Choosing the Right Thermocouple Type for Your Data Logging Application

A Practical Guide for ACR SRX6 Thermocouple Data Logger Users

Why Thermocouple Type Matters

Each thermocouple type is defined by the two dissimilar metals used to construct it, which in turn affect:

• Temperature range

• Stability and drift characteristics

• Sensitivity (mV output)

• Suitability for different environments (oxidizing, reducing, cryogenic, etc.)

Choosing the appropriate thermocouple type ensures better accuracy, longer service life, and better resolution with the SRX6 logger, which has a raw input range of approximately -5 mV to +70 mV.

Thermocouple Type Comparison

Type	Composition	Temp Range (°C)	Sensitivity (µV/°C)	Pros	Cons
J	Iron / Constantan	-210 to +760	~50	Low cost, good mid-range response	Susceptible to oxidation above 550°C
K	Chromel / Alumel	-200 to +1260	~41	Widely available, good general-purpose	Prone to drift in reducing atmospheres
T	Copper / Constantan	-270 to +400	~43	Stable and accurate in low temps	Limited upper range
E	Chromel / Constantan	-200 to +900	~68	Highest sensitivity among base metals	Slightly less common
S	Platinum Rhodium / Platinum	0 to +1450	~10	Excellent high-temp stability, chemically inert	Expensive, low output, requires cold-junction accuracy

(Note: Sensitivity values are approximate averages over moderate temperature ranges.)

Application Considerations

Type J

Good choice for general-purpose mid-range temperature applications such as HVAC, sterilization equipment, and food processing. Note the iron component is prone to rusting in moist environments.

Type K

A workhorse sensor used in metallurgy, kilns, and heat-treating due to its wide range. Slightly lower output means slightly reduced resolution on lower-end readings.

Type T

Ideal for cryogenic or low-temperature applications including freezers, refrigerated storage, and environmental chambers. Its higher mV output helps the SRX6 provide higher resolution at low temps.

Type E

High sensitivity makes this type beneficial when fine resolution is required, especially in low- to mid-range applications. Also performs well in inert atmospheres.

Type S

Typically reserved for high-temperature applications such as furnaces, incinerators, and semiconductor fabrication. Despite its low sensitivity, it offers long-term stability and low drift. Its cost and low voltage output make it suitable only when these characteristics are necessary.

Implications for ACR SRX6 Thermocouple Logger Users

ACR's SRX6 Thermocouple Logger applies a 16-bit ADC across a range of approximately 75 mV. Given that thermocouples typically produce up to 50 mV over their full range (often less in practice), selecting a type with higher sensitivity can significantly improve resolution in the target range.

For example:

• Type T’s ~43 µV/°C makes it very effective for fine resolution at low temperatures.

• Type E, with its high output (~68 µV/°C), can provide exceptional resolution, especially in mid-range environments.

However, this must be balanced with environmental considerations, durability, and calibration requirements.

Summary Table of Selection Guidelines

Application	Recommended Type	Rationale
Cryogenic Freezers	T	High output, low drift
General Industrial	K	Widely used, durable
High-Temperature Furnaces	S	Stability at >1000°C
Food Storage	T or J	Accuracy at low/mid temps
Fine Measurement Accuracy	E	Highest sensitivity

References

• NIST Thermocouple Reference Tables: https://its90.nist.gov

• ASTM Manual on the Use of Thermocouples in Temperature Measurement (4th Ed.)
  ASTM STP 470B, ISBN: 978-0-8031-1466-1