On 4/26/2017 10:23 AM, 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.

I did a lot of experimenting trying to use a thermostat to turn on
heat tape. You'd think that you could repurpose a mechanical HVAC
thermostat,
but I was unsuccessful. There's so little force that a tiny stress
on the case throws the setpoint way off. I didn't find any
mechanical
systems for sale that had anywhere near 1-2 degrees F accuracy or
setability or stability.

A digital HVAC thermostat won't go that low.
I shunted the thermistor and created a calibration chart
for the range around freezing.
Has battery backup, so that's a possibility. I don't remember
exactly why I didn't stick with that one. It may have
had something to do with wanting remote sensing inside the
pipe insulation.

This device
http://www.buyincoins.com/item/45040.html#.WQDMNLkpV7k
works, but it's not passive, so won't
work if the power fails. Battery backup could solve that problem.
If you put a switch in the power to the displays, it shouldn't
take much to battery back it.
There are other devices linked to that page that are packaged.

I finally replaced it with a plug-in device from the hardware store.
Looks like a three-way AC socket expander designed for controlling
heat tape.
Nowhere near the accuracy, but it does the job at the expense of
running the heat tape at unnecessarily high temperatures.

The solution to a problem starts with a clear definition.

Are you talking degrees F or C?
"Bang" suggest that your pump is frozen and the rotor locked.
Can you not protect that with a breaker?
What is the "system?" Interesting that it can tolerate
freezing other than the pump issue. Most pumps I know
about will bust if you freeze them.
Does the system have to reset itself when the temperature
goes up?
Do different parts of the system freeze/thaw at different
rates?
Is it always the same? For example, does the same part
of the system freeze first with an east wind or a west wind?
What are the consequences of a false trip?
How long does it take to unfreeze?
How many measurement points do you need?
How long does it stay frozen? Battery backup practical?

Using my best psychic vision to make up a scenario,
I'd power the pump using a relay. Power the relay from
the pump side, so if the power goes off, it stays off.
You might want some delay in that so it doesn't go off
on a power glitch. That implies some sort of battery
backup, but it doesn't have to hold more than a few seconds.

A thermostat similar to the one referenced above can do the
thermal shutdown. If you want it to come back up on its own,
you'll need battery backup for that as well.

Are we having fun yet?

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


Most of those don't have specs, but the cheap ones that do
are as bad as:
On-off Window : less than 16„ƒ
That's a mighty big hysteresis compared to the OP request.
Something to watch out for.