OT: dual input temp controller source

If you like thermocouples as sensors, any setpoint box that takes a thermocouple input will do it. Just put the hot junction on system A, and the cold junction on system B, and run the extension wires to a millivolt-ish sensing unit.

I'm looking for a dual input/differential temperature controller, in something like a 1/16 DIN size.

Oddly this seems to be a pretty uncommon device.

I found this

but from the look of that site, I'm guessing nobody stocks or sells those things.

Omron doesn't seem to have any dual input controllers either, and I don't need some PLC contraption- just something to send a signal when the difference between two temperatures exceeds a set point. Since the actual temps go up and down, a regular controller won't work as there is no fixed setpoint.

I think Omega had something that can do this, but it was pushing into extreme prices.

any ideas?

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Solar thermal collection systems use commonly available controllers that can switch on a differential of two temperatures and output 120vac to a pump. Mine also has a max temp to limit the high temperature. A variable output one was being created a few years ago, also with proportional AC output to differential. I would have to go to another site to get the brand name but a simple search should net you a few brands available on eBay or elsewhere.

It is a flat panel wall mount with two three thermal inputs (one just for looks), digital readouts some buttons, backlighting for about $150 a few year ago.

Not sure what you want to do with the o/p or if the pump control fits your end usage.

Cydrome Leader wrote in rec.crafts.metalworking on Mon, 6 Jun 2011 21:47:43 +0000 (UTC):

You need a comparator circuit.

I was thinking about just using an op amp and making this all analog, but like the idea of the hysteresis and ramp setting in those temp controllers, plus you get a readout, instead of having to write on some bezel next to a knob in black market and just writing in close values.

I'm still lost as to why this seems to be such an odd item. Does nobody else need something like this?

Really, not often at all.

A couple of times, I've made them for customers from ordinary temperature controllers by disabling the cold-junction compensation, but that might not be so straightforward with a modern controller where one does not have access to the firmware etc.

There are also some instruments with dual inputs, in which case you can get (potentially) higher accuracy**, but the instrument cost tends to be a bit high.

**Tbe output in mV/° of a thermocouple varies with temperature (they're nonlinear), so the trick of back-to-back sensors is only an approximate solution.

I've heard of flow sensors that work with a thermometer upstream of a heater, and a second thermometer downstream; when the flow is ON, the downstream sensor has higher temperature than the upstream. Those, though, were a balanced thermistor bridge sensor (difference precision but not individual temperature accuracy). Don't know if that's a massproduced item, however.

Two-terminal current source sensors, like AD590, can be used effectively, too. Inverting op amp, bias voltage to (+) input, to the (-) input connect T1 sensor (other end to +V) and T2 sensor (other end to -V), and a current-source bias (just a resistor to a known voltage source). You can dial in any absolute temperature-difference setpoint and (depending on the feedback resistor) get any gain, but again it doesn't tell you either temperature with any accuracy.

Two thermocouples connected back to back might do it, if you can tolerate a small shift in the differential over the operating range. What is it?

Connector bodies and wire:

Without strain relief, works fine for me: With strain relief if you are abusive: jsw

I just want to run a fan when I'm not home, and it's cooler outside. The building is brick, so the temp is alway sort of reversed from outside.

it's obviously nothing critical, I'm just baffled that there's nothing off the shelf for this, aside from those solar energy things somebody mentioned.

thanks for the links. It's really looking like some junk box project might be the only really viable option here.

You really need two dewpoint sensors as well so you don't pull in air that's slightly cooler but much more humid. I've asked the tv weathermen to add this to their reports and finally see them give the outdoor dew point. If it's below 55F I ventilate, above 65F the A/C may be better regardless of the temperature difference.

For quick estimates I consider 3% of humidity equal to 1 degree F when the dew point is ~ 60F or above, because I have a readout of indoor + outdoor temperature and humidity but not dew point. This may reflect my own tolerance, it's not from a psychrometer table.

jsw

Interesting point about the dew temp, never though about that.

I recall an awesome sling psychrometer in grade school, that was made with real mercury thermometers.

I have a cheap plastic one with alcohol thermometers that isn't such a risk to swing indoors.

I read the Radio shack weather station, compensate for the humidity difference and decide whether or not to open windows and the attic hatch. If I was going to automate it I'd probably use an old laptop so I could easily adjust the control algorithms and log and display the results. A laptop on AC draws much less power than a desktop. My old Armadas use ~12W.

jsw

Some other formulas and rule-of-thumb approximations:

They give 3.3% humidity per degree F: "For example, a relative humidity of 100% means dew point is the same as air temp. For 90% RH, dew point is 3 degrees Fahrenheit lower than air temp. For every 10 percent lower, dew point drops 3 =B0F."

jsw