pH/ORP calibration
As a rule, follow the procedure shown in the pH meter’s instruction manual. These procedures will vary depending on whether the meter is a simple type with manual adjustments, a microprocessor type or a pH transmitter.

Frequency of calibration
The frequency of calibration is a function of many factors. These factors include:

1. The accuracy required by the application.

2. The value of the off-specification product versus the cost of calibration.

3. The coating or abrasive nature of the application.

4. The stability of the pH electrode and pH meter as a system.

The frequency of calibration is really determined by experience. At a new installation, calibration might initially be checked every few hours or shift with the calibration changes noted in a log. As a pattern of longer stability is found, the time between calibration checks can be increased to once a day or once a week. Although the frequency of calibration is solely the responsibility of the user, once a week is the longest recommended interval between calibrations.

System calibration concepts
The pH electrode and the pH meter should always be calibrated as a system. Electronic calibration of a pH meter with a pH signal simulator checks the meter only and does not correct for imperfections of the pH electrode. Even if perfect when new, the performance of pH electrodes varies with time, usually in an unpredictable way. When changing electrodes or connecting an electrode to a different pH meter, re-calibration must be performed.

Single point calibrations
Single point calibrations involve the use of one pH buffer. They are the easiest to make but can provide misleading results. They should only be used for quick checking from time to time.

Two point calibrations
As their name implies, 2 point calibrations use 2 pH buffers: for example, buffers 7.00 and 4.00 or buffers 7.00 and 10.00. Two point calibrations correct for the pH electrode’s offset and span errors. Since both the offset and span vary with time, the 2 point method is the preferred one.

Grab sample calibrations
The grab sample calibration method is used when it is difficult or undesirable to remove an electrode from a system. This method involves obtaining a sample of the liquid being measured and noting the meter’s reading at that time. The sample’s reading is obtained by use of a calibrated lab or portable meter and that reading is compared to that of the on-line meter. The online meter is adjusted by the difference between the readings. It is important to use the difference between the readings because the system’s reading may have changed in the intervening time. It is important that the sample being measured by the lab meter be at the process temperature or erroneous results may occur.

Calibration procedures
Stepwise calibration procedures are noted in the pH meter’s instruction manual. The following suggestions will help make calibrations as accurate as possible:

1. Before placing the electrode in a new buffer, use an absorbent paper towel or clean absorbent cloth to blot, not wipe, off any liquid that clings to the electrode. This action will minimise carry-over that could contaminate the buffer.

2. Always use fresh buffers. Safely dispose of the buffers after they have been used for calibration. Do not return them to their bottles; this action could contaminate the buffers.

3. Stir the electrode in the buffer to make certain that the fresh buffer quickly reaches the electrode’s measuring surface.