Classification of Temperature

We will now learn about the classification of temperature. In addition, we will also understand the importance of temperature.

Importance of Temperature

Temperature is one of the most essential factors in process engineering to detect a hazardous condition in plant and in equipment. The Safety Integral Level (SIL) measures the safety instrument function. SIL specifies a target level of risk reduction. The International Electro-technical Commission (IEC) 615081 standard assigns SIL to the devices, which are able to eliminate device fault and detect the fault.

Classification of Temperature for Electrical Equipment

There are some insulation classes, which allow a maximum permissible temperature for the safety of devices. Electric instruments can be downsized by insulation technique of higher thermal endurance.

The following table shows maximum allowable temperature and the materials used for various types of insulation −

Insulation Classes Maximum Permissible Temperature (ᵒC) Material Used
Y 90 Cotton, silk or paper
A 105 Reinforced Class-Y materials with impregnated varnish or insulation oil
E 120 Combination of different materials
B 130 Inorganic material with adhesives
F 155 Class-B materials that are upgraded with adhesives, silicon and alkyd-resin varnish of higher thermal endurance
H 180 Inorganic material glued with silicon resin or adhesives of equivalent performance
C >180 100% inorganic material

Classification of Temperature for Hazardous Areas

The temperature classification describes the threshold temperature for the hazardous area. The value of minimum ignition temperature is classified from T1 to T6. This classification of temperature identifies a temperature that an instrument will produce at ambient environment temperature (40ᵒ C). The identified temperature is called maximum surface temperature.

  • T1 − The minimum ignition temperature > 450ᵒ C and the maximum surface temperature generated by the instrument is 450ᵒ C.

  • T6 − The minimum ignition temperature > 85ᵒ C and the maximum surface temperature generated by the instrument is 85ᵒ C.

Temperature Classification Minimum Ignition Temperature Temperature Maximum Surface Temperature
T1 >450ᵒC [842ᵒF] 450ᵒC [842ᵒF]
T2 >300ᵒC [572ᵒF] 300ᵒC [572ᵒF]
T3 >200ᵒC [392ᵒF] 200ᵒC [392ᵒF]
T4 >135ᵒC [275ᵒF] 135ᵒC [275ᵒF]
T5 >100ᵒC [212ᵒF] 100ᵒC [212ᵒF]
T6 >85ᵒC [185ᵒF] 85ᵒC [185ᵒF]

If there is any measure issue that occurs with the hazardous area instrument, it can be repaired. The repair section is of three basic categories.

Factory Repair Only

The device must go through some safety test procedures and returned to the factory.

Field Repair

It is difficult to instruct the end user technician practically. If there is any difficulty to solve the issue, an authorized factory personnel should be employed.

Field Repair by End-user − This involves direct replacement in terms of form, fit and function.

Temperature Measurement

Temperature measuring instrument is designed in accordance with the safety standards. In industrial environment temperature measurement is required for a wide variety of needs and applications. A large number of sensors and devices fulfill such demand. The measuring instruments are as follows −

  • Thermometer
  • Thermostat
  • Thermistor
  • Thermopile
  • RTD (Resistance Temperature Detector)
  • Thermocouple


1. In which insulation class the inorganic material with adhesive is used?

    a) Class E

    b) Class Y

    c) Class B

    d) Class A

Ans: c

Explanation − According to table 1, the inorganic material with adhesive can permit only 130ᵒC insulation which refers to Class B type.

2. What is the maximum surface temperature (in ᵒC) in the T4 type of temperature?

    a) 100

    b) 135

    c) 200

    d) 235

Ans: b

Explanation − According to table 2, the lower hazardous is a classification of temperature is T4 which allows 135ᵒC of maximum surface temperature to cause a hazard.

3. Which of the following is not a temperature measuring instrument?

    a) Thermocouple

    b) RTD

    c) Thermistor

    d) Barometer

Ans: d

Explanation − Thermocouple, RTD and thermistor are the temperatures measuring instrument but barometer measures air pressure.