose SI units are based on physical realizations, the temperature is defined on a theoretical set of conditions. The current working temperature scale is the International Temperature Scale of 1990 (ITS-90) and it is measured in degrees Celsius (^oC).

There are two methods widely used for calibrating thermometers:

• Fixed points calibration method
• Comparison to standard thermometers method

Fixed points method

A thermometer is calibrated by measurements at a series of temperature fixed points (freezing/melting points, triple points or vapour pressure points of pure materials). By using this method we insert the thermometer in a fixed point cell which provides the desired temperature point.

In the following table the most common fixed points according to ITS-90 are shown:

* Triple Point is defined as the point where Liquid, Solid and Gas are in equilibrium.

The water triple point is the most important and accurately realizable of the fixed points. The apparatus used for the realization is a glass flask nearly filled with very pure water and placed in an ice and water bath that contains the cell at or near the freezing point of water.

The fixed points method is the most accurate calibration method and it is used only in the highest quality calibrations. It is not commonly used by calibration laboratories since it is very complex and high cost.

Comparison method

This method of calibration is the most widely used. It is based on comparing the thermometer under test to a higher accuracy standard thermometer. The comparison usually takes place in a liquid bath or a dry block calibrator. Thermometers used as standards by comparison method are usually high accuracy resistance thermometers.

Several parameters must be taken into account when using the comparison method. The most important are immersion depth and homogeneity of the liquid or air where the thermometer is immersed. The immersion depth depends on the construction of the thermometer, the temperature difference between the bath and the surrounding atmosphere, the heat transfer capability of the bath and the temperature stability of the bath. Homogeneity depends on the equipment used. Better homogeneity is achieved by liquid baths, but we are able to improve homogeneity of dry block calibrators by inserting a metal equalizing block with thermowells to receive both the standard and the thermometer under test.

Temperature Probe & Thermometer Calibration Methods

Temperature probes and thermometers are used in all types of industry from food, to aerospace and industrial applications.

This category includes mostly direct reading temperature measuring instruments (glass thermometers, temperature sensors with incorporated indication, etc.). These instruments are calibrated using the thermal (temperature) calibration method. Depending on their accuracy, either the comparison method can be selected, or the fixed point method (for instruments requiring a high accuracy calibration.

Climatic chambers

They are chambers with controlled temperature. They are programmed to a set point and they also contain a temperature indicator. The comparison method is used for their calibration. But in this case, a number of standard thermometers, placed in specific positions inside the chamber, are required. There are special rules and procedures for climatic chambers calibration.

Furnaces, ovens, liquid baths

There are several types of instruments that belong in this category. They have a wide range of applications in several sectors such as food industry, medical industry, calibration laboratories, etc. Especially for the liquid baths, the liquid used depends on the desired temperature, so alcohols are used for temperatures below 0 oC, water from 0 oC to 80 oC and oils up to 300 oC. Their calibration is similar to the one used for the dry block calibrators and is based on the comparison method.

Chart recorders

They provide a hard copy record of the measured temperature. They may contain additional functions such as real-time display or alarm. Their calibration is performed by using the comparison method.

Data Loggers

They are similar to the chart recorders, but they are electronic measuring instruments and they do not provide paper-based records. They store the measurements into their memory. Data loggers may also contain real-time display, alarm outputs and other functions. Their calibration is similar the one used for the chart recorders.

Thermocouples

They consist of two dissimilar conductors connected together at the measuring junction. The temperature change at the reference junction causes a voltage to be generated. Depending on the type of the thermocouple (K, J, T, etc.) there are reference tables which correspond the generated voltage into temperature values. Thermocouples are calibrated by measurement either with fixed-point temperatures or, by comparison with a reference thermometer, in thermally stabilized baths or furnaces. Also, a combination of both methods can be used.

There are of course several other instruments used to either produce or measure temperature. All these instruments are used in many different applications and processes. No matter the purpose of the application, the only way to be sure about our temperature measurements is to use instruments which are properly calibrated from accurate reference sources.

Calibration of a Liquid in Glass Thermometer Example

Before commencing with the thermometer’s calibration, we must visually inspect the thermometer to ensure that there are no malfunctions such as gaps in the measuring liquid, errors in the measuring scale, etc.

We must perform the calibration in a laboratory with ambient temperature within 23 ^oC ± 3 ^oC and temperature stability, during the testing period, of ± 1 ^oC. We must make sure that the thermometer under test is left in the laboratory for sufficient time period in order to be conditioned to the lab’s environmental conditions.

The thermometer we want to calibrate has a measuring range of -10 ^oC to +50 ^oC and it is a partial immersion thermometer, which means that its bulb and a specified part of its stem must be inserted into the bath, in order to indicate correct temperature readings. The thermometer is a C accuracy class thermometer (± 0.5 ^oC MPE).

The calibration is performed by using a liquid bath, containing ethanol, for temperatures from -10 ^oC to 0 ^oC, a dry block calibrator (with its thermowells) for temperatures from 0 ^oC to +50 ^oC, and a Pt-100 Platinum resistance thermometer alongside a high accurate digital multimeter for measuring the resistance of our standard.

Firstly, we insert the thermometer under test and our standard thermometer in the liquid bath. We set the bath’s temperature at -10 ^oC and leave it to stabilize. When we achieve stabilization we take the readings of both the under test instrument and the standard. We repeat this procedure for the 0 ^oC temperature point.

We remove the thermometers from the liquid bath and insert them into the dry block calibrator. We set the dry block calibrator at +10 ^oC, leave it to stabilize and record the readings of both instruments. We repeat this procedure for the rest temperature points (+20 ^oC, +30 ^oC, +40 ^oC and +50 ^oC). (We can use different measurements points, less or more, depending on our procedures or on the customer’s specific requirements).

The calibration report will contain the measurement results in a table similar to the following:

In order for our calibration report to be complete, we must also include a column containing the uncertainties of measurements, which can be evaluated by using the document EA-04/2 “Expression of the Uncertainty of Measurement in Calibration”.