From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Quickie Thermal Probe
Date: Thu, 12 Dec 2002 17:21:27 -0800
NNTP-Posting-Date: Thu, 12 Dec 2002 17:21:28 EST
Organization: Bell Sympatico
Charles DH Williams wrote:
> In article <3DF8FDBE.34F2@Spam.Bots>, Mike Monett wrote:
> > Any words of wisdom from someone who has done this before?
> You are going about it the wrong way. Do a google search for "hot wire
> anemometer" and use a small thermistor, or a tungsten filament (break the
> glass of a small light bulb) as the sensing element.
> There is a brief mention plus a couple of links to get you started at
Thanks, Charles. A plain 4W bulb measures about 350 - 450 ohms and
increases just by holding it in your hand. It might be a bit too fragile
to poke in dark corners, but a grain of wheat bulb would be easier
to protect and might work also. I just measured one at 44.5 ohms, and it
also increased when I touched it briefly. This approach would use a bit
more battery power and would need an external amplifier.
The differential pair of transistors has the amplifier built in, would
be a bit less sensitive to ambient temperature changes, and might need a
bit less power. For example, running the sense transistor at 2 mW means
about 8 mW total, or roughly 1 mA from a 9V battery. The transistors
would run several degrees above ambient, which is what I think I'm