[time-nuts] temperature

Neville Michie namichie at gmail.com
Sun Dec 8 22:55:18 EST 2013


Hi,
I use a HP3468A multimeter to measure a PT100 platinum resistance thermometer. It gives me resolution of one mK, but calibration is another matter.
It is best to use a 4 terminal device, but 2 terminal into the 4 terminal input works well. Thermoelectric effects and the requirement for 1 microvolt stability
makes wiring them into your own circuit difficult. One of the great technical difficulties is to get a resistor to compare them against, it must be very stable,
have no thermoelectric effects and have a temperature coefficient in the order of one PPM. I always admire the way HP designed their ohm meters.
There are other issues, however. Whereas a volt meter can connect perfectly to a reference, a PRT can only report its own temperature.
That is no problem when you are working in a well stirred water bath, that will have the PRT at the same temperature as the object in the same bath.
When you get to measure air temperature you are into serious sampling errors, the PRT has some self heating and so is air velocity sensitive, and the air 
you are measuring may not be the same air as is over your OCXO or item of interest. There is a personal plume of warm air rising from an observer, so 
you must be careful with your measurement technique.
The same problems occur with quartz crystal thermometers, which is why they are not more commonly found in surplus.
A PT100 sensor is quite cheap, and their calibration is little short of brilliant. However a they would cost much more if their calibration is traceable.
For my use, I use an ice-point cell as a calibration check, with care you get 10mK accuracy. You only need the knowledge how to set it up, a blender to make ice slush,
and a picnic vacuum flask, to make your own calibration reference.
I use thermistors for air measurement, and calibrate them against the PT100 in a thermostatic water bath. Thermistors can be run with a very low 
level of self heating and they are very sensitive, their resistance changes 4% per Centigrade degree, and they come in high values like 100K ohm. You read 
them in a bridge circuit with a voltmeter, so they are many orders of magnitude easier to use than a 100 ohm PRT.
 They are made small enough to get them in close contact 
with the object to be measured. 
If you want to know about humidity measurement I can tell you much about that,
cheers,
Neville Michie

On 08/12/2013, at 12:40 PM, Mark Spencer wrote:

> Sorry if this is somewhat off topic, but I'd be interested in more details re precision temperature measurement devices.   Have been using an inexpensive USB temperature sensor for the last year or so to monitor the temperature in my lab and have been looking at the correlation between frequency shifts in some ocxo's vs temperature changes.   I should also start taking humidity measurements as well at some point. 
> 
> 
> Any pointers re suitable instruments to accomplish this that can be sourced via the usual surplus sources would be welcome.
> 
> Thanks in advance
> Mark Spencer
> 
> Sent from my iPad
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