[time-nuts] temperature sensor

Javier Herrero jherrero at hvsistemas.es
Mon Jul 21 12:02:32 EDT 2014


There are two types of NTC thermistors that are (very) frequently used 
in spacecrafts, the Measurement Specialties (traditionally YSI, Yellow 
Spring Instruments) 44907 and 44908, that are 10k @ 25ºC NTC: 
http://meas-spec.com/downloads/44908.pdf and 

Both are specified in error against temperature, being ±0.1ºC for the 
44908 and ±0.2ºC for the 44907, in the 0-70ºC range.

I suppose that these ones are obscenely expensive, but the "civilian" 
versions are the 44006RC and 44031RC, with the same specified 
tolerances, and the same calibration curve, and readily available (e.g. 
from Farnell). I suspect that the difference between the space qualified 
ones and the others are quality control and traceability, and perhaps 
(only perhaps) a different low-outgassing epoxy coating, but probably 
the sensor material is the same in all of them. The advantage is that 
you get a quite good intial tolerance, so calibration can be simplified, 
since you only need to calibrate the resistance measurement device with 
known resistances to obtain a ±0.1ºC error from the sensors.

The qualification mandated by NASA for these sensors refers mainly to 
MIL-PRF-23648 standard, that describes a load life test, consisting in 
application of maximum power specified for the themsitor during 1000h, 
intermittently (30min. on, 30min. off) and checking at 250, 500, 750 and 
1000h that the thermistor remains in the specified tolerance in the 
applicable specification sheet, so I suppose that these thermistors are 
quite good wrt long-term stability.

In applications where more stability is required, also Pt thermistors 
are used in spacecrafts (mainly Pt1000 and Pt2000, also others), but the 
problem is that depending on the wire run, they shall be measured using 
4-wire techniques, and also initial calibration is not so tight (when it 
is so tight, they tend to be also very expensive) and not so easy (a 
Pt1000 has a rough variation of 3ohm per ºC).



On 21/07/2014 16:12, Attila Kinali wrote:
> On Mon, 21 Jul 2014 04:39:51 -0700
> Alexander Pummer <alexpcs at ieee.org> wrote:
>> NTC are not that very stable, they are amorphous material winch could
>> recrystallize slowly and therefore change it's electrical behavior ,
>> PT100 style is more reliable since it is pure metal
> How long is the time constant for NTCs?
> I guess, it wouldn't matter for most of the measurements we do,
> as NTCs need to be "calibrated" before precision measurements
> anyways. Unless one measures over several months, or years.
> But on this timescales, i wouldn't really trust an off the shelf
> PT100 either. Not unless i measure its stability
> For use in GPSDOs and OCXOs, i guess it doesn't really matter,
> as long as the NTC stays within spec. There an external loop
> corrects for the variation/drift of the measurement.
> While we are at it: what is a good way to calibrate/characterize
> temperature sensors that is available to hobbyists?
> 			Attila Kinali

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