A thermocouple is said to be a ‘’simple’’ temperature measurement device. With a difference in temperature between its cold junction and hot junction, you have a voltage reading that gives you an indication of the temperature being measured. But is this really a simple device?
Well a detailed study of the thermocouple will reveal that it is not really a simple device as it seems. From the extension cables that could serve as antenna to pick up stray voltages, to ground loop problems, to advanced signal conditioning modules applied to curb noise and composition challenges in the thermocouple material, it does not appear to be a simple device in the real sense. However, the thermocouple is still a preferred device when it comes to measuring high temperatures and in certain applications where an RTD is not suitable.
How then do we use a thermocouple device successfully? Here I have itemized some practical application tips that help in making the thermocouple a success in its application:
Always examine thermocouple manufacturers’ specifications for conformity to standards, specified temperature ranges, and interchangeability. These are the basics of its application.
Reproducibility and interchangeability between brands of thermocouple should be examined. Errors due to thermocouple replacement are common in industry and should be avoided.
Ground loops are a common problem in thermocouple applications. Use isolated signal conditioning modules to avoid ground loops.
Tip 4 :
Always use thermocouple signal conditioning modules with appropriate input filtering. This has been shown to reduce or eliminate serious noise errors.
Each thermocouple wire connected to the sensing module must be at the same temperature. Module connectors should have no thermal gradients(temperature differences) across the individual connections.
Thermocouple behaviour depends on the materials’ molecular structure. Environmental conditions such as stress, chemical corrosion, radiation, etc that affect molecular structure anywhere along the length of the thermocouple wire can create measurement errors.
Use twisted pair extension wires and signal conditioning modules with adequate filtering to help avoid EMI (Electromagnetic Interference)and RFI (Radio Frequency Interference) errors.
Keep thermocouple lead lengths as short as possible.
Use the manufacturer’s recommended extension wires if long thermocouple leads are necessary.
Always observe colour code polarity. Also be aware that European manufacturers and North American manufacturers used opposite color code polarity conventions. Be sure to note the difference and take appropriate steps to avoid any polarity error due to this difference.
Avoid ‘’heat shunts’’ when installing thermocouples. Any heat conducting material, like large lead wires, may shunt heat away from the thermocouple, creating measurement errors.
Hostile corrosive environments combined with moisture and heat may cause corrosion, which can stimulate galvanic action and create electrochemical voltage errors. Consider the operating environment before deciding on the type of thermocouple to use.
Temperature measurement response time is significantly impacted by the thermocouple package encapsulation. e.g. thermocouples in a thermal well have a slow response time, which may cause undesirable hunting in a control loop. Determine whether the response time is critical for your application before deciding to use an encapsulated thermocouple package.
Endeavor to use signal conditioning modules with electronic CJC (Cold Junction Compensation) techniques that use temperature-sensing devices, which have thermal response times equivalent to that of the measurement thermocouples.
An exposed junction thermocouple is recommended for the measurement of flowing or static non-corrosive gas temperature when the greatest sensitivity and quickest response is required. In this type, measuring wires are unprotected and the response time is very fast.
An Ungrounded (insulated) junction thermocouple is more suitable for corrosive media although the thermal response is slower due to the air gap between junction and outer sheath. This type of construction provides best protection to thermocouple wires and it is electronically isolated construction. In some applications, where more than one thermocouple connects to the associated instrumentation, insulation may be essential to avoid spurious signals occurring in the measuring circuits.
An earthed (grounded) junction thermocouple is suitable for corrosive media and for high-pressure applications. It provides faster response than the insulated junction and protection not afforded by the exposed junction thermocouple.