[time-nuts] Temperature sensors and bridge amps

Florian E. Teply usenet at teply.info
Fri Nov 12 10:06:27 UTC 2010


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.

> 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



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