From: "Bill Sloman"
Subject: Re: Temperature Probe Interface / Coax cable Issues
Date: Mon, 20 Jan 2003 23:32:35 +0100
Organization: Planet Internet
NNTP-Posting-Date: 20 Jan 2003 22:33:12 GMT
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"Kevin Walton" wrote in message
> Summary - is this just impedence matching problems from using the
> wrong coax?
> I wanted to replicate a temp probe interface that I am using, which I
> have done and it works - ish.
> I didn't try to understand the circuit but it uses an ICL7660 and a
> TS912 IC, 2 variable resistors, a zener reference, resisters,
> capacitors and 2 1000UH SMD Inductors - one on each input wire.
> The sensing is done by a remote 1K0 3900ppm NTC sensor, connected by
> coax and a bnc plug/socket. The original coax is unmarked, 3.2mm
> external diameter, rubbery coated, 2.5m long.
> In my replacement probe the coax is RG 58 C/U, 5mm dia., 3.5m long.
> I calibrated the temp reading succesfully using the new interface and
> probe with the coax rolled into a say 20cm roll on the table, but then
> unrolling the cable caused significant change in the reading. Placing
> 2 sensor rolls of wire on top of each other caused the readings to go
> None of this occours in the original sensor, it can be rolled,
> unrolled, placed ontop of new sensor cale rolls with no problems.
> Is this an obvious problem that I just don't appretiate, such as
It might have something to do with the resistance of the braid forming the
outer screen of the coaxial cable.
Bare copper oxidises, and to get a good stable low resistance connection
through braid you need several milliamps of current.
Good coaxial cable usually uses silver-plated copper braid in the screen,
which is a bit better behaved.
For seriously good conductivity for low level direct current, think about
semi-rigid coax (RG402 or RG405) where the outer screen is a coper tube, or
Alcatel's Quickform, where the copper braid screen has been soaked with tin
to give much the same electrical effect. Both are ridiculously expensive.
If you are using BNC compression fittings on the cable, try taking them to
pieces - if you can do it without a spanner you've found your problem - and
reassembling them carefully, with every nut tightened up hard, with a
spanner. Better still, throw them out and get crimp fittings and a crimp
tool. At Cambridge Instruments we finally got sick of intermittent problems
caused by sloppy assembly, and went all-crimp.
Management bitched about paying for the crimp tools, but got pacified by
tales of million-dollar electron-beam micro-fabricators stalled in final
test until we finally worked out which BNC connector had been assembled
Bill Sloman, Nijmegen