Oyster-10, V1.9 11/09
3
pH Overview
pH is a unit of measure (ranging from 0 to 14pH) indicating the degree of acidity or
alkalinity of a solution. pH tests are the most commonly performed measurements in water
analysis, using the hydrogen ion activity of a solution to determine acidity or alkalinity.
Solutions with a pH less than 7 are considered acidic, solutions with a pH higher than 7 are
known as bases, and solutions with a pH of exactly 7 are considered neutral.
The pH scale is logarithmic, so, for example, if sample A is 1 pH less than Sample B, this
means that Sample A is 10 times more acidic then Sample B. A difference of 1 pH
represents a ten-fold difference in acidity.
ORP/REDOX Overview
ORP stands for
O
xidation
R
eduction
P
otential and represents the oxidizing or reducing
nature of a solution. The overall “reactive” tendency of a solution can be determined from
ORP measurements. ORP testing is becoming more common in waste-water and water
treatment measurement regimens. The unit of measurement for ORP is mV.
Calibration and Operation
pH Calibration (NOTE: ORP does not require calibration)
Before accurate measurements can be obtained, it is necessary to calibrate the meter with
the electrode. Typically, pH 7 is calibrated first and then one or more of the other buffers
(shown in the left column in the table below). The meter automatically recognizes all of the
buffers listed in the table below and calibrates itself to the buffer’s pH value.
1. Connect the pH electrode to the meter’s BNC connector input.
2. Open the top half of the meter to an appropriate viewing position.
3. Power the meter with the
POWER/CAL
button.
4. Place the electrode into a buffer solution (4, 7, or 10).
5. Press and hold the
CAL
key until the CAL display icon appears.
6. During calibration, CAL is shown on the display followed by SA
(saved).
7. When calibration is complete, the meter automatically displays END and returns to
normal operation mode. Repeat steps 1-5 as necessary. Note that it is not necessary
to turn the meter off and then back on again to calibration another reference.
Important calibration note:
Since pH electrodes can vary in performance, this meter
includes a special feature that displays an electrode’s slope in %. For example, if two
electrodes (one new and moderately used) are calibrated in the same pH 4 solution they’ll
both display pH 4 on the meter. This is because the meter will accept one pH value for a
range of measurement values as shown in the table below (in this case, any reading
between 1.68pH and 6.50pH will be accepted as 4pH calibration). The meter then displays
the slope % for the electrode under test. If the slope % is below 75% or above 125%, the
electrode must be replaced. A slope of 100% is ideal.
pH reading
Range of measurement
1.68
0.00 to 1.68
4.00
1.68 to 6.50
7.00
6.50 to 7.50
10.00
7.50 to 12.45
12.45
12.45 to 14.00
