OT: dual input temp controller source

On Monday, June 6, 2011 2:47:43 PM UTC-7, Cydrome Leader wrote:

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 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?

On Tue, 7 Jun 2011 20:18:57 +0000 (UTC), Cydrome Leader


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.

On Tuesday, June 7, 2011 1:18:57 PM UTC-7, Cydrome Leader wrote:

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.

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

http://usashinko.com/WC16

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?

========================

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.

--

Eric

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


You need a comparator circuit.
--

Dan H.
northshore MA.

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:
http://www.omega.com/toc_asp/sectionSC.asp?section=3DG&book=3Dtemperature

Without strain relief, works fine for me:
http://www.omega.com/ppt/pptsc.asp?ref=3DSMPW_SMP_HMP_HMPW&Nav=3Dtemg03

With strain relief if you are abusive:
http://www.omega.com/ppt/pptsc.asp?ref=3DSMPW-CC&Nav=3Dtemg03

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.
http://en.wikipedia.org/wiki/Psychrometrics

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.

http://parts.digikey.com/1/parts/2040287-humidity-sensor-5v-5-3-pin-chs-csc=
-20.html

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.

http://www.dataq.com/products/startkit/di145.html

jsw

Some other formulas and rule-of-thumb approximations:
http://en.wikipedia.org/wiki/Dew_point

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 °F."

jsw