[time-nuts] temperature

Tim Shoppa tshoppa at gmail.com
Sun Dec 8 23:22:22 EST 2013


Interestingly, HP for a long time sold"quartz thermometers" based around a
probe with a quartz crystal with a well characterized linear temperature
coefficient. They called the crystal cut "LC" (Linear Coefficient):

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-03.pdf

http://en.wikipedia.org/wiki/Quartz_thermometer


On Sun, Dec 8, 2013 at 10:55 PM, Neville Michie <namichie at gmail.com> wrote:

> 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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