[time-nuts] temperature

[Bob Camp]

Hi

The Quartz Thermometer died when somebody proved that hysteresis was a big deal on the probes.

Bob

On Dec 8, 2013, at 11:22 PM, Tim Shoppa <tshoppa at gmail.com> wrote:

> 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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