28
5 - CALIBRATION
Martek Instruments, Inc.
Depress the
SELECT
key to choose
"ZERO". The display will then prompt:
Then will ask the user to:
Since there is no solution in the reference
cell, the voltage displayed represents the zero
voltage set point of the reference cell. As men-
tioned earlier, the voltage values shown in
this manual are for example purposes only.
The normal voltage displayed when zeroing
the sensor should be approximately -1.00
Volt.). Once the voltage has stabilized, de-
press the
SELECT
key. The display will re-
spond with the following:
Input the 5-digit low (zero) conductivity
value by pressing the
SELECT
key. Once the
zero point has been entered, the Mark 22 will
automatically respond with the following
d i s p l a y :
and the "SPAN" prompt will be flashing.
5.2.2
Conductivity Span -
Calibration
There are two methods that can be used
to set the high (span) point for conductivity:
*
Electronic (dry) Method for calibration
*
Dynamic (wet) Method for calibration
.
The Electronic Method for Spanning
Conductivity
What sets the Mark 22 apart from other
calibrators is its ability to use alternative
methods for calibration (ASTM D5391-95) as
a substitute for standard reference solutions.
To do this requires precise resistance values
that can be traceable to NIST standards for
accuracy and reliability. Martek Instruments
provides a certified calibration plug with a
precise resistance value equal to 1.000
µ
S/
cm with each Mark 22. Additional "solution"
values (10.00 and 100.0
µ
S/cm) are available
from Martek. A precision resistance box with
an accuracy of at least + 0.05% may also be
used instead of the calibration plug.
In addition to a known resistance value,
the capacitance of the reference cell must be
known. The capacitance of the Martek Con-
ductivity Reference Cell is approximately 170
to 175 picofarads and this value is included
within the calibration plugs to compensate
for the Martek Reference Cell’s impedance.
[To determine the capacitance of other refer-
ence cells, consult the manufacturer or con-
nect an impedance meter to the dry cell to
determine its capacitance.]
NOTE: the following procedure is for a 1.000 uS/
cm solution.
First, remove the conductivity reference
cell that is hooked up to the Mark 22. If not
already connected, hook up the conductiv-
ity interconnect cable (P/N 603051) with the
span calibration plug (P/N 400549) to the
Mark 22. Next, return to the monitoring
mode by pressing the
EXIT
key. Place the
Mark 22 in the manual monitoring mode for
co ndu ctivity.
Now, press the
CELL
key. If not already
PLACE SENSOR IN
LO COND STANDARD
HIT KEY W/STABLE
-0.9998 VOLT
INP 5 DIGIT STD.
LO C = 000.00
µ
S
ZERO SPAN
CONDUCTIVITY
