Quick Answer
Thermocouple calibration is performed by comparison method: placing the thermocouple under test alongside a calibrated reference standard (SPRT or calibrated Type S thermocouple) in a temperature-controlled furnace or bath, and recording the EMF difference at defined temperature points across the thermocouple's range.
Key Takeaways
- Thermocouples are calibrated by comparison method in a furnace (high temp) or stirred bath (low/medium temp).
- Reference standard: SPRT (below 660°C) or calibrated Type S thermocouple (above 600°C).
- Calibration covers ITS-90 fixed points: ice point (0°C), tin point (231.9°C), zinc point (419.5°C).
- Thermocouple drift is permanent — regular calibration (annually) is mandatory for accuracy.
- Prism calibrates all standard and special thermocouple types: K, J, T, E, N, R, S, B.
Thermocouple Types and Temperature Ranges
| Type | Materials | Range | Common Use |
|---|---|---|---|
| Type K | Chromel / Alumel | -200°C to +1372°C | General purpose, furnaces, HVAC |
| Type J | Iron / Constantan | -40°C to +750°C | Old installations, vacuum, reducing atmospheres |
| Type T | Copper / Constantan | -270°C to +400°C | Cryogenic, food, pharmaceutical cold chain |
| Type E | Chromel / Constantan | -40°C to +900°C | High output, cryogenic to mid-range |
| Type N | Nicrosil / Nisil | 0°C to +1300°C | High-temperature stability, replaces K above 1000°C |
| Type R | Pt-13%Rh / Pt | 0°C to +1768°C | High accuracy, precious metals, glass |
| Type S | Pt-10%Rh / Pt | 0°C to +1768°C | Reference thermocouple, pharmaceutical kilns |
| Type B | Pt-30%Rh / Pt-6%Rh | +250°C to +1820°C | Very high temperature, smelting, ceramics |
Why Thermocouples Drift and Must Be Calibrated Regularly
Thermocouples drift due to changes in alloy composition caused by oxidation, contamination, and grain growth at high temperatures. Type K thermocouples used above 800°C can drift by +10°C to +20°C per year. In industrial furnaces (heat treatment, glass, ceramics), this drift leads to incorrect temperature control, product quality failures, and energy waste. Annual NABL calibration detects drift and allows correction.
Thermocouple Calibration Procedure at Prism
Reference Standard Preparation
Set up calibrated SPRT (for <660°C) or calibrated Type S thermocouple (for >600°C) as reference. Reference uncertainty: ±0.1°C.
Furnace/Bath Setup
Place reference and test thermocouples at same depth and adjacent positions in dry block calibrator, furnace, or stirred bath.
Stabilisation
Allow temperature to stabilize (±0.05°C for 5 minutes before reading).
Multi-Point Calibration
Record readings at 5–10 temperature points covering the thermocouple's application range.
EMF Measurement
Measure thermocouple EMF (in mV) with calibrated nanovoltmeter. Convert to temperature using ITS-90 reference tables.
Certificate Issue
Issue NABL certificate with as-found corrections, expanded uncertainty, and thermocouple tag number.
Frequently Asked Questions
How often should industrial thermocouples be calibrated?
Annual calibration is standard. Type K thermocouples used above 800°C in furnaces may need calibration every 6 months due to faster drift rates. Type R/S precision thermocouples used in reference applications need annual calibration and regular fixed-point verification.
Can a drifted thermocouple be corrected?
Yes. A thermocouple can be recalibrated to determine its current correction factors. These correction values can be programmed into the temperature controller/transmitter as a calibration correction table. However, heavily drifted or physically damaged thermocouples should be replaced.
What is the difference between thermocouple calibration and RTD calibration?
Both use comparison calibration method, but thermocouples generate an EMF (millivolts) measured with a nanovoltmeter, while RTDs have a resistance (ohms) measured with a precision resistance bridge. Thermocouples can cover wider temperature ranges but have higher uncertainty than RTDs at the same range.
Does Prism calibrate thermocouple extension wires?
Prism calibrates thermocouples as complete assemblies (thermocouple + extension wire + connector). For critical applications, extension wire type mismatch is a common source of error that calibration reveals.
What are MIMS (mineral insulated metal sheathed) thermocouple calibration options?
MIMS thermocouples (the industrial standard with stainless steel sheath) are calibrated at Prism by immersion in a dry block calibrator or furnace. The immersion depth must be ≥ 15× the sheath diameter for accurate calibration.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
Hours: Mon–Sat, 9:00 AM – 7:00 PM