Application Note

Fluke display Input Output
reading % Input voltage current
4.000
0
1
.000 V 4.000 mA
8.000 25 2.000 V 8.000 mA
1
2.000
50
3.000 V
12.000 mA
16.000 75 4.000 V 16.000 mA
20.000 100 5.000 V 20.000 mA
Calibrating the signal
conditioner
The following is a procedure
for calibrating a 1-5 V input, 4-
20 mA output signal
conditioner using the precision
current shunt constructed in
Figure 5.
Step 1: Setting the Fluke 707
to source 4-20 mA
1. With the shunt assembly in
the “source” jacks, power
the 707 on.
2. Check the display. If the dis-
play does not read 4 mA,
turn the 707 off and on
again while holding the
mode button for two sec-
onds. The display should
now read 4.000 mA.
Step 2: Calibrating
1. Place a precision multimeter,
such as the Fluke 187 DMM,
set to dc current mode, in
series with the output of the
signal conditioner as shown
in Figure 6.
2. Connect the test leads from
the precision shunt assembly
to the signal conditioner
input terminals, observing
proper polarity.
3. With the 707 set in the mA
source mode, the display
should read 4 mA (1.00 V
across the shunt).
4. Adjust the zero control on
the signal conditioner for an
indication of 4 mA on the
D
M
M
.
5.
Press the 0-100 % button on
the 707 until the display
reads 20 mA. Then adjust
the span adjustment on the
sig
nal c
onditioner until the
display on the DMM reads
20 mA.
6. Push the 0-100 % button
and the 707 should read 4.
V
erif
y the meter c
onnected
to the output sig
nal condi-
tioner reads 4 mA.
1
7. Calibration is now complete.
Note: Many manufacturers specify
a warm-up time prior to beginning
calibration.
Step 3: Checking linearity
Once zero and span controls
have been properly set, signal
conditioner linearity may be
verified using the following
procedure. This procedure will
check zero, 25 %, 50 %, 75 %
and span settings for signal
conditioner linearity.
1. With the precision resistor
assembly in place, adjust the
source current of the Fluke
loop calibrator to 4 mA (1.0
V) using the 25 % button.
The DMM displays 4 mA.
2. Using the 25 % button, step
the source current to 25 %,
50 %, 75 % and 100 % and
note the corresponding val-
ues. Table 2 shows the
correct values of output for a
linear signal conditioner. (If
values differ from that
shown in Table 2 by more
than the linearity specifica-
tion of the signal conditioner,
c
ontact the signal condi-
tioner manufacturer or
replace the device.)
Making fine adjustments
Many signal conditioners with
0-20 mA and 4-20 mA outputs
are notorious for zero and span
c
ontrol interaction
. If, when
checking linearity in the cali-
bration section, your output
meter displayed a value higher
or lower than 4 mA, perform
the following steps to affect the
required 4 and 20 mA display
on the output meter. Some
signal conditioners have non-
interactive controls, and thus
do not require this procedure.
1. Note the value above or
below 4 mA that was dis-
played on the output meter
when you returned to a
source value of 4 mA on the
707. Adjust the zero control
on the signal conditioner so
the value of the output
meter shows one-half the
difference of the remaining
mA value to 4 mA.
Example: If your display
reading at zero input in Step
7 was 3.50 mA adjust the
output (with 4 mA source
current) to display a reading
of 3.75 mA, which is one-
half the delta toward the
desired value of 4 mA; e.g
.,
4.00 - 3.50 = 0.50; 0.05 /
2 + 3.50 = 3.7
5, or one-half
the difference between the
reading and the desired
value.
L
oop calibration and maintenanc
e Fluke Corporation 5
707 with
shunt assembly
installed
Signal
con
ditioner
Lin
e power
Set DMM to
dc current
(connections
vary)
Fig
ure 6
1
If your output meter displayed a value higher
or lower than 4 mA, consult the “Making fine
adjustments
” section on this page.
Table 2. Correct value of output for a linear signal conditioner