[time-nuts] Environmental sensor recommendations.

[Dr. David Kirkby]

On 5 April 2018 at 15:44, John Green <wpxs472 at gmail.com> wrote:

> Why has no one mentioned thermocouples?
> I had some experience with thermistors a few years back designing thermal
> attenuators and equalizers for CATV. NTC thermistors can have a large
> change of resistance for a unit change in temperature. They aren't linear,
> but there are formulas for computing resistance vs temp. PTC thermistors
> have a much smaller change per unit change in temp., but are much more
> linear. And, they are susceptible to self heating, which makes things
> interesting. If I remember correctly, in my research something called an
> RTD was supposed to be the king when it came to accuracy and repeatability.
> As someone else has stated, the IC devices are supposed to be quite good,
> but you have to interface with them.
>


Keysights nano volt / micro ohm meter takes

https://www.keysight.com/en/pd-1000001296%3Aepsg%3Apro-pn-34420A/micro-ohm-meter?cc=ZA&lc=eng

can do direct SPRT, RTD, Thermistor, and Thermocouple measurements

SPRT = Standard Platinum Resistance Thermometer

I don't claim to know much about this, but the uncertainty quoted for the
SPRT probes with that meter is 0.003 deg C.

When I Google SPRT probes, I see they are incredibly expensive - many
thousands of USD each.

I'm a bit puzzled there seem to be a number of 3-wire platinum resistance
thermometers. I can understand using 4-wires for a Kelvin connection, but
can't understand the use of 3-wires.

Of course, for environmental monitor in a lab, one is most unlikely to need
very high accuracy.

Dave