pH sensor is designed to cover pH range from 0 to 14. You can use these value (pH0=4mA and pH14=20mA) to get a ballpark value but these will not be very accurate as each sensor has unique tolerances and efficiency tends to degrade over time.
To achieve greater accuracy please use two point calibration. Unlike traditional 4-20mA output sensors, it may be a little tricky as there are no extreme value pH solutions available (i.e. pH0 and pH14) so the calibration points must be any two of the following: pH4, pH7, pH10. To make the task easier always use pH7 as one of the point.
The pH7 point is important, as it is “neutral”. In an ideal world you would always get 12.0mA in pH7 solution but in real life acceptable output for brand new sensor in pH7 is between 11.0mA and 13.0mA. Calibration in pH7 also gives you offset value. OFFSET [mA] = pH7_output – 12.0 Acceptable offset is ±1.0mA, if you observe larger offset, the may be a need to refill reference solution and replace the salt bridge.
The second point would depend on your process. If you are operating mainly in high pH use pH10 point, alternatively if low pH is your main interest – use pH4. In case where you do not know application preferences – we recommend to use pH4 over pH10. The acidic nature of pH4 will help to clean sensor while in calibration.
The theoretical output should be 3.43mA lower for pH4 (or higher for pH10) compared to the real output in pH7. Using these two points you can calculate the efficiency: EFFICIENCY (%) = (pH7_output – pH4_output) / 3.43 Brand new sensor typically has an efficiency over 90% range, but may be as low as 83%. Over time the efficiency is expected to drop and once it reaches 75% or less the sensor may require maintenance. Cleaning is usually enough to restore efficiency, but if the number is still low replacing the salt bridge may help.