In general electronic instruments are used to measure chemical properties of water. As such, they are indirect methods of measurement, and therefore must be calibrated. This is essentially the same as synchronizing watches; each watch may tell time differences accurately, but the true time requires resetting or calibrating each watch to an agreed-upon standard. To calibrate chemical probes/instruments, two or more samples of known concentration are measured, and then a calibration factor is entered into the device (or applied to the final result) to adjust the output to agree with standard values. To save time, we will calibrate only the pH meter while out in the field.