[time-nuts] temperature sensor

[Bob Camp]

Hi

A “temperature sensor crystal” is very much the same thing as a normal crystal (except for angle of cut). The mounting is pretty much the same as the crystals you have seen before. The only thing you do to improve the thermal coupling is to do a backfill with something like helium. Backfill levels are low and they vary depending on the application and the cut of crystal. The thermal resistance isn’t great, but it’s good enough. You only have micro watts going into the resonator. Increasing the backfill would increase the damping and thus the resistance. That would increase the power dissipated in the resonator. This would defeat any gain you got from the increased backfill. 

For direct contact sensing, you use SAW devices rather than BAW’s. If you do things “right” you can put the SAW directly in contact with the “stuff” you are sensing. The thermal resistance in that case is essentially zero. 

Bob

On Jul 19, 2014, at 7:45 AM, Attila Kinali <attila at kinali.ch> wrote:

> On Wed, 25 Jun 2014 14:21:49 +0200
> "Bernd Neubig" <BNeubig at t-online.de> wrote:
> 
>> the time-nut approach for temperature measurement would be to use
>> a temperature sensor crystal - like the good old Hewlett-Packard guys 
>> did many years ago. If you do not look for ultra-linearity of the frequency
>> vs. temp response, there are several possible types of crystal cuts
>> possible. The simplest one is the Y-cut or the slightly rotated Y+5° cut,
>> which has a slope of about 90 to 95 ppm/K @ room temperature.
>> Smaller sensor crystals are tuning-fork type crystals, which 
>> come in the same small cylindrical package as normal watch crystals.
>> For further reading I have attached an application note for such a
>> crystal from AXTAL.
> 
> Do you have any data on the temperature resistance from case to crystal?
> The PT100 and NTC sensors have the nice property of having a very good
> thermal coupling between the sensor element and the case. But i suspect
> that temperature sensor crystals have a very small area that couples
> the crystal to the case (in order to get a high enough Q for the oscillator
> to work), which in turn limits the speed at which the sensor reacts to
> temperature changes.
> 
> 				Attila Kinali
> 
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