Instruments require careful calibration to produce any reliable data. Most instruments rely on some sort of measurement device that outputs a voltage or current that is recorded. This voltage or current must be calibrated to the physical property being measured.

For example, pH meters which measure voltage are calibrated by using buffer solutions of known pH. Two points usually are necessary - one to set the zero point of the voltage (which voltage is pH 7)(pH meters have a knob called calibration for this) and a second to set the range (pH units/volt) (pH meters have a knob called temperature or range for this second setting).

Temperature sensors are also calibrated at two points, frequently by using phase transitions. For temperature sensors used near room temperature, for example, the sensors may be immersed in boiling water to calibrate a high point and in melting ice to calibrate a low point. Melting metals may be used to calibrate thermocouples at high temperatures.

Sensors may also be compared to other carefully calibrated sensors. For instance, a thermistor reading may be compared to the reading from a carefully calibrated alcohol thermometer by immersing both in the same container until readings of both are constant.

Comparisons frequently are used to calibrate on-line sensors in industry. A sensor may be calibrated in the laboratory and then taken to the plant for side by side measurements with an on-line sensor. This method frequently is used for temperature sensors. Alternatively, a sample may be withdrawn from the location where a sensor monitors and is taken to a laboratory where a carefully calibrated sensor measures the same property. For instance, pH may be measured continuously on-line. Samples are withdrawn daily and measured in the laboratory using a pH meter that is calibrated carefully with buffers. The values then are compared to assure that the on-line electrode is reporting properly. On-line electrodes sometimes collect a coating which causes the pH to drift.