Instructional manual- HBPH-MK2
– UK-004
Page 5
Calibrating the Sensor
When you receive your sensor, it is not calibrated which is needed to provide a precise result. The sensor
calibration is made in the PLC, PC, display or were you translate your mA signal into a pH value. So, the
calibration is in principle a formula which calculate your pH value based on the mA signal. The parameters
in the formula are based on measurements done with two well-known solutions. Two pH buffer solutions,
7 pH and either 4 pH or 10 pH, and a DC milliammeter are required. The calculations below are done with
pH 7 and pH 10. The process is similar for other buffer pH values.
1.
Disconnect the red wire at the instrument or power supply and connect it to the multimeter (-)
black.
2.
Connect the multimeter (+) red to the instrument or power supply red wire output terminal.
3.
Rinse the probe and place it in 7 pH buffer solution. Allow the temperature of the buffer and probe
to stabilize at room temperature.
4.
Write down the reading on the multimeter as mA
pH7 (in the example called: ”
m
ApH7”). The
reading should be between 11 and 13 mA. If not, the probe is defective, or the salt bridge needs to
be replaced
5.
Rinse the probe and place it in 10 pH buffer solution. Allow the temperature of the buffer and
probe to stabilize at room temperature.
6.
Write down the reading on the multimeter as mA
pH10 (in the example called: ”
m
ApH10”).
7.
Reinstall
The pH value is calculated using this formula: pH= K * mA C
Example:
The two constants can be calculated like this for 7 and 10 pH solutions:
The noted currents for the two solutions in the example are: mApH7 =11.08mA and mApH10=14.47mA
K = (pH value1-pH value2) / (mApH7 - mApH10) =7 - 10 / 11.08 - 14.47 = -3/-3.39 =0.885
C = pH value1 - K* mApH7 = 7
–
0,885 *11.08 = - 2.81
The formula looks like this: pH = 0.885 * mA signal - 2.81
Checking the Sensor
The operation of the 2-wire, 4-20 mA sensor can be checked by a few simple measurements. Two pH buffer
solutions, 7 pH and either 4 pH or 10 pH, and a DC milliammeter are required.