Three-Wire RTDs
Measurement circuits that accept three-wire inputs minimize
the effects of lead wire resistance as long as the outer legs
are equal. However factors such as terminal corrosion and
loose connections can still create significant differences
between the lead resistances seen by the measurement circuit,
because only one ohm of difference between the legs results
in an error of 4.7?F (2.6?C).
As shown in Figure 4.10g, the lead wire C acts as a
sense lead and is part of both halves of the bridge and
therefore cancels out at balance. The lead wires A and B
are in different halves of the bridge and therefore at null
balance R3 = B ? A + RTD. Therefore, now the lead-wire
error is no longer the total lead resistance (A + B), but only
the difference between their resistances (B ? A).
This is a major improvement in reducing the lead-wire
error and is sufficient for the needs of most industrial applications
where the lead-wire lengths are short. However, it is
not a complete solution because wire resistances are guaranteed
only within a 10% tolerance; therefore, if A and B are
identical wires of identical lengths, their resistances can still
differ within the 10% tolerance. So if nominally they both
are 5 ?, in reality one could be 4.5 and the other 5.5 ?. If
this were the case, the difference of 1 ? would still introduce
an error. With a 100-? platinum RTD that error would correspond
to 1/0.385 = 2.6?C.
If the purpose of the temperature measurement is to calculate
the exothermic heat release of a batch reactor, this
error might still be too much. In that application the temperature
rise through the reactor jacket is about 5?F and the span
usually selected for the differential temperature transmitter
is 10?F (5.6?C). In order to identify the end point accurately,
the total heat release must be determined to within 0.5%
maximum error. Because the total heat release is calculated
by multiplying coolant flow with its temperature rise, the
flowmeter itself will contribute 0.25% in error and therefore
one must measure the temperature rise within 0.25%.
An error of 0.25% over an actual measurement of 5?F is
0.0125?F (0.007?C). This is such a small error limit that even
three-wire RTD transmitters may not meet it (their usual error
limit is about 10 times higher). For this reason, in laboratory
situations or for other high-precision measurements, one
might consider the use of four-wire systems, which completely
eliminate the lead-wire effect.
Four-Wire RTDs
Using a four-wire measuring circuit eliminates the above
problem. The design engineer should consider any of the
leading brands of temperature transmitters that accept fourwire
RTD inputs. Direct connection to remote devices with
three-wire extension cable will often produce errors that can
be significant and will vary with environmental conditions.
Four-wire RTDs can be connected either to a null-balance
bridge or to a constant current source. Both will be described
here. Figure 4.10h illustrates a four-wire null-balance bridge.
It operates by switching a triple-pole double-throw switch and
making alternate null-balance measurements in the two configurations.
In one configuration, lead A is measured together
with the RTD resistance, while in the other configuration it is
lead B, so they cancel out completely and the actual value of
the RTD resistance is determined as (R3a + R3b)/2.
Microprocessors and advanced electronics make it feasible
to provide this level of sophistication, but complexity
still costs money, so these designs are relatively expensive;
in addition, they are still limited by contact resistance
considerations. Even the best (gold-plated) switching contacts
contribute some contact resistance. The difference
between these resistances does introduce some miniscule
errors whenever one uses a switching configuration to make
a resistance measurement.
Another way to eliminate the lead-wire error is to use a
constant current source (CCS) in a four-wire RTD configuration.
These miniaturized CCS packages are available at
relatively low costs and provide an accurately constant current
flow of about 2 mA or less to avoid self-heating errors.
As shown in Figure 4.10i, in this configuration the bridge
itself is replaced by a DVM, which measures the resistance
of only the RTD and is insensitive to lead-wire effects as
there is no current flow through the connecting wires. The
source lead resistances (A and B) contribute no error because
the voltage drop is not measured along them.
For the maximum in precision, it would be prudent to make
sure that the current flow (Ic) through the RTD is constant and
that the DVM does draw any current (i = 0), and also to cancel
out the thermocouple (TC) junction voltages at points #1 and
#2. This is necessary because as the two wires (platinum RTD
and copper lead) at #1 and #2 form TC junctions, the millivoltages
they generate will also be registered by the DVM.
This effect is eliminated by offset compensation. The offset
voltage generated by the unintended TC junctions is measured
by the DVM when the CCS circuit is opened and, therefore,
Ic = 0. The smart RTD readout memorizes the voltage sensed
when no current is flowing and corrects the total reading by
that amount when the CCS is connected and Ic is about 2 mA.
In general, two-wire RTDs are only used in heating,
ventilation, and air conditioning (HVAC)-type secondary
applications, three-wire RTDs are still used in some processing
industries, and four-wire RTDs are used in most highprecision
services or in the laboratory.
