Application Note

6 Fluke Corporation L
oop calibration and maintenanc
e
2. Set the source current of
the 707 to 20 mA using the
0-100 % button. Note the
“output” meter display read-
ing. Adjust the span control
of the signal conditioner
one-half the delta from
20 mA.
Example: If the output dis-
play reads 21 mA, adjust the
span control to 20.5 mA
(one-half the delta to the
required value of 20 mA).
3. Repeat this “one-half step”
process until the required
output is obtained. There are
signal conditioners that have
non-interactive controls that
do not require this proc
edure.
Verifying process scaled,
indicators
Scaled process indicators are
used to provide information
about a process either locally
or to a control room located a
distance away. These indicators
typically take a mA measure-
ment in series with the 4-20
mA loop signal or measure a
1 V to 5 V drop across a 250
ohm resistor in series with the
4-20 mA signal (a 4-20 mA
signal through a 250 ohm
resistor produces a 1 V to 5 V
drop).
For indicators with a mA
input, the direct mA current
output of a 705, 707, 715, 787
or 789 can be applied directly
to the input of the indicator. For
voltage input indicators, a 715
is ideal with its direct voltage
output. Or use a resistor across
the output of a 7
05, 7
07, 787
or 789 as described previously
(“Using Fluke loop calibrators
as a voltage sourc
e
”).
This example shows how to
use the 707 to verify a mA input
indicator. Make connections as
shown in Figure 7.
1. Power the 707 up; the default
output should be 4 mA. Note
the indication, (digital or ana-
log) which should be
approximately 0 %.
2. Depress the 0-100 % button
on the 707; it is now in the
span check mode and is out-
putting 20 mA. Note the
indication (approximately
1
00 %).
3.
If testing the linearity is
necessary, use the 25 %
button to step the mA output
in 4 mA steps and record the
indications
.
4.
To calculate the errors in per-
cent, use the formula:
Nominal - actual/span x 100
N
ominal is the ideal value,
actual is the rec
orded meas
-
urement and span is 1
6 (4-20
mA = a 16 mA span).
Example: If the indication
with 0 % applied is 1 %, cal-
culate error as such: 0-1/16 =
.0625 X 100 = 6.25 % error.
Calculate error based on the
rec
orded indications and
c
ompare to the tolerances for
the indicator
. If any of the
calculated errors are too
large, adjustment may be
necessary. Normally, there
are at least two adjustments
for analog indicators: zero
and span. The zero adjust-
ment is typically on
the
faceplate of the indicator.
5. With an output of 4 mA from
the 707 adjust the zero indi-
cation. Span adjustment is
either a hard adjustment or
accomplished by bending a
linkage on the meter move-
ment. Refer to the manu-
facturer’s proc
edure for this
adjustment.
6.
Apply a 20 mA signal to the
indicator and make the
adjustment as specified. Once
the adjustment is c
ompleted,
re-verify the indicator and
confirm the adjustments had
the desired effect. If the indi-
cator still fails the test, it will
either need to be readjusted
until a satisfactory result is
attained, or replac
ed
.
Voltage input indicators
The procedure for using the 707
to verify voltage input indicators
is almost identical to the proc
e
-
dure outlined. The primary
difference is the addition of the
prec
ision 250 ohm adapter
.
+
–
Indicator
input
50%
0%
100
%
Figure 7