Hi > On May 19, 2016, at 12:46 PM, Attila Kinali <[email protected]> wrote: > > On Thu, 19 May 2016 07:40:15 -0400 > Bob Camp <[email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
