Thermocouples & pyrometers

Thermocouple

A thermocouple consists of two wires of dissimilar material which are joined together at one end, called the hot junction. 

When the hot junction is heated, a small voltage will appear across the other ends of the wires, called the cold junction.

The higher the temperature difference the higher the voltage developed.

The wires vary from readily available materials such as iron or copper to very expensive alloys such as platinum/rhodium. The pairs are selected to give millivolt outputs to meet particular temperature ranges and are each designated by a letter identifier, such as type K, R, N, etc.

Type K is the most widely used in industry and is preferred for kilnforming. Type N is now preferred for pottery kilns, having supplanted the platinum based type R because of cost. However, type R is still preferred for glass crucible furnaces because they can exceed 1300°C and failure of the thermocouple could be very costly if a crucible were lost.

Fig 50-01.

At its most basic a thermocouple may comprise the two wires, welded together at one end, threaded through ceramic insulators and terminated with a terminal block, as in Fig 1.

Fig 50-02

To provide additional protection against mechanical damage or harmful atmospheres, the thermocouple is often put inside some form of sheath which can be high temperature metal alloys or ceramic.

For glass kilns a 6mm diameter metal sheathed type K thermocouple as in Fig 2 is ideal. They can be straight or bent at any reasonable point and fitted with plastic or woven wire screened extension cable of whatever length.

Pyrometer

A pyrometer consists of a thermocouple connected to an indicating device, calibrated in degrees C or F.

An analogue indicator (one with a scale and pointer), will have a scale calibrated to a particular type of thermocouple and MUST NOT be used with any other type without being re-calibrated or adjusted.

Produced voltage drives the indicator and as such the required thermocouple length will be specified. Varying this length will vary the voltage drop along the lead, thus altering the voltage at the cold junction and the accuracy of the indication.

Some battery powered free standing pyrometers don’t draw power from the thermocouple, but use the battery power to generate a voltage opposing the thermocouple output. When both voltages are equal the generated voltage is measured and displayed as a temperature value. No current flows in the T/C lead so its length can vary widely without affecting calibration.

Powered digital controllers operate in the same manner, so for them the T/C or extension lead length is not critical.

Many modern digital controllers contain the data for a wide range of sensor input types and can be configured to use any of them; so there is no need to order different instruments for different thermocouple types.

However, it is important that the instrument is correctly set for the thermocouple type being connected; to not do so could result in costly damage.

As an example, an instrument calibrated for a type K thermocouple, if connected to a type R couple will show 450ºC when the kiln temperature is actually about 1650ºC.

I have seen the outcome of this actual mis-configuration and oh! what a mess.

Fortunately for kilnformers, type K is the default input type set into most instruments; but be sure and confirm this.

Thermocouple polarity

A thermocouple has a positive and a negative terminal, just like a battery. When connected positive to the positive on the indicator the instrument will show a rise as the thermocouple hot junction is heated. If the indicator goes the wrong way, reverse the connections at ONE end; preferably at the thermocouple end, because that gets disturbed less frequently.

Extension cables

Widely varying distances between measuring point and indicator, and the high cost of some of the wires, has led to a range of insulated cable pairs being available for use with the more common thermocouple types.

Each wire is given a colour code to denote polarity and the outer sheath another colour to denote type. However, because of the wide range of couple types, and of conflicting national standards, one should not make assumptions about what the colours denote, but should refer to the literature and the supplier. 

Extension cables are intended to be used in situations of low temperature so their insulation and ratings differ from that used for T/C cables. These range from 105°C (221°F) for PVC through 260°C (500°F) for Teflon to 480°C (890°F)  for fibreglass.

Because extension cables can be run adjacent to power cables, with a consequent risk of induced stray voltages affecting instrument performance, most extension cable has some form of in-built screening.

This can be either a metal foil inside the outer sheath or a woven wire covering on the outside.

It is important that this screen is earthed at some point, but, it must not be earthed at more than one point. Should a fault develop in the power system, a defective earth could cause the full fault current to flow through the screen, with dire consequences for the instruments.

Using extension cable as a thermocouple

Fig 50-3

Despite the temperature limits for extension cable mentioned above, fibreglass insulated type K extension lead is frequently used as a T/C material.

Whilst the accuracy is not as high as for T/C wire, using extension lead in the manner shown in Fig 3 overcomes this problem.

In this case the test was to measure the variation in heat absorbency between clear and black glass during a firing.

Both the pieces of cable were cut from the same coil, temperatures were both measured on the same instrument, so any difference in readings must be due to difference in temperature measured.

The insulation is stripped back and the hot junction is formed by welding the two ends together. The heat causes the fibreglass to break down but it retains its insulating property until dislodged by moving. 

This is definitely a once-off use, but the cable is cheap and the process is accurate.