Steve101 Posted February 18, 2015 Report Share Posted February 18, 2015 Hello, I am setting up temperature control application. Controlling a furnace using V350-35-TR20 (PID Control), thermocouple type K (Omega KQXL-116G-9 M ) input to TC module IO-ATC8. Have problem with temperature accuracy. TC reads correctly when no power is being applied to furnace, as soon as power is applied to heating elements temperature/process value jumps dramatically, the higher the temp the further off it is, up to 100°. when power is removed temp drops back to being accurate as verified against other controllers. I am using type k shielded cable. Have tried grounding shielding at either end of cable. Have many furnaces wired in similar fashion to stand alone temperature controllers without this problem. Any Thoughts? Quote Link to comment Share on other sites More sharing options...
Michael_P Posted February 18, 2015 Report Share Posted February 18, 2015 Hi Steve, I have seen similar conditions in the past, This sounds to be a ground condition. Please ensure the TC is connected to ground at the signal source. Please ensure the furnace is connected to ground as well. In the past I have seen a similar condition with a TC in a motor case, after further examination there was a ground strap that was loose, causing the condition. It may be helpful to test with a different TC to confirm it is the field wiring and not the hardware. Thank you Quote Link to comment Share on other sites More sharing options...
Walkerok Posted February 18, 2015 Report Share Posted February 18, 2015 Hi Steve, The way you phrased your original question it sounds like you are using an electric oven. If electric, I assume that there are SCRs that control the temperature of the oven. The SCRs do generate noise as they switch on and off and it is possible for the noise created to put in some amount of voltage into the earth ground of the entire oven. Is your thermocouple a grounded or ungrounded type (if it is grounded change to an ungrounded type and this will probably have a positive effect)?. It is very possible that no amount of shielding and grounding of drain wires will help this condition because the source of the noise is the threads of how you secure a connection into your oven. (I am guessing that you have a metal connector of some sort at the thermocouple end). If possible .... to test this, suspend the thermocouple in the oven without making any grounded connection (don't let the thermocouple/sheath touch anything metal). If your issue goes away then the noise is in the ground of the oven and you need to make your thermocouple connection with a non conductive material (ceramic) and this will fix the problem. The better your oven is grounded the less significant the noise will become, but there are situations where the overall noise can not be brought to zero and electronic filtering should be employed. The historical manifestation I have seen with noise being on the ground of the equipment is as you describe it. It is not necessarily a quickly rising/falling signal but can be just an overall shift either up or down when power is provided to the SCR electric element controller. The better you ground the smaller the shift will be in magnitude. Good Luck Keith Quote Link to comment Share on other sites More sharing options...
Steve101 Posted February 19, 2015 Author Report Share Posted February 19, 2015 Michael, Keith, Yes, you were both right, the furnace was not grounded properly. After grounding furnace better problem went away. Thanks for the help, Good Info. Steve Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.