I have used thermistors in this application with good luck. The self 
heating characteristics combined with the resistance relationship to 
temperature works well.

Larry

At 12:21 AM 2/9/2010, you wrote:
>The transistor body provides thermal mass, true.  However, it may be an
>asset depending on the application and the range of temperatures and the
>rate at which the temperature changes.  "Dithering" temperature sensors
>making the electronics (and whatever they control) go on and off "quickly"
>may not be the best for the application.  So quick response as an asset may
>or may not be advantageous.  (Thought I'd stir up the pot a bit).
>
>In my experience thermal responses of several to tens of seconds have been
>more than adequate.
>
>Rich
>
>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
>Dario Greggio
>Sent: Monday, February 08, 2010 4:15 PM
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Measuring flow using a temperature sensor (and maybe a
>heater)
>
>Bob Blick ha scritto:
> > A hot wire exposed directly to the air is pretty responsive. The wire
> > material is also probably a relatively poor conductor of heat, so the
> > temperature is not heavily influenced by the connecting wire. As opposed
> > to a transistor, encased in plastic, with copper leadframe and
> > conductors muddling the accuracy by bleeding heat into the thermally
> > isolated parts of the device.
>
>I see Bob.
>I was considering a BD139-or alike, metallic case.
>Since the sensor woudl be smaller (the aforementioned SOT), I should use
>something else. Or maybe move to a TC74 (TO220) and use a BDX53 BJT.
>
>--
>
>Ciao, Dario
>--
>Cyberdyne
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Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.mchipguru.com

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