I finally received the two Platinum Resistance Thermometers I had ordered:
http://www.ebay.com.au/itm/400244691889$8 each including postage. They claimed to be Grade A. They have a 1000 ohm resistance at 0 °C and 1384 ohms at 100 °C with a linear slope in between. The curve of platinum is not exactly linear but close - the exact formula is at the end of this post. The sensor is 30mm long and 5mm diameter. Sealed with epoxy, so not useful for extremely high temperatures. Should be OK up to 200 °C at least, but I am mainly interested in under 100 °C temperatures. Platinum sensors are usually available at 100 and 1000 ohms, and the 1000 ohms are a bit easier to use with 2 wire resistance measurements.
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5775.0;attach=18384)
So how good are they?
The definition of Grade A is
Tolerance = ±[0.13 +0.0017 *|t|] °C . (With t measured in Centigrade)
At room temperature, the two matched resistance to 0.02%. That corresponds to a temperature difference reading between the sensors of 0.05 °C. Not bad.
I then checked the 0 Deg °C. The water temperature of a glass full of chilled water and ice is a very accurate 0 °C and it is not affected by factors such as air pressure. I was using rain water (non urban - almost no dissolved minerals or pollutants) so the water temperature between the ice should be 0 °C to an accuracy better then 0.01 °C.
Both sensors measured -0.02 °C. Well within the Grade A spec of +/- 0.13 °C.
Boiling point is very hard to check accurately, so I will not bother checking that. For one thing, I am at 1100 meters altitude, so our boiling point is well below 100 °C anyway.
I have to say I am extremely impressed. Looks like they are very comfortably within the Grade A specs, so for $8 a real bargain. They are perfect for use with my standard resistors, and I have also received my metal film resistors that I was wanting to do temperature and stability tests with. At 100mV test voltage, the self-heating is not a problem (much less then 1 °C error) , but if I want to be paranoid, I can test at 10mV which is 0.1uW of heating.
Richard
Edit: If you want the true Temp Curve:
ASTM Standards E 1137 for Industrial Platinum Resistance Thermometers specifies that the resistance-temperature relationship for such devices for the range 0 °C to 650°C, to within the tolerance mentioned above, will be described by the equation:
R(t) = R(0)[1 + At +Bt
2]
Where:
t = temperature (to ITS-90), °C,
R(t) = resistance at temperature t, (= 1000 ohms for my sensors)
R(0) = resistance at 0°C
A = 3.9083 * 10
-3(°C), and,
B = -5.775 * 10
-7(°C
2).
The European DIN standard that has now become IEC Standard 60751 uses slightly different numbers, but the end result is basically the same. For a fourth order accurate curve, see this document:
http://www.uniteksys.com/Graphics/CalVan.pdf