How to Select a Temperature Instrument

Detection of the process production process is the primary means of understanding and controlling industrial production. We need to understand the whole picture of the process accurately and maintain it to ensure the production process is smooth.

Temperature instrumentation failure is relatively standard, but as long as you master the working principle and overhaul methods, you can also complete overhaul and installation work quickly.

Today, I want to share with you the knowledge of the eight commonly used types of temperature instrumentation, including the working principle, installation methods, product selection, the instrument’s composition, and the process’s uses, matters needing attention.

Temperature Instruments

1. Temperature Instrument Selection

1.1 Accuracy Class

For general industrial thermometers: choose 1.5 or 1 grade.

For precision measurement and laboratory thermometers: choose 0.5 or 0.25 grade.

1.2 Measuring Range

The highest measured value is at most 90% of the instrument’s upper limit of measuring range; the average estimated value of this upper limit is 1/2 or so.

The pressure thermometer’s measuring value should be within the instrument’s measuring range of the upper limit value of 1/2 ~ 3/4.

1.3 Bimetallic Thermometers

Working pressure and accuracy should be preferred to meet the requirements of the measurement range.

The diameter of the case is generally selected φ100mm; in poor lighting conditions, the location of higher and farther observation distance places should be selected φ150mm.

Instrument shell and protection tube connection generally should be selected as a universal type, but also in accordance with the principle of convenient observation of axial or radial type.

Fixed Bimetallic-Thermometer

1.4 Pressure Thermometers

Suitable for in-situ or in-ground display at low temperatures below -80°C, where close observation is not possible, where vibration is present and where accuracy is not required.

1.5 Glass Thermometers

Used only for special occasions where measurement accuracy is high, vibration is low, there is no mechanical damage, and observation is easy. However, glass mercury thermometers should not be used due to mercury hazards.

1.6 Base Type Instrument

In situ or on the ground to install measurement and control (regulation) instrumentation, using the base type temperature instrumentation is appropriate.

1.7 Temperature Switch

Applicable to temperature measurement needs contact signal output occasions.

2. Selection of Centralised Temperature Instrumentation

2.1 Detection Elements

  • According to the temperature measurement range, select the appropriate graduation number of thermocouple, RTD, or thermistor.
  • Thermocouples are suitable for general applications, RTDs for vibration-free applications, and thermistors for occasions requiring fast measurement response.
  • According to the requirement of response speed of the measurement object, the following time constants of the detection (measurement) element can be selected:

Thermocouple: 600s, 100s and 20s at three levels;

RTD: 90-180s, 30-90s, 10-30s and <10s four levels;

Thermistor: <1s.

  • According to the use of environmental conditions, according to the following principles of selection of junction boxes:

Ordinary type: places with good conditions;

Splash-proof and waterproof type: wet or open places;

Flame-proof type: flammable and explosive places;

Socket type: only for special occasions.

  • Threaded connection can be used in general, and flange connection should be used for the following occasions:

Installation of equipment, lined pipes, and non-ferrous metal pipes;

Crystallization, scarring, clogging, and corrosive solid media:

Flammable, explosive, and highly toxic media.

  • Thermocouples and RTDs used on special occasions:

Temperature higher than 870 ℃, hydrogen content greater than 5% of the reducing gases, inert gases, and vacuum occasions, choose tungsten rhenium thermocouple or blowing thermocouple.

Equipment, pipeline wall, and the surface temperature of the rotating body, the choice of surface or armored thermocouple, RTD.

Containing hard solid particles of media, the choice of wear-resistant thermocouple.

The duplicate detection (measurement) element protection casing requires multi-point temperature measurement and the choice of multi-point (branch) thermocouple.

Armored thermocouples can be used to save unique protection tube material (such as tantalum), improve the response speed, or require the detection (measurement) of element bending installation.

2.2 Transmitters

The choice of transmitters is based on the acceptance of standard signals to display the instrumentation supporting the measurement or control system.

To meet the design requirements, it is recommended that the integrated measurement and transmission transmitter be used.

Temperature transmitter

2.3 Display Instrument

  • Select a single-point display for general indicators; choose a digital indicator for multi-point displays. For requirements involving access to historical data, opting for a general recorder would be appropriate.
  • Signal alarm system: Using the indicator or recorder with a contact signal output is appropriate.
  • Multi-point recording should use a medium-sized recorder (such as a 30-point recorder).

2.4 Selection of Accessory Equipment

When multiple points share a display instrument, a reliable quality switch should be selected. When using thermocouples to measure temperatures below 1600℃, if the temperature change at the cold end affects the accuracy of the measuring system, and if the supporting display instrument lacks automatic compensation for cold end temperatures, it is necessary to select a cold end temperature automatic compensator.

2.4.1 Compensation Wire

  • According to the number of thermocouples, graduation number, and the use of environmental conditions should be selected to meet the requirements of the compensation wire or compensation cable.
  • According to the use of environmental temperature selection of different levels of compensation wire or compensation cable:

-20 ~ +100 ℃ selection of ordinary level.

-40 ~ +250 ℃ selection of heat-resistant grade.

There are intermittent electric heating or electric solid, magnetic field places that should use shielded compensation wire or cable.

The design should accommodate the length of the reciprocating resistance value and support display instruments, transmitters, or computer interfaces enabling the determination of external resistance input by ensuring an appropriate cross-sectional area for the compensation wire.

3.Conclusion

In the chemical industry, the enterprise is inseparable from various instruments. Instrumentation in the chemical production process plays a vital role in detecting, displaying, recording, or controlling process parameters.

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