piclist 1998\10\01\071206a >
Thread: Sensors
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face BY : Nicholas Irias email (remove spam text)



For the temp sensor, you probably want a nickel-chrome to nickel aluminum
thermo couple.  The cost of these arises not so much from the wire junction,
but the housing that is used to protect the lead wires and the junction
itself from physical damage.  I use one of these thermocouples to measure
the temp of molten aluminum, and the sensor I use is in a 3 foot long
protective tube to keep the aluminum off the wires - cost around $90.  If
you have a more controlled environment (maybe not immersing in a molten
metal or other corrosive material) so that you do not need such a large
protective housing, the thermocouple will be cheaper.

To use a thermocouple you need a rating curve in mV/degree.  But note that
1) at some point the thermocouple wires will transistion to copper wire, and
2) your reading of mV from the copper leads, when translated to degrees C,
is actually the difference in junction temperatures.

Unless the nickel alloy to copper junction temp will be constant, you will
want to monitor that secondary junction with something like a DS1620 located
at that junction.  You can then compare that secondary junction temp to the
temp at which the thermocouple rating curve was developed.  If the
thermocouple was calibrated with the secondary junction at 0 degrees (in an
icewater bath), and your secondary junction temp is measured as 25 degrees
C, just add the 25 degrees C to the temp reading you get from the
thermocouple's calibration curve.

You may need an amplifier.  These junctions can be read easily with a DVM,
but I havent tried reading with a PIC A/D, which wants a relatively low
source impedance.

Don't have any info for the other sensors.  What are you building anyway, a
Venus probe or some other sort of re-entry vehicle?

-Nicholas

-----Original Message-----
From: Neil Calitz <EraseMEncalitzspamBeGonespamspam_OUTOLDMUTUAL.COM>
To: @spam@PICLISTTakeThisOuTspam.....MITVMA.MIT.EDU <KILLspamPICLISTEraseMEspamMITVMA.MIT.EDU>
Date: Wednesday, September 30, 1998 5:21 AM
Subject: Sensors


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