Sensor correction

Sometimes a sensor such as a thermocouple cannot be fitted precisely at the desired measuring point but must be placed nearby. Most modern programmable controllers allow the operator to insert a correcting value to represent an estimated or known difference in temperature between the point of installation and the point where the temperature is wished to be known.

The amount of correction or offset needed to be inserted is small and that accepted by a controller is usually limited to less than 100 degrees;more usually  only 10 degrees or so.

The sensor correction function finds application in places such as oil refineries and chemical processing plants, where operations run continuously at constant temperatures. With all controllers harmonised, alarms can be set to trigger when minor deviations occur.

It should be understood that the inserted value applies to all readings across the full range of the instrument; akin to offsetting the zero on kitchen scales to compensate for the weight of a bowl.

At first glance this could be considered ideal for a glass kiln; the glass temperature is sought and the thermocouple cannot accurately measure it, so let’s offset the PV indication to allow for the different positions of glass and thermocouple.

Unfortunately, the situation in a kiln is rather different to that in the processing plant. In a kiln, the difference between the glass temperature and the PV reading on the controller will vary considerably during the course of a firing.

As a result, the entering of a particular sensor correction or offset value into the controller may reduce the error at one point in the firing but increase the error at others. The reading at room temperature will always be at error by the amount of the offset.

Kiln builders are able to activate this function when configuring a controller. That they generally choose not to do so is because it’s use can only lead to confusion and is likely to produce less rather than more success when kilnforming.

Skutt is an exception to this practice. In their ‘Operating Manual for Glass Kilns’, on page 19 they give instructions for using their TCOS (thermocouple offset) feature to " calibrate the thermocouple when it is reading consistently and predictably incorrect". (the output of a thermocouple can be changed only by changing the composition of one or both of the pair of wires)

Oh to be able to peer into a kiln and tell the temperature to within a few degrees. What an ability.

This instruction is accompanied by a caution to consult with a Skutt technician before doing so to avoid the risk of permanent damage to the kiln. In reality, damage to the kiln is less likely than damage to the glass.

Most modern instruments have this capability buried somewhere inside. With good reason, most kiln builders keep it buried