Rosemount 2700 Instruction Manual Download Page 17

Page: 17 / 94

3-2

MODEL 2700

SECTION 3.0

CALIBRATION

Do Not attempt to standardize sensors in any other way
than as described in this manual.

pH Calibration

When the sensor selected is a pH sensor, there are three
possible ways to calibrate. The first of these is a single
point calibration. Here, the user manually enters a buffer
value, then places the sensor into the buffer and presses
the return key.

The second type of calibration available is a two point
manual calibration. This calibration is performed the same
way as the single point described above, but for two val-
ues. Generally a buffer near the lower end of the operating
range (“Low Buffer”) is done first. Then, a buffer near the
high end of the range (“High Buffer”) is done next.

The Solu Cube Analyzer also employs an “Auto-Buffering”
feature designed to simplify pH buffer calibration. The
“Auto Buffering” technology allows the Solu Cube pH
Analyzer to identify any of five standard pH buffers and
calibrate itself appropriately. In operation, the user simply
exposes the pH electrodes to a pH buffer selected from
the table below, and instructs the analyzer to perform
either a “Low Buffer” or “High Buffer” operation. The Solu
Cube Analyzer then proceeds to monitor the electrode
inputs for a stable reading. Only when the electrode signal
is absolutely stable can a maximum accuracy calibration
be made. Once the required stable input is obtained, the
Solu Cube Analyzer identifies the selected buffer based
upon the electrode signal, and calculates the precise
buffer pH value based upon the buffer sample tempera-
ture.If the user has selected the “Low Buffer” function, the
Solu Cube Analyzer completes the operation by setting its
electrode offset value (commonly referred to as standard-
ization). If the user has selected the “High Buffer” function,
the operation is completed by setting the electrode gain
(slope).

ORP Calibration

A one-point calibration may be performed on ORP
Sensors. To calibrate an ORP sensor, first place it into
the ORP Standard Solution for 15 minutes or more. From
the “Program” menu choose the appropriate Sensor
Number. Next, choose “Simulator” (“Temperature”can
be chosen for a temperature calibration). Next, the
instrument will ask you to “Connect Generator To
Electrodes.” Here, the ORP Standard Solution being
used is acceptable (a signal generator may be used
instead of the solution, but is not necessary). Press
“Enter” to begin calibration. When finished the unit will
now show a screen that says “Adjust mV Reading To
Agree With Generator Output”. Here, use the arrow keys
to adjust the resulting reading to match the known ORP
Standard Solution (or mV generator, if that is being
used). Next, press “Enter” to complete the calibration.

(1) One type of commonly used ORP standard solution
is the saturated quinhydrone solution. This can be made
quite simply by adding a few crystals of quinhydrone to
either pH 4 or pH 7 buffer. Quinhydrone is only slightly
soluble, so just a few crystals will be required. The
resulting solution is yellow-colored. The resulting poten-
tials, measured with a clean platinum electrode and sat-
urated KCl/AgCl reference electrode, should be within
±20 millivolts of the value shown below. Solution tem-
perature must be noted to insure accurate interpretation
of results. The ORP value of saturated quinhydrone
solution is not stable over long periods of time; there-
fore, these standards should be made up fresh each
time they are used.

CAUTION

The solution used during the following
check is an acid and should be handled
with care. Do not let the solution come in
contact with skin or clothing. If contact
with skin is made, immediately rinse with
clean water.

(2) A second ORP standard solution can be prepared
from the following recipe: Dissolve 39.2 grams of
reagent grade ferrous ammonium sulfate, FE(NH4)2

(SO4)2 • 6H2O, and 48.2 grams of reagent grade

ferric ammonium sulfate, FeNH4(SO4)2 • 12H2O, in

approximately 700 milliliters of water (distilled water is
preferred, but tap water is acceptable). Slowly and
carefully add 56.2 milliliters of concentrated sulfuric
acid. Add sufficient water to bring the total solution
volume up to 1000 ml. This standard ORP solution,
although not as simple to prepare as the quinhydrone
recipe, is much more stable, and will maintain its milli-
volt value for approximately one year when stored in

pH (25

Volume

Part No.

4.01

One Pint (473 mL)

905-3506

One Pint (473 mL)

905-3501

7.00

One Pint (473 mL) of Concentrated

905-3581

Solution to be Diluted with
Distilled Water. Makes 25 Pints
(11.8 Liters)