[time-nuts] Environmental sensor recommendations.

Bob kb8tq kb8tq at n1k.org
Thu Apr 5 12:25:47 EDT 2018


Hi

Thermocouples are *really* low output voltage devices at “normal” temperatures.
That gets you in to fairly fancy measurement setups ( no “just strap it to an ADC 
input” stuff). They also require cold junction compensation. They measure offset 
temperature to that junction. You do a lot of work to get just one sensor to operate.
That can be done with various chips from various outfits. It’s still not cheap. The 
net result is no more accurate or higher resolution than the other stuff we have 
discussed. The place thermocouples come in are for very high or very low temperatures.
If you want to know if it’s really 2,876C, a thermistor is not what you want to use.

RTD’s are indeed very stable long term. Properly handled they can do a real good 
job. They are basically a strain gauge. Mount them wrong (yes, I have a few hundred
examples of this) or treat them wrong ….. they don’t do quite as well. If you really
want to get accuracy out of them, you run a triple point cell to calibrate them before
use. Yes, this is getting off into temperature nuts territory. 

Heading back a bit to the original question: 

What *does* impact your frequency or time standard? 

1) Temperature 
2) Pressure
3) Humidity
4) Load
5) Supply voltage
6) Acceleration (gravity)
7) Stress / strain (bending)
8) Time ( aging )
9) EMI ( injection locking )

I suppose that list could go on for quite a ways. The obvious one on the list that
has not been addressed yet is voltage. 

Bob

> On Apr 5, 2018, at 10:44 AM, John Green <wpxs472 at gmail.com> wrote:
> 
> Why has no one mentioned thermocouples?
> I had some experience with thermistors a few years back designing thermal
> attenuators and equalizers for CATV. NTC thermistors can have a large
> change of resistance for a unit change in temperature. They aren't linear,
> but there are formulas for computing resistance vs temp. PTC thermistors
> have a much smaller change per unit change in temp., but are much more
> linear. And, they are susceptible to self heating, which makes things
> interesting. If I remember correctly, in my research something called an
> RTD was supposed to be the king when it came to accuracy and repeatability.
> As someone else has stated, the IC devices are supposed to be quite good,
> but you have to interface with them.
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