Temperature: RTD vs Thermocouple

Q: Which standards or codes apply?

IEC 60751 (RTD Pt100), IEC 60584 (thermocouples), ASME PTC 19.3 TW (thermowell strength), ISA-RP for installation.

Quick pick (TL;DR)

≤ ~600 °C and you want accuracy & stability → RTD Pt100 (3-wire, Class A).
Higher temperature, vibration, fast response or tight space → thermocouple (type K general, N for stability, J/T for special).
Use a head-mounted 4–20 mA + HART transmitter.
Always fit a thermowell sized for insertion depth and verified by the ASME PTC 19.3 TW wake-frequency calculation.

Step 1 — RTD vs thermocouple

	RTD (Pt100)	Thermocouple
Range	−200 to ~600 °C	up to ~1700 °C
Accuracy	High, stable	Lower, drifts
Response	Slower	Fast
Vibration	Less robust	Robust
Use for	Control ≤600°C	Furnaces, high T

Step 2 — Wake-frequency check

Thermowell in flow (PTC 19.3 TW)

Why it matters flow sheds vortices behind the well vortex frequency f_w must avoid resonance Vortex frequency f_w = St · v / D St ≈ 0.22 , v = 3 m/s , D = 0.019 m f_w = 0.22 × 3 / 0.019 = 34.7 Hz Acceptance f_w / f_natural < 0.8 (in-line limit) if f_natural = 120 Hz → 34.7/120 = 0.29 → OK If it fails → shorten U, use stepped/tapered, thicker tip

Vortices can resonate & snap the well off — PTC 19.3 TW keeps the frequency ratio safe.

Step 3 — Installation details

Insertion ~⅓ pipe ID into flow.
3-wire RTD; 4-wire for lab accuracy.
Head transmitter reduces noise.

Thermowell wake-frequency calculator

Vortex shedding frequency check

f_w = St·v/D; keep f_w / f_natural < 0.8.

Strouhal St

Velocity v (m/s)

Tip OD (mm)

f_natural (Hz)

Screening — full ASME PTC 19.3 TW also checks dynamic stress. If fails: shorten U / stiffen.

Common mistakes

Skipping the wake-frequency check.
Insertion too shallow.
Thermocouple where RTD is needed.

Requisition & datasheet

Process, temperature, pressure, velocity.
Sensor, transmitter, thermowell, wake check.

Frequently asked questions

What should I know about Temperature Measurement?

≤ ~600 °C and you want accuracy & stability → RTD Pt100 (3-wire, Class A). Higher temperature, vibration, fast response or tight space → thermocouple (type K general, N for stability, J/T for special). Use a head-mounted 4–20 mA + HART transmitter. Always fit a thermowell sized for insertion depth and verified by the ASME PTC 19.3 TW wake-frequency calculation.

Which standards or codes apply?

IEC 60751 (RTD Pt100), IEC 60584 (thermocouples), ASME PTC 19.3 TW (thermowell strength), ISA-RP for installation.

Standards — IEC 60751; IEC 60584; ASME PTC 19.3 TW; ISA-RP.

Temperature Measurement