On 1/14/08, Bob Blick <bbblick@sbcglobal.net> wrote:
> Hi Alan,
>
> "Assumed by calculation" seems to me to have a certain
> cost, just as an extra wire would have a certain cost.
> I'm wondering if the availability of 3 wire sensors
> has more to do with the engineer who came up with the
> idea being less bright than he should have been. 3
> conductor and 4 conductor wire, not much difference in
> price.
>
> Actually, using a 3 wire sensor could be much more
> expensive than using a 4 wire sensor. With a 4 wire
> sensor, you measure the voltage across two terminals.
> With a 3 wire sensor, you also need to measure the
> drop.
>
> Definitely the 3 wire idea was from a
> less-than-perfect engineer.

No, :)

It doesn't need any "computation". All remote displaying devices using
3 wires to RTD have a "calibrating resistor" onboard.  Calibrating
algorithm is much easy than you imagine and does not need any
computation (OK, if you can't live without computations you may do
it...).

The temperature is measured on the field using the same three wire RTD
and the same displaying device but with very short wires (near zero
errors).
Or the RTD is replaced with a metrological two wire rezistive box .
The value read on the field (or the value of the set resistance from
the box)  is assumed as a "real temperature".

On the remote end,  the technician needs only to adjust the resistor
for one temperature (better near the end of scale) untill it has the
same readings as the field device (or the set resistance-temperature
dependence)

easy and clean,

Vasile


>
> Cheerful regards,
>
> Bob
>
>
>
> --- "Alan B. Pearce" <A.B.Pearce@rl.ac.uk> wrote:
>
> > >What abnout the drop on the shared wire? I don't
> > see
> > >how this is very useful, why not just use four
> > wires
> > >and do it right?
> >
> > The drop on the shared wire is assumed by
> > calculation from sensing the wire
> > on the other end. I have yet to become aware of an
> > installation where the
> > difference in wire length is enough to upset the
> > calculation.
> >
> > There is no reason why this is 'wrong'. 4 wires are
> > really only needed when
> > measuring very low resistances or extreme accuracy
> > is required.
> >
> > --
> > http://www.piclist.com PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist