Calibration - pH and ORP
Instruction Manual
February 2014
LIQ_MAN_6081-P
32
Calibration - pH and ORP
from a point as close as possible to the sensor.
7. Press MENU. The main menu appears. Choose Calibrate.
8. Choose pH.
9. Choose Standardize.
10. The top line shows the present reading. Use the arrow keys to change the pH reading in the
second line to match the pH reading from the referee instrument.
11. The screen at left appears if the entered pH was greater than 14.00 or if the mV offset
calculated by the transmitter during standardization exceeds the reference offset limit
programmed into the transmitter. The display then returns to step 10. Repeat the
standardization. To change the reference offset from the default value (60 mV), see section
8.4.
12. If the entry was accepted the display returns to step 9.
13. To return to the main display, press MENU then EXIT.
10.5
Procedure–Entering a Known Slope Value
1. If the electrode slope is known from other measurements, it can be entered directly into the
transmitter. The slope must be entered as the slope at 25°C. To calculate the slope at 25°C
from the slope at temperature t° C, use the equation:
slope at 25 °C = (slope at t°C)
Changing the slope overrides the slope determined from the previous buffer calibration.
2. Press MENU. The main menu appears. Choose Calibrate.
3. Choose pH.
4. Choose slope.
5. The screen at left appears briefly.
6. Change the slope to the desired value. Press ENTER.
7. The slope must be between 45 and 60 mV/pH. If the value entered is outside this range, the
screen at left appears.
8. If the entry was accepted, the screen at left appears.
9. To return to the main display, press MENU then EXIT.
10.6
ORP Calibration
10.6.1 Purpose
1. For process control, it is often important to make the measured ORP agree with the ORP of a
standard solution.
2. During calibration, the measured ORP is made equal to the ORP of a standard solution at a
single point.
10.6.2 Preparation of ORP standard solutions
ASTM D1498-93 gives procedures for the preparation of iron (II) - iron (III) and quinhydrone
ORP standards. The iron (II) - iron (III) standard is recommended. It is fairly easy to make, is not
particularly hazardous, and has a shelf life of about one year. In contrast, quinhydrone standards
contain toxic quinhydrone and have only an eight-hour shelf life.
Iron (II) - iron (III) standard is available from Rosemount Analytical as PN R508-16OZ. The ORP of
the standard solution measured against a silver-silver chloride reference electrode is 476±20mV
at 25°C. The redox potential is -476±20mV at 25°C.
10.6.3 Procedure
1. Press MENU. The main menu screen appears. Choose Calibrate.
2. Choose ORP.
3. The top line shows the actual ORP or redox potential (Live). Once the reading is stable, change
the number in the second line to the desired value. Press ENTER.
4. The screen on the left will appear briefly.
5. The display returns to the Cal Sensor screen. Press EXIT. Choose the other sensor and repeat
steps 2 through 4.
