On 2017-04-26, Jim Wilkins wrote:
"mike" wrote in message
news
On 4/26/2017 3:02 AM, unk wrote:
I want to cut AC power if the temperature drops below a
close-to-freezing
temp, and I want it to work even if there's no power (takes care of
the
power fails first then system freezes then power is restored - bang
goes
pump motor). This needs to be accurate to about 1-2 degrees.

[ ... ]

http://www.ebay.com/bhp/capillary-thermostat

Not the same, but it reminds me of something used for
temperature controlling a circulating water bath.

It is a mercury thermometer with one wire through the bulb and
contacting the mercury in the bulb. The other end is a very fine wire,
inside the bore of the capillary, and adjusted up or down by turning a
magnet over the top end of the device. (That turns a wire wrapped around
the fine wire, and moves it up and down as desired.

The limiting factor is that the wire and mercury cannot
interrupt more than a 100 uA of current, so you would need battery
backup to a circuit to amplify the current to open the power relay to
the pump.

The bath that I first saw one in, and that I use one in (thanks
to a hamfest) was German made. I lucked into the thermometer part
later, at a tools flea-market, because the original thermometer had been
broken in the assembly before I got it. (I picked it up with the idea
of controlling temperatures for color photo processing.) It was settable
to well less than one degree F.

The box mine came in has a label from Fischer Scientific, and is
marked:

Thermoregulator
Roto-Stat

Cat No, 15-180-5

Following that, I find the following:

http://www.philadelphiainstrument.com/thermoregulators.asp

Download the pdf instruction sheet. You will find two of the
listed models to be within your range needs:


================================================== ====================
62451 -35 to 500 F 0.005 F
62542 -35 to 135 F 0.02 F
================================================== ====================

No clue as to the prices. I guess that you'll have to call
them, and be sitting down when you call. :-)

Good luck,
DoN.

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