[time-nuts] HP5065A environmental sensitivities

[Attila Kinali]

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 errorprone.

Also, composite sensors have higher uncertainties and drift
then single sensors. Even more so: the integrated temperature sensors
are intended for use with the main sensor as a compensation tool. 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.

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.

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.

> 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.

			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