Thermocouple offest with resistor
Thermocouple offest with resistor
(OP)
I have a control system that only works between 70 and 100 degrees F. I want to put an inline resistor in the thremocouple leads that is variable to change the offest of the thermocouple reading so I can make my process operate in a higher range.
Any suggestions on what to use to do this? Sizing?
thanks
Dave
Any suggestions on what to use to do this? Sizing?
thanks
Dave





RE: Thermocouple offest with resistor
Suggest that a new control system be pursued if it is feasible. If it can be done with an off the shelf PID system, it's only a couple hundred bucks. But that is a guess since there are no details on the control system needs or specs.
Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
THanks again
Dave
RE: Thermocouple offest with resistor
A thermocouple generates a small voltage (millivolts) and this can certainly be offset by another small voltage. Thermocouple simulators are simply resistive dividers from a voltage source. Typically the thermocouple is screwed or plugged into a connector, the temperature of which determines the "cold junction". Cold junction compensation is applied by measuring the temperature of this block. If you split one of the thermocouple leads you get an X-to-copper then copper-to-X pair of junctions, where X is the thermocouple material in one lead. Provided these two junctions are at the same temperature you do not create any thermal offset.
If you knew that one thermocouple lead was grounded at the amplifier you could put the back-off resistor in this lead and use a resistor up to the power rail to create the offset. The backoff resistor would want to be around 10 ohms, for example. The power rail resistor would be large enough to give just the right amount of backoff, perhaps a few millivolts. If both thermocouple inputs are relatively floating then you could still use the same backoff resistor but using a floating power supply such as a small Lithium battery to provide the offset. Thermocouple systems are usually not complicated and the signal bandwidth is often so low that you do not have to worry unduly about noise.
However, buying a new controller is still good advice
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
The reason you cannot do this is the resistor value
would depend entirely on the input impedance of the
thermocouple amplifier.
If you knew this was a linear resistive input
impedance you could calculate a resistor but it
is unlikely to be linear resistive.
more trees less bushes
RE: Thermocouple offest with resistor
Just sticking a series resistor in the line is a sin and a crime against nature. We all pay for our sins, one way or another in the end.
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
Seems adding a resistor is just making a quick fix that will probably do more harm than good and not really solving the issue at hand.
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
RE: Thermocouple offest with resistor
I hope your burns heal quickly from that 188 degree water bath! You can reduce your expenditures and help your buget by turning down your water heater. Hope ya find the humor in this!
Scott
In a hundred years, it isn't going to matter anyway.