The selection of temperature instruments must be based on measurement units, installation method, material compatibility, sensing element type, and accuracy requirements. The following specification defines the technical criteria to ensure reliable measurement performance and suitability for various industrial process conditions.
1. General Requirements
1.1 Measurement Unit
All temperature measurements shall be expressed in degrees Celsius (°C).
Instruments shall provide direct-reading indication unless otherwise specified.
1.2 Accuracy
Temperature transmitters should preferably meet ±0.5% accuracy class.
Higher accuracy classes may be required for critical control loops or quality-sensitive applications.
2. Installation Requirements
2.1 Piping Installation (Full Pipe Flow Applications)
Instruments may be installed vertically or at a 45° angle to the pipe wall.
For 45° installations, insertion shall be against the flow direction to minimize flow-induced vibration and improve thermal response.
Insertion depth (length of the sensing element inside the pipe wall):
Minimum: 50 mm
Maximum: 125 mm
2.2 Equipment Installation
For vessels, tanks, or equipment walls, the sensing element shall be immersed at least 150 mm beyond the inner wall.
2.3 Special Applications
For flue ducts, furnaces, refractory-lined equipment, or high-temperature chambers, the installation method shall be determined based on actual operating conditions, thermal gradients, and structural constraints.
3. Sensor Element and Material Selection
3.1 Thermowell Material
Thermowell material shall be selected according to:
Process temperature and pressure
Corrosion resistance to the measured medium
Flow velocity and vibration conditions
High-parameter applications (high temperature, high pressure, high velocity) require vibration and strength calculations for the thermowell.
3.2 Sensors for Ambient or Surface Temperature
Thermowells are not recommended.
Use armored (mineral-insulated) or assembled RTD/thermocouple sensors for faster thermal response.
3.3 Selection of Temperature Sensing Elements
RTD (Resistance Temperature Detectors)
Suitable for applications requiring high accuracy, stable output, and fast response.
Preferred type: PT100, operating range –250°C to 850°C, optimized for –200°C to 600°C.
Thermocouples
Recommended for wide temperature ranges or vibration-intensive environments.
Common types: K-type, S-type, selected based on measurement temperature range and process medium.
For temperatures >870°C and reducing atmospheres with hydrogen content >5%, a gas-purged thermocouple is recommended.
4. Measurement Range Requirements
4.1 Local Temperature Instruments
The maximum process temperature shall not exceed 90% of the instrument’s upper range limit.
The normal operating temperature should be within 50% of the instrument range for optimal resolution and accuracy.
4.2 Low-Temperature Applications (Below 0°C)
The upper limit of the selected instrument must fully cover the minimum ambient temperature to avoid freezing-related measurement deviation.
5. Instrument Structure Selection
5.1 Local Indicating Instruments
Bimetal thermometers are preferred for on-site indication.
A universal (adjustable angle) connection between the housing and thermowell is recommended for flexible orientation.
For corrosive or outdoor applications, ensure the selected housing and seal materials meet IP rating and environmental protection requirements.