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TC-08 USB

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TC-08 USB

Postby Stuby6565 » Wed Aug 26, 2015 9:30 pm

We have three TC-08 usb dataloggers running, each has several thermocouples connected and one module each for mv logging from a CT with mv output. We use Transformers to create current flow in each of the three loops and thus monitor current with the mv signals and temperature at intervals eleswhere on the three seperate loops. We use multi channel session logging and have done so for many years. I now have an unfamilar temperature signal when we switch in the third Transformer and input 300 amps, the temperature on the third TC08 drops by 10 degrees, when the transformer is on its of cycle everything jumps back up? The other two loggers work with no issue during there seperate on/off cycles. I have tried changing channels, thermocouples, tc-08, mv modules to no availe? It only occurs when the third transformer kicks in? The transformers are controlled by a refernence thermocouple on each loop, one of the thermocouples is in contact with the cable core as it needs to measure the core temperature, this goes through the Tc08, the transformer thermocouple is in contact with the sheath and is fully insulated. I believe the contollers, pc, transformers and TC08 are on the same earth loop. So before i switch over the transformer controller is there anything else I may need to consider? Any pointers would be appreciated.I have done this many times before with no issue, unless there is an earthing issue I cant think what is causing this.
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Re: TC-08 USB

Postby Martyn » Fri Aug 28, 2015 8:31 am

You appear to have covered most possibilities. The only remaining one is that when you switch in the third transformer it is inducing a signal on the wires, taking the device outside the common mode operating volatge (+/-7V).

Try carefully routing the wires to see if this helps.
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Re: TC-08 USB

Postby Stuby6565 » Sat Aug 29, 2015 9:37 pm

Voltage less than 5v, all three loops set up exactly the same on seperate transformers and controllers to measure three seperate loops. I have disconnected the controller thermocouple and all works well, as soon as i reconnect it alls not well, controller thermocouple switches in transformers to generate current and thus heat, this is connected to the core, a seperate thermocouple is also connected to core and thus the tc08 for logging. I think i must have a loop feedback or noise coming through from somewhere between the controller and TC08, not something we have come across before in the many years we have been doing this, never even seen it on the old Rs232 versions which we still use when workload needs arise. The only other thing it may be is the phasing the transformers are running on and if the PC/ loggers are on one of the same feeds, I'll check this next week when loop is on a cool cycle. Thanks for the reply, it started me thinking about something else.
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Re: TC-08 USB

Postby Martyn » Wed Sep 02, 2015 2:55 pm

Even though a voltage measurement is less than 5 volts, it can be possible for noise to be picked up by the wiring that adds a common mode voltage which is outside the operating range of the device.
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