> So it turns out its a 3 wire RTD. So if I understand it correctly, one wire is the excitation voltage, one is return and one is the voltage drop across the RTD ?
>
>
> Spehro Pefhany <
KILLspamspeffKILLspaminterlog.com> wrote:
> Quoting Mike Harrison :
>
> > On Thu, 3 Jan 2008 09:55:38 -0800 (PST), you wrote:
> >
> >> I've used the 2 wire RTDs before...essentially measure the voltage
> >> across the device for the temperature, but I've been asked to look
> >> at 3 wire or full bridge RTD's. Now, they talked like they are
> >> one and the same but I think a full bridge is a 4 wire? 4 wire
> >> seems simple enough in that it has a constant current source to
> >> get better accuracy but what about 3 wire devices? A current
> >> source that dumps into a common ground? Just need some
> >> clarification on that.
> >>
> >
> > With 4 wire, you apply current on one pair and measure voltage
> > differentially on the other. Easiest
> > way is to apply a constant-ish current, and take the ADC reference
> > as the voltage across a precision
> > series resistor in the excitation leg - this gives a direct reading
> > of resistance independent (
> > within reason) of the excitation current, avoiding the need for
> > expensive voltage references.
> >
> > 3-wire makes the reasonable assumption that the voltage drop on both
> > excitation wires is the same.
> >
> > Assume the wires to one end are A,B and the other is C
> >
> > Apply current to A, with C grounded
> > Measure voltage across B & C - this is the RTD voltage plus the
> > voltage drop on the C wire
> > Measure voltage between A and B - this is the voltage drop on the A
> > wire. Subtract this value from
> > the BC reading to get the 'real' RTD voltage value.
>
> Another approach is to use a second current sink that's twice the
> value of the main one, and just apply that to the compensation
> lead. That causes the leadwire resistance to cancel out without
> any calculation, and with a single-ended input, provided, of course,
> that the leadwires are of equal resistance. Otherwise there's an
> error proportional to the difference in the leadwire resistances.
> With 100R (or less) sensors, narrow sensing ranges, and long runs
> of relatively small-gauge wire, the leadwire resistance can be
> substantial compared to the sensing range, so differences from
> different lots of wire can have a significant effect. Nonetheless,
> 4-wire sensing is rarely used outside of lab situations.
>
> Best regards,
> Spehro Pefhany
> --
> "it's the network..." "The Journey is the reward"
>
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