[time-nuts] temperature sensor

[Bernd Neubig]

Hi Bob,

see my answer to Attila, which I have sent a minute ago.

Bernd  DK1AG

-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] Im
Auftrag von Bob Camp
Gesendet: Samstag, 19. Juli 2014 15:00
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] temperature sensor

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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