Updated: May 31
In this article, I am going to discuss Temperature Measurement which is an important part of any process control system. To measure temperature is essential.
Temperature measurement is the most critical measurement. in some plant-like oil and gas, steel plant, power plant, etc. were to measure temperature accurately is essential.there are many types of temperature measurement are available which can be the non-contact type or contact type of measurement.
Non-contact type measurement
Non-contact type temperature measurement like pyrometer or infrared thermometer which use infrared light to detect temperature from distance. when infrared light comes in contact with the hot or cold body which is to be detected then rays get back to device which is converted to usable quantity.pyrometer also come with different types as another website also provides about their types in detail I am not going for further explanation.
Contact type Measurement :
Contact type of temperature measurement especially electronic devices which are most commonly used nowadays. let have a look at such device
1. RTD (Resistance Temperature Detector)
As the name defines its clear introduction that with a change in resistance temperature changes. it determines temperature by measuring the resistance of a pure electrical wire. This wire knows as the temperature sensor. If you have a range of measurement from -250 to 850 °C then it becomes handy as RTD provides the most accurate measurement with linear characteristic. the following result shows the formula of calculating temperature,
Rt = Ro (1 + α (t - to))
Where α is sensitivity at which resistance change with the per degree of temperature,
Rt and Ro are resistance value at t and to temperature
In RTD copper, nickel, platinum are used to measure temperature.out of them platinum is most commonly used as it provides good linear characteristic over a wide range of temperature.
In platinum PT100, PT1000, PT500 are used. PT100 means it has 100 ohms resistance at 0°C. Which has a sensitivity of 0.385 Ω/°C. RTD comes in different type we can use them as per accuracy requirement. which we will discuss in another article.
When two dissimilar metal are twisted together it produces an emf which is directly proportional to temperature difference one is called hot junction other called cold junction which in further gives output in a low level of voltage signal (mV) which is also known as Seebeck effect. hot junction is refer as measuring junction while cold junction where the connection is done.thermocouple is used where high-temperature measurement is required. one Chromel and one constantan are commonly used metal to measure temperature.
thermocouple comes in many variant like J, K, E, T, N, B, R, S. with each different type temperature ranges are different but everyone contain same schematic with different dissimilar metal can be used.
like RTD thermistor work totally in the opposite way. A thermistor is a negative temperature coefficient type measurement, as well as positive temperature coefficient like RTD.with increase in temperature resistance, is decreased in case of the thermistor.
They can be found in everyday used appliances such as fire alarms, ovens and refrigerators also in automotive application to measure oil and coolant temperatures in cars. They are also used in digital thermometers.