Selection of temperature instruments

1. General principles

01. Unit and scale (scale)

The scale (scale) unit of a temperature instrument is unified in Celsius (°C).

02. Detect (measure) the insertion length of the component

The selection of the insertion length should be based on the principle that the detection (measurement) element is inserted into a representative position where the temperature of the measured medium is sensitive to change. However, in general, in order to facilitate interchangeability, the length of the first to second gears is often selected uniformly for the entire device.

When installing on flue, furnace and masonry equipment with thermal insulation materials, it should be selected according to actual needs.

The material of the protective cover of the detection (detection) element should not be lower than the material of the equipment or pipeline. If the protective sleeve of the shaped product is too thin or not resistant to corrosion (such as armored thermocouples), an additional protective sleeve should be added.

Temperature instruments, temperature switches, temperature detection (measurement) components and transmitters installed in inflammable and explosive places with live contacts should be explosion-proof.

Second, the selection of the local temperature instrument

01. Accuracy class

General industrial thermometer: choose class 1.5 or class 1.

Precision measurement and laboratory thermometers: Class 0.5 or 0.25 should be selected.

02. Measuring range

The highest measured value is not greater than 90% of the upper limit of the measuring range of the instrument, and the normal measured value is about 1/2 of the upper limit of the measuring range of the instrument.

The measured value of the pressure thermometer should be between 1/2 and 3/4 of the upper limit of the measuring range of the instrument.

03. Bimetal thermometer

When meeting the requirements of measuring range, working pressure and accuracy, it should be preferred.

The diameter of the case is generally φ100mm. In places with poor lighting conditions, high positions and long viewing distances, φ150mm should be selected.

The connection method between the instrument shell and the protective tube should generally be a universal type, or an axial type or a radial type can be selected according to the principle of convenient observation.

04. Pressure thermometer

It is suitable for on-site or on-site panel display with low temperature below -80 ℃, unable to observe closely, with vibration and low accuracy requirements.

05. Glass thermometer

It is only used for special occasions with high measurement accuracy, small vibration, no mechanical damage and convenient observation. However, mercury-in-glass thermometers should not be used due to mercury hazards.

06. Base instrument

For on-site or on-site installation of measuring and control (adjustment) instruments, base-type temperature instruments should be used.

07. Temperature switch

It is suitable for occasions where contact signal output is required for temperature measurement.

3. Selection of centralized temperature instrument

01. Detect (measure) components

(1) According to the temperature measurement range, select a thermocouple, thermal resistance or thermistor with the corresponding graduation number.

(2) Thermocouples are suitable for general occasions. Thermal resistances are suitable for vibration-free applications. Thermistors are suitable for occasions requiring fast measurement response.

(3) According to the requirements of the measurement object for the response speed, the detection (measurement) elements of the following time constants can be selected:

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

Thermal resistance: 90～180s, 30～90s, 10～30s and <10s grade four;

Thermistor: <1s.

(4) According to the environmental conditions of use, select the junction box according to the following principles:

Ordinary type: places with better conditions;

Splash-proof, waterproof: wet or open-air places;

Explosion-proof: flammable and explosive places;

Socket type: for special occasions only.

(5) In general, the threaded connection method can be used, and the flange connection method should be used for the following occasions:

Installation on equipment, lined piping and non-ferrous metal piping;

Crystallization, scarring, clogging and strong corrosive media;

Flammable, explosive and highly toxic media.

(6) Thermocouples and thermal resistances used in special occasions:

In the case of reducing gas, inert gas and vacuum where the temperature is higher than 870℃ and the hydrogen content is more than 5%, tungsten-rhenium thermocouple or blowing thermocouple is selected;

The surface temperature of the equipment, the outer wall of the pipeline and the rotating body, select the surface or armored thermocouple and thermal resistance;

For medium containing hard solid particles, wear-resistant thermocouple is selected;

In the protection casing of the same detection (measurement) element, when multi-point temperature measurement is required, multi-point (branch) thermocouples are selected;

In order to save special protection tube materials (such as tantalum), improve the response speed or require the detection (measurement) component to be bent and installed, an armored thermocouple can be selected.

02. Transmitter

Transmitters are selected for the measurement or control system matched with the standard signal display instrument.

In the case of meeting the design requirements, it is recommended to select a transmitter that integrates measurement and transmission.

03. Display instrument

(1) A general indicator should be used for single-point display, a digital indicator should be used for multi-point display, and a general recorder should be used if historical data is required to be consulted.

(2) For the signal alarm system, an indicator or recorder with contact signal output should be selected.

(3) A medium-sized recorder (such as a 30-point recorder) should be used for multi-point recording.

04. Selection of auxiliary equipment

(1) When multiple points share one display instrument, a switch with reliable quality should be selected.

(2) Thermocouples are used to measure the temperature below 1600°C. When the temperature change of the cold junction makes the measurement system unable to meet the accuracy requirements, and the supporting display instrument has no automatic cold junction temperature compensation function, the cold junction temperature automatic compensator should be selected.

(3) Compensation wire

a. According to the number of thermocouples, the graduation number and the environmental conditions of use, the compensation wire or compensation cable that meets the requirements should be selected.

b. Select different levels of compensation wires or compensation cables according to the ambient temperature:

-20～+100℃ choose ordinary grade;

-40 ~ +250 ℃ select heat-resistant grade.

c. In places with intermittent electric heating or strong electricity and magnetic fields, shielded compensation wires or shielded compensation cables should be used.

d. The cross-sectional area of the compensation wire should be determined according to the reciprocating resistance value of its laying length and the external resistance allowed by the supporting display instrument, transmitter or computer interface.