30
5 - CALIBRATION
Martek Instruments, Inc.
Once the displayed voltage has stabilized,
press the
SELECT
key. The display will
prompt with the following message:
Enter the raw conductivity value of the
reference solution by pressing the
UP
or
DOWN ARROW
key. You can then move
forward to the next digit by pressing the
SELECT
key and move back to a previous
digit by pressing the
EXIT
key. The position
of the decimal point will vary when other
cell constants are used.
NOTE: If the solution is not flowing through the
cell, the voltage displayed may slowly start in-
creasing due to polarization. Care should be
taken to press the
SELECT
key as quickly as
possible after the solution has been placed in the
reference cell and as soon as the voltage appears
to have stabilized.
Once the last digit has been entered, the
Mark 22 will automatically return to the cali-
bration menu:
and the "pH" option will be flashing.
Therefore, unless solution temperature in
your conductivity reference cell is exactly
25
o
C, you must measure the solution tem-
perature and apply the following formula:
Solution Conductivity @ 25
o
C X [1 + .02(T - 25)] =
Raw Conductivity
For example, let’s say you are using a ref-
erence solution with a certified conductivity
value of 30 microsiemens/cm @ 25
o
C. After
measuring the solution with a thermometer,
you determine the temperature of the solu-
tion to be 19
o
C. If you input these numbers
into the formula stated above, the equation
will look like this:
30 uS/cm X [1 + .02(19 - 25)] = 26.4 uS/cm
After processing the equation your an-
swer should be 26.4 uS/cm. Again, this is
the raw conductivity value of the reference
solution and this is the value that you would
enter into the Mark 22.
To span the conductivity value, place the
reference solution in the conductivity refer-
ence cell. It is desirable that the solution be
flowing through the cell, but if this is imprac-
tical, fill the cell completely with solution and
seal the ends.
Now, enter the calibration mode and
choose the "COND" option. Next, choose the
"SPAN" option and press the
SELECT
key.
The display will prompt with the following
message:
Then will ask you to:
PLACE SENSOR IN
HI COND STANDARD
HIT KEY W/STABLE
+0.5120 VOLT
INP 5 DIGIT STD.
HI C = 0.0000 uS
TEMP
COND
pH
TIME
