[time-nuts] HP5065A environmental sensitivities

[Bob Camp]

Hi

> On May 19, 2016, at 12:46 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Thu, 19 May 2016 07:40:15 -0400
> Bob Camp <kb8tq at n1k.org> wrote:
> 
>> One advantage of doing all the compensation off of a single sensor is that 
>> *if* there are cross effects and *if* you can characterize them, you can 
>> correct them out. Put another way, if the pressure reading changes by 
>> 0.01% per C, having a reasonable idea of the temperature of the sensor lets
>> you take care of that. 
> 
> But munching everything into a single system makes the thing mathematically
> intractable. Seperating the values, compensating them for induced errors
> first and then using them is much easier and less error prone.

Having all the outputs from a single sensor or multiple sensor makes the math 
no harder or easier. If the pressure sensor moves 0.01% per C with a separate
pressure sensor and a separate temperature sensor …. the math is the same as
if it comes off a single device. You have the same effect to train out and the same
math to do it.


> 
> Also, composite sensors have higher uncertainties and drift
> then single sensors.

That’s often the difference between a $10,000 sensor and a $1 sensor rather
than an integrated part. 

> Even more so: the integrated temperature sensors
> are intended for use with the main sensor as a compensation tool.

Which is one of the reasons they work for that purpose. 

> The specs
> of the temperature sensor are good enough if the drift/hysteresis of the
> temperature sensor is less than the one of the main sensor. That you can
> read out the temperature sensors value is more a goody for those applications
> when a temperature sensor is needed but not high accuracy/precision required.

Again, a delta between a $10,000 sensor and a $1 sensor is indeed that you can 
likely read the Fluke / Hart setup to a lot higher level of accuracy. 

> 
> What is usually a good approach is to use the temperature sensors on
> barometric and hygrometric sensors only for their temperature compensation.
> At most, use the temperature sensor for cross checking and detecting faults.
> 
> For real temperature measurements, I would use a wirewond Pt sensor
> on a 24bit ADC with a stable reference.

I would prefer a set of at least three SPRT’s each with their own readout and to calibrate
them each with an independent triple point cell. That most certainly would do a *much* better
job of producing accurate temperature. Of course, that’s not going to fit into a modest $10K 
budget. 

> 
> Temperature effects are by far the largest effects we have to deal with.
> Having a stable and reliable measurement system for temperature is not
> only worthwhile, but actually a requirement before you start compensating
> anything else

But is +/- 0.0001 K  good enough or do we need +/- 0.000001 K. Oddly enough, it turns out
that you can do a really good job compensating most frequency sources with a much more 
modest *relative* accuracy. It also turns out that gradients will get you *long* before anything 
much below 0.1C applies. 

> 
>> Things like sensor drift and sensor hysteresis … that’s not quite so easy to
>> take care of. The only hope there is that they are small enough to be
>> neglected. The same issue with hysteresis is actually a big limit on
>> humidity compensation of some devices. They adsorb water vapor at a very
>> different rate than they desorb. 
>> Modeling that can be really messy. 
> 
> Hysteresis can be properly modeled and compensated. The problem is, that
> the math behind it becomes nasty very quickly. Often just using a simple
> second order diff equation system and letting a Kalman filter estimate
> the parameters is easier than modeling a full memory system... under the
> condition that one can excite the system reliably and isolate/estimate
> the other effects well enough.

It’s generally not the math that is the issue…..Try modeling a humidity effect that takes < 1 minute
to adsorb and a couple of months do desorb. Hysteresis in OCXO’s can turn out to be dependent
on rate, range, starting point, and air flow direction. Something over a thousand independent 
test profiles (six directions, eight rates, ten ranges, ten starting points ..)  may be needed to fully train it out. 

Bob

> 
> 			Attila Kinali
> 
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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