Thermocouples – An Introduction

What is a thermocouple sensor?

A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire.

What are the different thermocouple types?

A thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. There are high temperature calibrations R, S, C and GB. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Although the thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. That is, a very thin thermocouple may not reach the full temperature range. Click here to view a complete reference table for each thermocouple. The table includes international color codes for thermocouple alloys, temperature range and limits of error for almost every kind of thermocouple.

How do I choose a thermocouple type?

Because a thermocouple measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry. The following criteria are used in selecting a thermocouple:

· Temperature range

· Chemical resistance of the thermocouple or sheath material

· Abrasion and vibration resistance

· Installation requirements (may need to be compatible with existing equipment;  existing holes may determine probe diameter)

How do I know which junction type to choose?

Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed (see graphic below:”Thermocouple Tip Styles”). At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation (see table below).

Room-Temperature Insulation Resistance
Ungrounded Thermocouple

Nominal Sheath Diameter

Applied dc Voltage min. Insulation Resistance min
Less than 0.90mm (0.03in.) 50V 100M Ohms
0.80-1.5mm (0.030 to 0.059in.) 50V 500M Ohms
Greater than 1.5mm (0.059in.) 500V 1000M Ohms

The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to dry, noncorrosive and nonpressurized applications.

What is response time?

A time constant has been defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple is the fastest responding. Also, the smaller the probe sheath diameter, the faster the response, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.

Application

Atmosphere

Material

Maximum Temperature

Oxidizing

Hydrogen

Vacuum

Inert

304 SS

900°C (1650°F)

Very Good

Good

Very Good

Very Good

Inconel 600

1148°C (2100°F)

Very Good

Good

Very Good

Very Good

Thermocouple Ranges and Limits of Error

The table below shows temperature ranges and accuracy for J, K, E and T thermocouples. Click here for additional thermocouple types.

clip_image002[1]

Thermocouples Samples

2705352_08-general_thermocoupleimages (2)

Thread-Mounted-Thermocoupleimages

thermocouple_headimages (1)

images (1)images (4)

Comments are closed.

%d bloggers like this: