[time-nuts] Temperature sensors and bridge amps

[Bruce Griffiths]

Florian E. Teply wrote:
> On Thu, 11 Nov 2010 21:38:39 -0800 (PST)
> Perry Sandeen<sandeenpa at yahoo.com>  wrote:
>
>    
>> List,
>>
>> My following comments are am exploratory thought process of which I
>> don’t profess to know the answers. Perhaps in the future experiments
>> will provide some.
>>
>> So here we start.
>>
>> The Ni1000 SOT temperature sensor is a nickel based unit that has a
>> basic resistance of 1K ohms at 20 degrees C and a 6+ ohms (approx)
>> change per degree.
>>
>> The sensitivity of a standard platinum 100 ohm sensor is a nominal
>> 0.385 ohm/°C.
>>
>>      
> Well, to make comparison a fair game, i'd suggest to take the relative
> resitance change instead of the absolute one. For one, one could take
> a PT1000 instead of a PT100 so both the Nickel and the Platinum start
> out at the same initial value of resistance. By that, the platinum
> resistor gets to 3.85 Ohms/°C, roughly half the sensitivity of nickel.
>
>    
>> Wrote:<  Nickel sensors are more stable than thermistors, but not as
>> stable as platinum. The cost is more attractive than Pt, tho.
>>
>> Agreed.  The platinum price I was able to find was about $30 each.
>> So the Ni1000 is one tenth the price.
>>
>>      
> Surely one could find nickel resistors for less than 3 bucks each, but
> platinum isn't that expensive either. Depending on how they're built
> (bare resistor or housed in stainless steel, cables attached or not),
> one can find them for about 6 USD / 4 EUR at well-known sources and
> for something like 10 USD / 7 EUR packs of five off eBay. For plain
> resistors, that is. On the other hand, at quick glance i haven't found
> nickel resistors at all. Temperature control modules that can work
> with them, sure, but no resistors. Strange...
>
>    
>> Wrote: I'd consider staying analog with a DC bridge and a PID control
>> op-amp. You don't need a highly accurate voltage source for the
>> bridge because null is null, whatever the excitation voltage. Of
>> course, you'll want a stable null for the op-amp, too.
>>
>> I don’t know what a PID is but I agree about using a bridge circuit.
>>
>> Wrote:<need a highly stable set of bridge resistors for a stable
>> temperature. In the old days, precision, stable resistors were wound
>> on ceramic forms by soldering a loop of e.g. constantan wire to the
>> lead wires at each end of the form. Then you pull the loop at the
>> center so that you can wind it on the core in a non-inductive manner.
>> <Snipped>
>>
>> I have a number of them salvage from older test equipment.  Using
>> them in a small enclosed temperature control module is really
>> impractical.  One can easily buy 50 PPM/ degree metal film
>> resistors.  Probably sorting the two other branch resistors from a
>> batch of 10 with a 4 1/2 digit or greater resolution DVM can provide
>> an extremely well matched set.
>>
>> Let’s assume for example we want 80 C. for our oscillator.
>>
>> The Ni1000 is rated as 1482.5 ohms at 80C  and 1489.1 ohms at 81C
>> resulting in a change of about.0066 ohms per milli-degree.
>> As stated earlier, the standard platinum 100 ohm sensor is a nominal
>> 0.385 ohm/°C. or .000385 ohms per milli-degree.
>>
>> Although the platinum sensor is superior can such a low value of
>> change be used practically in a bridge circuit made by us time-nuts?
>>
>>      
> In a bridge circuit, you don't measure resistance directly, but use the
> voltage that appears across the bridge. So for a 100 ohms element,
> you'd usually have ten times the current flowing in that branch
> compared to a 1kohm element. That again works out to the same voltage
> swing. And as said, there are platinum sensors around with 1kOhm, also
> have seen 2k and IIRC 5k. On the other hand, the high-value platinum
> sensors generally aren't available in the highest accuracy bins.
>
>    
>> Another question is are we over-engineering a regulating circuit for
>> the crystal, as in how sharp is the turning point?  Will this be gold
>> plating a Yugo?  I have no idea.  I’m bringing this up for discussion.
>>
>>      
> Umm, is there such a thing as over-engineering to a time-nut? ;-)
> How sharp the turning point is strongly depends on how the crystal is
> cut. IIRC, SC-cut crystals tend to have a quite flat turning point
> somewhere around 50-80°C. Can't give you numbers on that though.
> But i'd guess to get to the point of zero tempco of the crystal and
> stay there, one would need to get within less than one degree C
> sonsistently. So you'll want to know the temperature with accuracy of
> at least 0.2°C. The thermostat will need fine-tuning anyways in order
> to accomodate all the unknowns like exact turnover temperature of your
> crystal, tempco of the other resistors, whatever.
>
>    
Due to manufacturing tolerances not all crystals (eg Sc cut crystals) 
exhibit a turnover temperature.
>> Wrote:<  Don't even think of using any kind of variable resistor to
>> adjust the bridge null. What you want is a stable temperature near
>> the value that gives the least crystal tempco.
>>
>> Agreed.  But I have a question.
>>
>> If one was using the Ni1000 couldn’t one use say a 20 turn 10 ohm
>> ceramic trimpot swamped with a 10 ohm resistor or a low value Beckmen
>> 10 turn pot to find the center of the turning point?
>>
>>      
> In my opinion, for finding the turning point a trimpot is okay. But
> then replace the trimpot with a suitable combination of fixed
> resistors. Those of course should be as stable as reasonably possible.
> You don't want your thermostat to react to variations in ambient
> temperature. Other than those needed to keep the crystal temperature
> constant that is. Question is: should the thermostat circuit be located
> inside the chamber being held at constant temperature or outside?
>
>    
>> The last unknown for me is what type of op-amp does one use?
>>
>>      
> I'd go for a low-noise type. What exactly is needed from the opamp
> might again depend on the intended circuit. Usually you'll want good
> power supply rejection as well as good common-mode rejection while
> bandwith probably won't be too much of a problem.
>
> HTH,
> Florian
>
>    
There's also the option of using a copper thermometer element.
NIST have used wire wound copper resistors in the past for measuring the 
temperature of fused silica capacitors for example.
One could even etch such sensors into a circuit board (a flexible 
substrate may be an advantage).

Bruce