Thermocouple calibrators - what's inside?

[peranders]

How is a thermocouple calibrator designed?

Do I need more than a accurate voltage? Do I have to know any reference temperature? Is it senistive with choice of metals in conductors?

[Netlist]

Hi, Peranders

Could this help you?
http://www.google.be/search?hl=nl&ie...+made+of&meta=

/Hugo

[jackinnj]

used to calibrate things like laboratory voltmeters, etc. -- made by Fluke, Keithley, HP.

[ALW]

Peranders,

Good question - assuming you want a thermocouple emulator : -

You would need an accurate voltage, that corelates accurately with the Seebeck coefficient of the thermocouple you need to emulate.

This coefficient is non-linear, so you need to calibrate at each temp emulation setting.

Some thermocouple amps have DC present at their inputs and need the thermocouple DC resistance present to work, so the o/p impedance and a DC current path similar to that of a thermocouple needs to be present too.

The output should use a standard thermocouple plug / socket.

I've got one at work, I haven't been able to get it apart yet

Andy

[roddyama]

Hi Peranders,

The output of a thermocouple is nonlinear and the output voltages (<100mV) vary with different thermocouple types. Therefore a calibrator is not a simple matter. Here is a link to the Omega technical reference page. There is a wealth of information on all sorts of temperature measurement devices available here.
http://www.omega.com/temperature/Z/zsection.asp

Originally posted by peranders
How is a thermocouple calibrator designed?

Do I need more than a accurate voltage? Do I have to know any reference temperature? Is it senistive with choice of metals in conductors?

The direct answers to your questions are:
Yes, you will need to assign a linearized temperature to a non-linear output from the thermocouple.
Yes, sort of, as you will need to have a lookup table each type of thermocouple. The table will provide the volt-to-degrees conversion values
Yes, each different bi-metal pair has a different output and V/T curve.

Hope this helps.
Rodd Yamashita

[peranders]

I think you missunderstood me. I ment the calibrator itself, the electronics. The calibrator is used for calibrating thermocoupling amplifiers.

[roddyama]

Originally posted by peranders
I think you missunderstood me. I ment the calibrator itself, the electronics. The calibrator is used for calibrating thermocoupling amplifiers.

Hi Peranders,

I'm not sure what a "thermocoupling amplifier" is. Hopefully, we are talking about the same thing.

The thing is that it’s not the thermocouple that gets calibrated, it’s the device or unit that reads the voltage the thermocouple is putting out as it varies with temperature. The thermocouple itself cannot be altered so there is no way to “calibrate” it.

There are two ways that thermocouple measuring devices are “calibrated” in the field or in the lab so there are two main type of calibrating devices. The first, as Andy alluded to, is the TC emulator or simulator. It is used in place of the thermocouple in question by emulating the voltage of various types of thermocouples at selected temperatures. An accurate voltage that represents the output of the thermocouple at a specified temperature is generated and input to the measuring unit. The measuring unit is then calibrated based on this simulated thermocouple. If you were to make one of these, you would have to have the voltage-to-temperature tables at hand and be able to generate the voltage that is equivalent to the thermocouples output at the selected temperature.

The second type of “calibrator” is really just an accurate measuring unit with an accurate temperature “reference” to correlate to. The “calibrator” will measure the voltage from the thermocouple which is in proximity to a reference temperature measuring device, convert that voltage to a number with temperature units, and compare that temperature to that of the reference. The reference can be a thermometer in an ice bath (zero degrees C), or it can be a measuring device of higher accuracy then the typical thermocouple such as an RTD (Resistive Temperature Device). Of course, the reference has to give you a reading of temperature. This type of device would be used to calibrate the TC simulators described above.

Rodd Yamashita

[peranders]

Originally posted by roddyama

Hi Peranders,

I'm not sure what a &quot;thermocoupling amplifier&quot; is. Hopefully, we are talking about the same thing.

The thing is that it’s not the thermocouple that gets calibrated, it’s the device or unit that reads the voltage the thermocouple is putting out as it varies with temperature. The thermocouple itself cannot be altered so there is no way to “calibrate” it.

There are two ways that thermocouple measuring devices are “calibrated” in the field or in the lab so there are two main type of calibrating devices. The first, as Andy alluded to, is the TC emulator or simulator. It is used in place of the thermocouple in question by emulating the voltage of various types of thermocouples at selected temperatures. An accurate voltage that represents the output of the thermocouple at a specified temperature is generated and input to the measuring unit. The measuring unit is then calibrated based on this simulated thermocouple. If you were to make one of these, you would have to have the voltage-to-temperature tables at hand and be able to generate the voltage that is equivalent to the thermocouples output at the selected temperature.

The second type of “calibrator” is really just an accurate measuring unit with an accurate temperature “reference” to correlate to. The “calibrator” will measure the voltage from the thermocouple which is in proximity to a reference temperature measuring device, convert that voltage to a number with temperature units, and compare that temperature to that of the reference. The reference can be a thermometer in an ice bath (zero degrees C), or it can be a measuring device of higher accuracy then the typical thermocouple such as an RTD (Resistive Temperature Device). Of course, the reference has to give you a reading of temperature. This type of device would be used to calibrate the TC simulators described above.

Rodd Yamashita

What I mean is: Is it possible to make a Fluke 714 a little bit cheaper than the Fluke?

The calibrator is used for calibrate tc inputs.

[fmak]

Originally posted by peranders
How is a thermocouple calibrator designed?

Do I need more than a accurate voltage? Do I have to know any reference temperature? Is it senistive with choice of metals in conductors?

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Easiest way is to use a certified mercury thermometer (of the Beckman type, very long). Othwise you can use boiling point and freezing point of different substances, water, tin, lead etc.

[peranders]

Originally posted by fmak

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Easiest way is to use a certified mercury thermometer (of the Beckman type, very long). Othwise you can use boiling point and freezing point of different substances, water, tin, lead etc.

I don't want to calibrate with temperature rather with an electronic devise.