What kind of wire is this?

[grant fair]

I would like to make a Schmermund RTD thermometer, as described Scientific American's Amateur Scientist. See:

http://sciam.com/print_version.cfm?arti ... 9EC588EF21

In that article, the "stem wires" connecting the RTD to the outside world are described as nickel.

However, the inventor of the thermometer makes and sells these on Ebay, where he states:

"Stem wires that connect the detector to the hookup wires at the handle can be another source of thermal leakage. The stem wires used in the Schmermund Thermometer are 100 microns in diameter, (the thickness of a human hair), and of an alloy with very low thermoelectric properties for connection at both ends."

Does anyone know what this alloy is? Where could I buy a small quantity of this (30 ga)?

Thanks,
Grant

[Chris Smith]

http://www.google.com/search?sourceid=n ... hermo+wire

[grant fair]

Thanks, Chris-

The thermometer uses a platinum RTD (resistance temperature detector) as the temperature sensor. So I am not looking for thermocouple wire per se. Of course, any two different metals in contact with each other will generate a small voltage. I think what the inventor means is that the stem cell wire generates a fairly low thermoelectric voltage when it is touch with platinum. I have googled around but have not found what materials would have this characteristic when used with platinum. I was hoping someone could take one look at the Ebay description and know from that what he was using.

I wondered if it was Invar, a nickel alloy which expands very little when heated. It also can be welded easily. Schmermund spot welds the RTD leads to the "stem wires", so this might mean it is Invar. But I don't know about it's reactivity in relation to platinum.

So I am still wondering.

Grant

[Chris Smith]

My thermo company told me to “neverâ€

[rshayes]

Thermoelectric voltages can be avoided by using an AC bridge to measure the resistance of the RTD. Alternately, the measurements can be made with positive and negative polarity and the two measurements averaged. Using a four terminal technique, as described in the Scientific American article, will remove the effects of the resistance of the connecting leads. This will allow using high resistance wire of small diameter to avoid conducting heat through the leads.

One type of bridge circuit which uses a four terminal connection for accurate measurement of low resistances is the Kelvin Bridge or sometimes the Kelvin Double Bridge.

Power may have to be applied in short bursts (possibly milliseconds) to avoid self heating of the RTD.

Omega makes this type of sensor and probably has an extensive application section in their catalog devoted to it.

[grant fair]

Hi rshayes -

Since I don't have an ohmmeter over 4.5 digits, the bridge will likely be the way to go. Thanks for answering some concerns/questions which have arisen.

The inventor specifies an Omega RTD with a resistance of 1000 ohms (at the usual temp for specifying RTD's). He suggests a maximum current of 1/2 ma.

I will have to read about the bridges you identify and see what additional questions I have. Thanks again.

Grant