At 06:49 AM 1/15/2008, you wrote:
>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

Leadwires don't typically contribute more than a small percentage
of the sensor resistance, so we're talking about dealing with the last
few degrees of error (and, often more importantly, the drift of
that error since the leadwires are themselves RTDs and are
exposed to ambient temperature variations).

Often we can put a bit of electronics in the field and use only
two wires by converting temperature to a current loop, which is
better again. Then the RTD itself can be 2-wire or 3-wire, but the
leads are so short there is no concern. And in modern times, the
old current loops are giving way to various and nefarious digital field
buses.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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