want accurate mechanical temperature switch

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.

Reply to
unk
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Can you accurately set and measure its trip point?

Reply to
Jim Wilkins

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

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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?

Reply to
mike

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Reply to
Jim Wilkins

Check out solar water heating controllers. They're designed to activate/deactivate pumps based on multiple sensors or snap switches, including freeze sensors. You can reverse the contacts if needed with an external relay.

Reply to
Reflektor

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

Reply to
mike

That's why I asked how accurately he could measure temperature. The hysteresis is necessary if the thermostat has to switch high current, because the differential gives it the energy to snap open quickly to break the arc.

Home heating thermostats compensate for the hysteresis with an internal heater.

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Reply to
Jim Wilkins

[ ... ]

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:

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.

Reply to
DoN. Nichols

I've got an old mercury relay thermostat that I put in the garage to keep the temp at ~45-50 F. It doesn't go that low, but I tipped it sideways and it works. I don't know if it would go all the way down to 32 F. (and it won't have 1-2 degrees of accuracy.) I guess one could experiment in a frig or freezer. It'd have to drive some other AC power relay... I don't quite understand his no power spec.

George h.

Reply to
ggherold

How will the board continue to work without backup power?

As for the mercury-column thermometers controllers which I pointed to earlier are NO, closing when the temperature reaches the set point. But -- with the limit of 100 uA through the mercury and moving set-point contact, you need at least a transistor to control the current through the relay, and you *will* need power -- AC power supply, or a battery backup -- to make it continue to work during power outages.

And, since I don't know what the price is for these, I have no idea how affordable they will be for the task -- but they are fragile. They *do* have a hysteresis of less than a degree F.

Enjoy, DoN.

Reply to
DoN. Nichols

...

That's the point; it doesn't need to.

Reply to
dpb

I just remembered the words to search for.

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-jsw

Reply to
Jim Wilkins

On 04/29/2017 6:21 AM, Jim Wilkins wrote: ...

Interesting...the interesting group from the spec table.

Highest Standard Temperature Nominal Tolerances Setting Differential Open Close oF oF oF

  • Available in SPST construction only.

The biggest downside would appear that the only distributor I could find that even listed a price quotes $250/ea; the rest are "Quote". One knows when in that ballgame the answer is never going to be one one wants... :)

Reply to
dpb

...

To amplify, these boards retain their settings on power-off so if use the NO contact and the "COOL" side so closes on T>Setpoint, when power comes back on the relay won't close to restore power to AC if the temperature is below the turn-on setpoint.

Which use begs the question of why OP would leave the AC energized once it's even remotely close to freezing, anyway...a solution looking for a problem here.

Reply to
dpb

I've purchased small qtys from Grainger in the past, not sure what they stock these days. I don't think they we're more than $10 to $15.

I recently purchased a selection on ebay from China to build a Murphy switch for my tractor for unattended running of a PTO generator. It's a '54 Farmall with thermo-syphon cooling system -- no water pump or ports for a sensor, so I hope to find a place where a contact switch will work.

I can't vouch for the calibration accuracy, but they're cheap enough to experiment with.

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Reply to
Ned Simmons

I guessed that it was a DIY solar water heater. For a one-off home project there are thermal switches in discarded refrigerators and air conditioners that might be suitable. Some are open-on-rise and may require a relay, perhaps a solid state one to avoid opening during the pump's starting surge.

-jsw

Reply to
Jim Wilkins

If it's a solar water heater, then products like this with built in freeze protection are readily available.

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Under "frost protection" it seems to ignore drain down/back valves, which are common in plain-water, uninsulated-collector setups.

Reply to
Intervention

...

Probably better guess--I see on reading again what I skimmed as AC" was "AC power"...

Reply to
dpb

It's for a cottage that is available to a number of people. It has electric heat, which is *supposed* to be set at 5 degrees above freezing when the users leave. They are supposed to drain the water as well.

But sometime they don't set it, and sometimes they don't drain the water system, and sometimes the power goes out.

What I want is a system that will open a valve to drain the system whenever

a) the power goes out, or

b) the temp gets too close to freezing.

I can do a) with a relay or contactor (which will also cut power to the pump) and a electrically operated valve, but for b) I need a switch that will cut power to the relay.

Educating the users has not worked (twice, counting the number of times pumps broke and pipes-had-to-be-replaced). A machine you only have to educate once.

Reply to
unk

I gave you several suppliers and search terms. Usually switches need to close near freezing to turn on heat, except for refrigeration thermostats which open when cold. However the fixed-setting replacements I found had a wide tolerance, which is why I would use an adjustable thermostat and test its setting, differential and repeatability.

-jsw

Reply to
Jim Wilkins

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