Lead-wire resistance in two-wire measurements causes
potentially large temperature measurement errors.
Other types of resistance measurements include three-wire
and four-wire resistance measurements. Four-wire
measurements are preferred in temperature applications
because they eliminate lead wire resistance from the
3 wire rtd sensor have a 1 postive(red) and 2
negative(white).whenever 3 wire rtd used for long distance
that means 10 meter wire..that time wire resistance also
increased.(r=pl/a).in 4 wire rtd sensor have a 2
postive(red) and 2 negative(white).4 wire rtd used for long
distance that time the red and red wire have short.so the
resistance variation is equal.
Most RTD’s incorporate a third wire with resistance R3.This
configuration provides one connection to one end and two to
the other end of the sensor. Connected to an instrument
designed to accept 3-wire input, compensation is achieved
for lead resistance and temperature change in lead
resistance. This is the most commonly used configuration.
If three identical type wires are used and their lengths are
equal, then R1 = R2 = R3. By measuring the resistance
through leads 1, 2 and the resistance element, a total
system resistance is measured (R1 + R2 + RE ). If the
resistance is also measured through leads 2 and 3 (R2 + R3),
we obtain the resistance of just the lead wires, and since
all lead wire resistances are equal, subtracting this value
(R2 + R3) from the total system resistance (R1 + R2 + RE)
leaves us with just RE, and an accurate temperature
measurement has been made.
A 4-wire configuration is also used. Two connections are
provided to each end of the sensor. This construction is
used for measurements of the highest precision.With the
4-wire configuration, the instrument will pass a constant
current (I) through the outer leads, 1 and 4.
The voltage drop is measured across the inner leads, 2 and
3. So from V = IR we learn the resistance of the element
alone, with no effect from the lead wire resistance. This
offers an advantage over 3-wire configurations only if
dissimilar lead wires are used, and this is rarely the case.
Still another configuration, now rare, is a standard 2-wire
configuration with a closed loop of wire alongside (Figure
5). This functions the same as the 3-wire configuration, but
uses an extra wire to do so. A separate pair of wires is
provided as a loop to provide compensation for lead
resistance and ambient changes in lead resistance.
How much should be the values and/or percentages of these
given below factors for a good controller ?
Steady state error
Oscillations & vibrations