Grounded or Ungrounded Thermocouple ?
One question that has to be answered for any thermal system that uses a thermocouple is whether to use a grounded or an ungrounded sensor. This seems to be an area of some confusion so here are some points to consider when you are faced with this situation.
Let's look at the construction of the two types: The grounded junction is formed with the sensor alloys welded to the sheath at the junction. The actual grounding occurs when the sensor is mounted to the vessel with a fitting. The ungrounded junction is formed by welding the sensor alloys together and insulating the junction from the thermocouple sheath.
Thermal response, reliability, grounding problems, and cost all are factors to consider.
Grounded junctions typically transfer heat 40% faster than ungrounded. If your process is relatively stable this may not be an issue. However, in a rapidly changing system this response time difference can be critical.
Reliability between grounded and ungrounded sensors is very similar except at elevated temperatures. As you approach 2000F grounded junctions are more affected by thermal stress due to the expansion rates of the different materials. This can lead to premature failure particularly in rapid thermal cycle systems.
The ungrounded thermocouple is the easiest way to avoid ground loop problems. Ground loops occur when the sensor is grounded and the instrument being used does not have an isolated input. The ground loop provides an unwanted conductive path from the sensor which the instrument reads as the sensor input. An ungrounded sensor is very effective at reducing grounding problems, but care should be taken at elevated temperatures to be sure that the system is truly isolated.
Ungrounded sensors are more difficult to manufacture and are generally $2 to$20 more expensive than grounded units of equal diameter. Very small diameter ungrounded sensors require specialized equipment and training to produce properly. However, the elimination of grounding problems may be well worth the cost.
If you have questions or problems please contact us at 800-728-2800. We have been working on thermal systems for the last thirty years and may have some ideas that will help you out.
For More Information Contact:
James H. Laas Company, Inc.
Tel: 800-728-2800
FAX: 563-359-3766
Email: info@jhlaas.com
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