[time-nuts] VRefs - I'm a bigger Voltnut than a timenut..

[Neville Michie]

Hi,
Temperature is one quantity that is (or was) determined by physical  
properties of substances.
I have a platinum resistance thermometer, but alone it is useless.
The simplest way of using it is to connect it in 4 wire mode to a HP  
3468A multimeter,
and it measures the resistance with a resolution of about 1mK.
Connect it to another HP 3468A and it varies by less than 1mK.
So the HP 3468A measurement is robust.
Use a widely disseminated formula for the resistance of thermal pure  
platinum
and it seems to measure temperature.
Construct an ice-point cell with a dewar flask, shaved ice, a  
stirring rod and a syphon tube
and we seem to have calibration to better than 10mK.
I then built a hypsometer to boil water in.
The temperature of the steam did not shift 1mK in several hours.
But you have to use a barometer (Fortin pattern) to find the  
atmospheric pressure,
correct for the density of mercury,
correct for the expansion of the scale,
get an estimate for local gravity from
the Geoid, apply an altitude correction.
The result was that a $10 platinum resistance thermometer seems to be  
accurate to 1mK
but the ice-point calibration uncertainty may add plus or minus 5mK.

The big difficulty with temperature measurement is that you need  
circulating thermostat
baths to immerse things in to compare temperatures to any accuracy.  
You only need
one or two of those in your shack to have no room for your time  
standards, voltage standards etc.
cheers, Neville Michie










On 01/12/2008, at 12:24 PM, Predrag Dukic wrote:

>
> Obsession for precision is the same whether one is after Hertz,
> Volts, or Degrees of Celsius (Or Fahrenheit).
>
> The methods and technology are similar...
>
> I am now trying to fix two HP2804A quarz thermometers, thet are
> giving some error codes, and do not work.
>
> More or less they are oscillators, and measure of temperature is
> measured by frequency shift  between reference and external  
> oscillator.
>
> Reference frequency is unusual 10.000.000,8 in one of them (marked
> exactly so)  and 10.000.001  in the other.
>
> (Yes, yes, not a typo: it is 10MHz plus 0.8 Hz or plus 1.0 Hz)
>
> So precise temperature and precise frequency cross their ways...
>
>
> By the way, does anybody have op/serv manual for this thermometer?
>
>
> Predrag Dukic
>
>
>
>
>
>
> At 16:18 30.11.2008, you wrote:
>> Hello,
>>
>> there's currently no easy way to convert a frequency into Volt, i.e.
>> other than by cryo Josephson junction array or realizing Ohm by a
>> Klitzing quantum hall effect device. Otherwise, FLUKE would have
>> implemented that already in their Calibrators/Standards. They  
>> still rely
>> on Artefact Calibration, i.e. ovenized longterm stable Zener  
>> References
>> and Reference Resistors.
>>
>> The 5720A uses a stacked double Zener reference on a proprietary  
>> hybrid
>> circuitry, giving 1-2 ppm/year stability, the 7001 and 732B use the
>> LTZ1000 (Zener above BE junction), as the 3458A, the 732A uses the
>> Motorola device (Zener below BE junction), and the elder 332D and  
>> 335D
>> use similar, but less longterm stable references.
>> The LM199, AD587 and I think also the Geller device may only be  
>> used as
>> transfer standards, having poor longterm stabilities of 20ppm/yr.  
>> or more.
>>
>> Therefore, the LTZ1000 only may serve as a really affordable  
>> standard,
>> i.e. below 8ppm longterm stability, for private use.
>>
>> I was lucky to get some LTZ1000 samples, and used the standard  
>> circuit
>> from the LT datasheet for creating a small PCB. I purchased  
>> commercially
>> available wire wound resistors (TK 3, 25ppm/yr) for the 5 necessary
>> reference resistors and two OP07. Those resistors should give less  
>> than
>> TK 1 and 1ppm/yr additional drifts.
>>
>> Compared to my HP34401A, the circuitry showed quite good short term
>> stability below 1ppm, but shielding still has to be improved, as the
>> LTZ1000 circuit is susceptible to external EMC disturbances. This  
>> Volt
>> reference then costs less than 100 Euros/Dollars in total.
>>
>> I designed further circuitry (with TK 0.3 and TK 0.1 precision metal
>> foil resistors from Alpha and Vishay, chopper amp, precision switches
>> and Teflon insulated wires) to have the possibility for precision
>> transfers of the 7,2V reference voltage to 100mV, 1V, 10V, 100V,  
>> 1000V
>> level.
>> Only then, a Volt Standard is complete.
>>
>> All this stuff will emulate or replace the commonly needed 732A  
>> (ref),
>> 720A (Kelvin Varley), 752A (Reference Divider) and 834B (Null  
>> Detector).
>>
>> Got no time yet to finish and improve the system, and to do the  
>> repair
>> on an old 332B standard, but would like to share my knowledge, if
>> anyone's interested.
>>
>> Frank Stellmach
>>
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