I wonder how you KNOW you'd fill every nook and cranny to eliminate an air=
=20
pocket.

I think the way they do it normally, is to put everything in a metal,=20
o-ring sealed stainless round (i.e. read as "strong") tube, and fill it=20
with a "silicon oil" (non-conductive hydraulic fluid is fine), which can't=
=20
be compressed. Then you don't have to worry about anything, since the=20
pressure is equalized within the tube. AND, the oil will transfer the=20
temperature nicely for you (the glue won't).

I'd be leery of a glue filler.. I'd guess that at 40m the material might=20
get squeezed like a toothpaste container...

Fun job!

--Bob

At 10:31 AM 8/26/2003 +0200, you wrote:
>Hi
>I was asked to make a temperature meter to measure 40m down in the sea.
>Quick hack, rigid device.
>Thinking about simply using a standard 10mV/=B0C semiconductor temperature=
=20
>gauge, and DMM module.  3-pin gauge, but 4 lead wire to have one extra at=
=20
>temp gauge GND for reference for DMM.  The temp sensor directly to end of=
=20
>cable, plus SMD cap, encapsulate using schrink tubing with melting glue=20
>carefully so no air is left and it is truly waterproof.
>
>Now it hits me: are theese chips immune to pressure?  (No drift in=20
>measurement?)
>You know every mateial is *slightly* compressible - not only gases!  So,=20
>how does this affect the chip??
>
>I could use PT100, 4-wire and ready PT100 module, but total cost is too=20
>high, and I don=B4t know about pressure sensitivity.
>
>Wire cost is substantial.  I need to use rigid polyurethane wire for=20
>mechanical strength in this app, and 3- and 2-wire are significantly=
 cheaper.
>So i am thinking of using the 3-pin device, with a cap and few components=
=20
>around so i can use a 2-wire connection thus: |: feed power, wait, measure=
 :|
>Comments?
>
>/Morgan
>--
>Morgan Olsson, Kivik, Sweden
>
>--
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--------------
Bob Axtell
PIC Hardware & Firmware Dev
Tucson, AZ
1-512-219-2363

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