Remote temp measurement

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With the current discussion about temp measurement.

I would like to know what anyone would suggest for remote temp measurement.

I want to measure a solar collector and water storage system.

Using a two wire lead, what's a good device to put at the far end of the
wire.

Calibration of the remote device will be hard and measurement of the
resistance of the two wire lead will need to be taken into account.

Thanks for any ideas.


hamilton

Re: Remote temp measurement

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Simply build a phantom powered sensor logging module which sits at the far
end with the sensor on a four wire (excitation and sensing pairs)
measurements system and communicating the readings back to you over the
digital link that shares the pair of wires with the power supply (AC or DC).

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Re: Remote temp measurement

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How far is the "far end"?  Generally, you don't want to have a "naked"
analog sensor at the end of a long leash of unknown properties, or in
a noisy environment.  Instead, you'll need a complete thermometer, so
you can send the data in some more robust, usually digital format.

If your line isn't too long (< couple of meters), a Dallas/Maxim
1-wire sensor might be worth trying.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: Remote temp measurement
A thermomistor in a Whetstone bridge will do it. A thermistor is merely a
resistor that changes
its resitance quite dramatically as the temperature changes (All resistors
do that, which is something
you don't really want in an electronic circuit, but a thermistor does it on
purpose and fairly predictable).
They are quite cheap, Radio Shack sells 'em for a buck or so. They are
rather small and very sensitive.
In order to convert the resistance to a voltage you need a socalled
Whetstone bridge.
Google if you want the theory behind the bridge.

Greetz

Waldemar

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measurement.




Re: Remote temp measurement
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AD590 if 1000 foot is far enough. Search for AN273 on analog devices site.


Re: Remote temp measurement
You can use a kind of DS1820 device. It is one-wire sensor which may be
powered through signal line. The question is: how far it is supposed to be?
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measurement.



Re: Remote temp measurement
Hi Hamilton,

The low tech and cheap solution would be a PTC or NTC resistor
(resistive temp sensor) as Waldemar suggested. However, you don't need a
Wheatstone bridge. Calibrate that sensor in house before putting it up
there, unless it comes calibrated. Also, briefly short the two wires at
the far end and have someone read the resistance if it's hard to get
there and you don't want to do two trips. Alternatively you could short
at the other end and measure from the sensor location but that means
dragging a meter up there. I have a nice pocket meter for such jobs.

You could measure the sensor resistance with a dual slope conversion. Or
sacrifice a multi-meter that has that and a display built in. Some even
come with a PC interface. Don't forget to lop off the cable resistance.
Another good idea is to place a cap across the line where you connect
your meter to muffle RF noise.

Regards, Joerg

http://www.analogconsultants.com

Re: Remote temp measurement
On Mon, 18 Oct 2004 22:34:14 GMT, Joerg

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Or use four wires. Two wires will carry the (constant) loop current
through the sensor and the remaining two wires will connect to the
sensor terminal at the sensor end and using high impedance voltage
sensing on the other end. Since practically no current will be flowing
in the voltage sense lines, there will be no voltage drop in these
lines.

With a constant current source driving the loop will produce a voltage
on the sense lines directly proportional to the resistance.
Alternatively, you could use feedback to keep the voltage at the
voltage sense line constant by altering the loop current and measuring
it, so that the resistance is inversely proportional to the loop
current.

With resistive sensors, watch out for sensor self heating due to the
loop current, thus a constant dissipation system would be better.

Use twisted pairs for the loop and voltage sense lines, use ferrites
around the wires close to the display unit and use UHF rated SMD
bypass capacitors across the sense wires just inside the display unit
to keep the RFI out (including any cellular phone signals), which
otherwise could disrupt the measurements by self-rectification inside
the display unit.

Paul
  

Re: Remote temp measurement

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What temperature range? What accuracy and resolution? How far away is
it? How often do you need to read it?

Without those details, I would suggest that a 0.5 degree accuracy, 0.1
degree resolution, solution would be a Dallas 18B20, an MSP430F1120, PIC
or whatever, a TC55 3V3 regulator, and a transistor across the two
wires. Supply at say 6V supplied through a resistor at the receiving
end, use a comparator  to sense the volatge across this resistor. Send
slow NRZ, it should be happy over several hundred metres. One processor
could deal with several sensors.

Paul Burke

Re: Remote temp measurement
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Yes, the expected temperature range needs to be known.  My knee-jerk
answer to OP was Dallas 1-wire thermo too since I've used them with very
good results for several years.  "Solar collector" set off warning bells
here because certain parts of solar collectors get nasty hot, hot enough
to make toast out of a DS1820, for example.  On the other hand, I've had
excellent results "talking" to Dallas 1-wire thermos through twisted
wire-wrap wire or telephone extension cord at distances ranging from
less than 1 inch to about 17 meters.

Re: Remote temp measurement

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All these "analog" answers from a bunch of talented embedded (digital)
engineers!

Asan old analog layabout I'd go for
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3065
or
http://www.national.com/appinfo/tempsensors/products.html#digital
or
http://www.analog.com/en/prod/0,,766_814_TMP05%2C00.html



martin

 Serious error.
   All shortcuts have disappeared.
      Screen. Mind. Both are blank.

Re: Remote temp measurement
Hi Martin,

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May I be excused from this? I am an analog engineer...

Sometimes the analog path is lower in cost. In this case it might even
be more reliable since as far as I understand Hamilton wants to place
the probe quite remotely. That means things could be fried when a
thunderstorm rolls through. A plain old resistive temp sensor at the far
end and a cap at the near end would be pretty hard to beat here. You
could even roll the cable through a toriod numerous times, ground it,
whatever, to make it most reliable.

Regards, Joerg

http://www.analogconsultants.com

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