Corrective Action (continued)
The thermocoupIe may not be touching the bottom of the protection tube. This will result in an air gap between the thermocouple and the protection tube and will tend to insulate it somewhat. Thermocouples should be checked to insure that they are not close to a heating element nor too remote from the load.
Care should be taken in relocating thermocouples that have been installed by an equipment manufacturer. The manufacturer may have a specific reason for placing the thermocouple in the installed location.
Stray Signal Pickup
The indicator may go upscale or downscale when power is turned on to the process. First, insure that this is not a TCB offset as may occur in millivoltmeter type instruments.
Check for, and remove, all grounds except any that may occur at the thermocouple hot junction. A check should be made to insure that the thermocouple leadwire is not run in the same conduit with power wiring. If possible, the source of the stray pickup should be isolated from the thermocouple and its leadwire.
If it is found that the stray pickup cannot be isolated from the thermocouple leadwire or thermocouple, a 2 mf capacitor may be used between the thermocouple, either plus or minus and ground. It may also be found necessary to place a 2 mf capacitor between plus and minus. These capacitors should be located at the instrument terminals.
In the case of d-c voltage pickup in the thermocouple or its leadwire, all grounds should be removed from the thermocouple circuit and then the thermocouple itself grounded to eliminate the effects of this d-c pickup.
Barber-Colman Company, Loves Park, IL.