3
CALIBRATION
Direct calibration in the signal loop is usually not practical due to the
difficulty in varying the measured parameter and the confusing interaction that
occurs between span and offset adjustments. However, the APLCL can be
quickly and easily bench calibrated using a commercially available current
calibrator or the calibration set-up shown below.
CALIBRATION PROCEDURE
The procedure outlined below, minimizes span/offset interaction and
simplifies calibration. In Steps 1, 2 and 3 the unit is
“nulled”
to zero readout
with zero input signal current. In Steps 4 and 5, the span adjustments are made
to establish the required slope of the transfer curve.Then in Step 6, the transfer
curve is shifted up or down as required by setting the offset adjustments. In Step
7, the final
“tweaking”
adjustments are made at minimum and maximum signal
current. Setting the decimal points in Step 8 completes the calibration.
Before calibrating, the READOUT SPAN (R
S
), SWING CURRENT (I
S
)
must be determined.
R
S
=
(Max. Numerical Display)
-
(Min. Numerical Display)
(Disregard Decimal Points)
I
S
=
(Current @ Max. Display)
-
(Current @ Min. Display)
Example
: Readout is to be 5.00 @ 4 mA and 15.00 @ 20 mA.
READOUT SPAN (R
S
) = 1500
−
500 = 1000
SWING CURRENT (I
S
) = 20 mA
−
4 mA = 16 mA
ADJUSTMENTS
1. Turn off all coarse offset and span adjustment switches
(S2-S10 down)
.
S1 has no effect when zeroing and can be in either position.
2. Apply zero current by opening the external zero current switch. Adjust
the indicator to read zero using the fine offset adjustment
(R.H. side,
front panel)
.
3. Close the external zero current switch and set the SWING CURRENT,
I
S
,
(16 mA in the example)
by adjusting the power supply voltage and
the external fine adjust pot. Then, turn the fine span control
(front, left)
to get a near zero reading
(Adjustment fully CCW)
4. With the SWING CURRENT I
S
, applied to the input, set up a
combination of coarse span adj. switches
(S6-S10)
to obtain a display
readout closest to the READOUT SPAN desired
(1000 in the example)
.
Set the exact READOUT SPAN with the fine span adj.
(front, left)
.
5. Repeat Step 2 to see if the zero value has shifted. If it has, re-zero with
fine offset
(front, right)
, then repeat Step 3 and 4.
6. After the span has been adjusted, set the signal current to the minimum
level
(4 mA in example)
. Then set the offset add/subtract switch
(S1)
,
the coarse offset switches
(S2-S5)
and the fine offset control
(front,
right)
to obtain the readout corresponding to this minimum current
value
(500 in the example)
.
7. Adjust the input signal current to its maximum value to see if the proper
readout is obtained
(1500 @ 20 mA in the example)
. If the readout is
slightly off, adjust the fine span
(front, left)
to obtain the true reading.
Then, recheck the reading at minimum input current
(4 mA)
and
readjust fine offset
(front, right)
if necessary. Repeat the maximum and
minimum readout adjustments until the unit displays the proper readout
at both extremes.
8. Set decimal points as desired using the three switches on the side of the
case and replace the front panel sealing screws. The unit can now be
installed.
APPLICATION EXAMPLES
Example 1:
An APLCL is to be calibrated to match a flow transducer whose
output is 10 mA @ 0 GPM and 50 mA @ 1375 GPM.
READOUT SPAN (RS) = 1375
−
0 = 1375
SWING CURRENT (IS) = 50 mA
−
10 mA = 40 mA
ADJUSTMENTS (Refer to the transfer curve below)
A Null the unit to zero readout @ 0 current per Steps 1, 2, and 3 of the
calibration procedure.
B Set the coarse and fine span adjustments to get a readout of 1375 @ 40
mA per Steps 4 and 5.
Note: With the full standard swing of 40 mA, the
coarse span switch reference markings can be used to determine
settings as follows:
S7 ON (1050) + S9 ON (260) = 1310 Span set with switches.
375 (needed) - 1310 (with SW’s) = 65 w. fine span adj.
C Set offset to readout 0 @ 10 mA per
Step 6.
Note: The readout observed
when the 10 mA min. current is first
applied can be used to determine the
offset switch settings.)
In this
example the readout will be
(+)
344
when the 10 mA min. current is first
applied. Applying -344 offset then
reduces the readout to zero @ 10 mA.
D Check readout at max.
(50 mA)
and
min.
(10 mA)
and fine tune
(tweak)
as required per Step 7.
Example 2 (Negative Slope):
A level measuring device puts out 6 mA when
a storage tank is full and 15 mA when the tank is empty. The APLCL is to
readout 90.0 tons at full tank and zero when empty.
READOUT SPAN (RS) = 900
−
0 = 900 (Disregard Decimal Points)
SWING CURRENT (IS) = 6 mA (@ max rdg) - 15 mA (@ min rdg) = -9 mA
In this case, the signal current is reverse [Term 3
(-)
with respect to Term
5
(+)
] causing the readout to go
“down” (increasingly negative)
as the
negative current increases.
ADJUSTMENTS
A Null the unit per Steps 1, 2 and 3.
B Set slope of transfer curve with span
adjustments to get readout of -900 @
-9 mA per Steps 4 and 5.
C Move transfer curve up by applying
(+)
offset per Step 6 until readout is
+900 @ -6 mA.
D Check extreme readings per Step 7,
0 readout @ -15 mA and +900
readout @ -6 mA.
Set D.P. Switch S1 and replace front
panel sealing screws.
Example 3 (± Display):
A differential pressure transducer has a range of
±1500 PSI with a 4 to 20 mA output
(-1500 @ 4 mA, +1500 @ 20 mA)
.
READOUT SPAN (RS) = +1500
−
(
−
1500) = 3000
SWING CURRENT (IS) = 20 mA(max) - 4 mA(min) = 16 mA
Note: Since the display readout is limited to 1999 numerical indication,
the full READOUT SPAN of 3000
cannot be obtained during zero
based span adjustment. However,
dividing both the READOUT SPAN
and SWING CURRENT by two, i.e.
1500 readout @ 8 mA, allows the
span adjustment to be made for the
proper transfer curve slope.
ADJUSTMENTS
A Null the unit per Steps 1, 2, and 3.
B Set transfer curve slope with span
adjustments per Steps 4 and 5, to get
a readout of +1500 @ 8 mA.
C Apply
(-)
offset per Step 6 to get a
reading of -1500 @ 4 mA.
D Check min. and max. extremes and
tweak if required to get desired
readout @ 4 and 20 mA per Step 7.
MAX/MIN CHK. OFFSET SP
AN SETTING ZERO
