Temperature is the physical property of a system, which underlines the common notions of "hot" and "cold." The material with the higher temperature is said to be hotter and that with a lower temperature is said to be colder. Temperature is the measure of the average kinetic energy of the particles in a sample of matter. In other words, temperature is the measure of activity and the frequency of collisions of molecules. Temperature, which is nothing but the degree of hotness or coldness, is measured in such units as °Celsius, °Kelvin and °Fahrenheit. Being a fundamental unit like mass and length, it cannot be measured using basic standards and can only be derived by measuring the change of state of the sensing device. International Practical Temperature Scale (IPTS) provides easy, convenient and consistent measurements by using uniform and self-consistent methods; there are nine reproducible fixed points available and these are called primary standards for temperature measurement. Instruments and sensors were developed to sense and display temperatures on the IPTS between these nine primary standards of temperature measurement. The table lists the different types of sensors in vogue.
Devices used to measure temperature
Any measuring device employed for temperature measurement has to be checked at regular intervals to confirm that its characteristics are retained over use and there is no deviation. If there is any deviation, then it should be confirmed that it is within the acceptable limits. Such a test is possible by subjecting the sensing device to a standard device whose accuracy is known. Several standard devices were developed over a period of time for the testing and calibration of sensors.
It is a simple device used for testing sensors in lower ranges. It is a well known fact that water oils at 100°C and if a sensor such as dial gauge or RTD sensor is thrust into boiling water, then the gauge should read 100°C and the output of RTD should be 138.51 ohms if measured with a multimeter. The limiting factors are that the water evaporates continuously and if care is not taken, then the bath will go dry soon and the measurements can go only upto 100°C.
To enhance the working temperature, oil is used instead of water and the working range can go upto 400°C. An efficient stirring system should be there to maintain uniformity of temperature. The main drawbacks of the oil bath are :
· Emission of toxic fumes
· Unsuitability for field use
- Long time required to reach high temperature
Dry block temperature bath
The limiting factors such as working range and emission of toxic fumes were overcome by the development of dry block temperature bath. In this, a metal block is used as a heated medium and the selection of the metal block depends on its ability to conduct heat, retain its dimension after repeated heating. The metals suitable for use are copper and brass. An electrical heater of suitable wattage is inserted into the block along with a sensor to measure the temperature. The output of the sensor is also inserted into the block to measure the temperature. The output of the sensor is fed to a controller, which will control the voltage to the heater through a suitable final control element. The heated block will have holes to a suitable depth for the insertion of test sensors and gauges. Care has to be taken while machining the block so that the heater will transfer heat rapidly to the block and the transfer of the heat from block to test sensor will also be quick. This is possible only if the holes are reamed and the clearance between the inserts and bore is minimal. This entire assembly should be housed in a suitable enclosure, making it lightweight and compact.
Calibrating of sensors and gauges
The calibration of sensors and gauges requires the following aids:
1. 6.5 or 8.5 digits digital multimeter
2. Master sensor (required for the cross verification for the block temperature)
3. Thermocouple/RTD reference table
After switching on the dry block temperature bath, set the required temperature. Insert the test sensor along with the multimeter. Allow the block temperature to reach the set value and stabilise. Normally, it should take about 10 minutes for the output of the test sensor to stabilize and in some cases, it may take more time also. Note the reading and convert the same into Celsius or Fahrenheit by referring to the table.
For cross verification, if required, the master sensor may also be inserted into the block along with the test sensor for comparison. For sub-zero temperatures, sensor calibration, dry calibration, dry block model for lower temperature is available and it employs Peltier cooling elements as a cooling medium to bring down the block temperature. For the dry block temperature, bath is an ideal tool for calibration laboratories as it takes less than 20 minutes to reach 650°C and about an hour to cool down to the ambient temperature. It also helps to keep the laboratory clean and tidy as it does not use any oil and also makes the laboratory a safe place to work in. The awareness about dry block temperature calibrators is slowly setting in, in India, and soon there will be greater demand as water and oil baths will be eased out slowly. The availability of indigenous dry temperature calibrators should speed up this transition.