5. PRT in 4-wire half bridge
A
4-wire
half
bridge
is
the
best
choice
for
accuracy
where
the
platinum
resistance
thermometer
(PRT)
is
separated
from
other
bridge
completion
resistors
by
a
wire
length
having
more
than
a
few
thousandths
of
an
Ohm
resistance.
Four
wires
to
the
sensor
allows
one
set
of
wires
to
carry
the
excitation
current
with
a
separate
set
of
sense
wires
allowing
the
voltage
across
the
PRT
to
be
measured
without
the
effect
of
any
voltage
drop
in
the
excitation
wires.
This
arrangement
cancels
out
both
the
effect
of
the
wire
length
and
differences
in
resistance
of
the
excitation
wires
going
out
to
and
returning
from
the
sensor.
shows
the
circuit
used
to
measure
the
PRT.
The
10
kΩ
resistor
allows
the
use
of
a
high
excitation
voltage
and
low
voltage
ranges
on
the
measurements.
This
e
nsures
noise
in
the
excitation
does
not
have
an
effect
on
signal
noise,
and
that
self
heating
of
the
PRT
due
to
excitation
is
kept
to
a
minimum.
Because
the
fixed
resistor
(R
f
)
and
the
PRT
(R
s
)
have
approximately
the
same
resistance,
the
differential
measurement
of
the
voltage
drop
across
the
PRT
can
be
made
on
the
same
range
as
the
differential
measurement
of
the
voltage
drop
across
R
f
.
The
result
of
the
four
wire
half
bridge
Instruction
is:
the voltage drop is equal to the current (I), times the resistance thus:
The
PRTCalc()
instruction computes the temperature (°C) for a DIN 43760 standard PRT from
the ratio of the PRT resistance at the temperature being measured (R
s
) to its resistance at 0 °C
(R
0
). Thus, a multiplier of R
f
/R
0
is used with the 4-wire half bridge instruction to obtain the
desired intermediate, R
s
/R
0
= (R
s
/R
f
x R
f
/R
0
). If R
f
and R
0
are equal, the multiplier is 1.
The fixed resistor must be thermally stable. The 0.8 ppm/°C temperature coefficient would result
in a maximum error of 0.035 °C at 125 °C. This measurement is ratiometric (R
s
/R
f
) and does not
rely on the absolute values of either R
s
or R
f
.
The properties of the 10 kΩ resistor do not affect the result. The purpose of this resistor in the
circuit is to limit current.
4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
8
