Temperature Sensors

Chapter from the Instrumentation & Control Design Guide.

TEMPERATURE SENSOR & THERMOWELL DESIGN

Temperature Sensor Type Selection

Type	Range	Accuracy	Response Time	Best Application	Limitations	Typical Cost
RTD (Pt100)	−200 to 850°F	±0.5°F	Moderate (5-30 sec)	Precision measurement, lab	Slower response, self-heating	Medium
Thermocouple Type K	−330 to 2,280°F	±2-4°F	Fast (1-10 sec)	General industrial, wide range	Less accurate than RTD	Low
Thermocouple Type J	−350 to 1,400°F	±2-4°F	Fast	Reducing atmospheres	Oxidizes above 1,000°F	Low
Thermocouple Type N	−450 to 2,300°F	±2-4°F	Fast	High temp, nuclear/aerospace	Less common inventory	Low-Medium
Thermocouple Type S/R	32 to 2,640°F	±2.5°F	Moderate	Very high temperature	Fragile, expensive	High
Bimetallic (local)	−50 to 1,000°F	±1% of span	Slow (minutes)	Local indication only	No remote reading, slow	Low
Infrared/Pyrometer	0 to 5,000°F	±1-2%	Instant	Non-contact, rotating equipment	Emissivity must be known	High
Skin TC	−50 to 1,500°F	±5-10°F	N/A	Pipe/vessel wall temperature	Installation critical, lag	Low-Medium

Thermowell Design Considerations

Thermowell Purpose: protect sensor from process fluid, allow sensor removal without process isolation

Design Standards: ASME PTC 19.3 TW-2016 (Thermowells — Performance Test Codes)

Key Design Checks:
1. Wake Frequency Calculation: thermowell natural frequency must be >125% of vortex shedding frequency
• f_shedding = S × V / d (S = Strouhal number ~0.2, V = flow velocity, d = tip diameter)
• If too close to natural frequency → resonance → fatigue failure
• Mitigation: shorter length, larger root diameter, stepped/tapered well

2. Steady-State Stress: bending from flow-induced drag + pressure
• Must be within material allowable stress at operating temperature
• Critical for high-velocity gas service

3. Immersion Length: minimum 10× tip OD for accurate reading
• In small pipes: may need elbow installation or angled insertion
• Too shallow = conduction error (reads closer to pipe wall temperature)

4. Material Selection: match process metallurgy
• Carbon steel: general service to 750°F
• 316 SS: corrosive, high temperature to 1,500°F
• Alloy C-276: severe corrosion, acid service
• Ceramic: extreme temperature, oxidizing atmospheres

Source: Instrumentation_Control_Design_Guide_v1.xlsx · sheet “Temperature Sensors”

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