Conduction error is a common problem for small application contact temperature sensors.
Temperature sensors can be very accurate devices. However sometimes poor probe design or bad installation can easily give a system significant temperature reading errors. A typical cause of conduction error is inadequate immersion into the application.
Sensors usually need protection sheaths and always use signal cables. Both items will conduct heat away from a system. Probe leads and sheath will pass through a thermal gradient at some point. If the thermal gradient is close to the measuring point, then the sensor will be influenced to some degree by ambient conditions. The signal will be low in hot environments and high in cryogenic applications.
Having said that, we sell to many applications where a sensor is surface mounted or is very short. Some error will be inevitable. If absolute accuracy is not demanded by a process and the application conditions do not vary much day to day, some engineers will not worry about conduction error. If you are measuring a potential fault condition and just need to know if it is significantly hotter than yesterday then conduction error is not a problem.
Reducing Conduction Error
Occasionally surface or low immersion temperatures need to be more accurate. Conduction error cannot be avoided, but great sensor design can reduce the error. Strategies include:
Thin signal wires in higher resistance materials such as Nickel (Not suitable for thermocouples)
Thinning any metallic outer sheath as much as possible
Designing a thermal insulator section in the sheath
Using insulation blanket inside a sensor assembly
Place lagging around the sensor sheath and leads
Maximising thermal contact area with the application
There is a compromise between immersion and conduction error. There is no absolute definition or calculation for adequate immersion. As a rule of thumb an immersion of 10 times sheath diameter should usually be adequate. There is no substitute however for application experimentation if conduction error is to be minimised while also minimising immersion.
We hope you enjoyed reading about the causes of conduction error. Our team have great knowledge of this subject and take error into consideration when designing our highly accurate probes. If you are looking for custom built probes with high accuracy, then look no further than Peak Sensors.
Peak Sensors Ltd,