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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