Calibration

Calibration is a marking a scale on a measuring instrument. This involves establishing the relationship between indications of a measuring instrument and standard or reference quantity values, which must be applied.

Calibration methods will depend on the equipment being used, but they usually involve use of a standard. Calibration is essentially the process of comparing measurements. One measurement is of a known size or correctness, e.g. a reference standard. The other is on the device or instrument being calibrated.

Calibration is important for some types of measuring apparatus. Volumetric glassware is calibrated during manufacture and marked with the maximum error (e.g. ±0.05 cm3). However, a thermometer might give precise readings, i.e. very close together, but may be several degrees away from the true value because it has not been properly calibrated.

Calibration is required for:

• Any instrument that has moving parts, such as an analogue ammeter or voltmeter, because movement can upset the balance of the instrument

• Anything that can be affected by temperature change

• Electronic equipment, as the performance of various electronic components can change over time; for example, the glass membrane in a pH electrode can be affected by deposits (dirt, oils and grease, protein, inorganic materials) and should be calibrated before each use.

To calibrate your 25 mL pipet, use a clean pipet and water for which the temperature is known and constant (temperature-equilibrated water). Using an analytical balance, weigh a clean 125 mL Erlenmeyer flask with a rubber stopper in the neck and record the mass.

Calibration of a thermometer can be done by using two set points: freezing point and boiling point of water. Mix ice and water and leave it for a few minutes; the thermometer should read 0ºC. Then place the thermometer in boiling water; the thermometer should read 100ºC. Any deviation from these values should be allowed for when taking temperature readings with the thermometer.

Rinse the electrode with deionized water and blot dry using a piece of tissue (Shurwipes or Kimwipes are available in the labs). 3. Place the electrode in the solution of pH 7 buffer, allow the display to stabilize and, then, set the display to read 7 by adjusting cal 1. Remove the electrode from the buffer.

Press and hold the "RE-ZERO" key to enter the calibration mode. Then you need to release the "RE-ZERO" key when the balance displays either "CAL or CAL 0". Once this is done, press the "MODE" key to change the calibration mass value. Enter the calibration mass value using the following keys: "MODE" key is used to move digit to enter a number; "RE-ZERO" key is used to change a number.  Verify that there is nothing on the weighing pan. Finally, press the "RE-ZERO" key.  The balance measures the zero-point.

Drain water slowly until the meniscus is at the 0.00 mL mark. Touch the tip of the burette to the side of a beaker to remove the drop hanging from the tip. After about a minute, to allow for drainage, make an initial reading of the meniscus, estimating the volume to the nearest 0.01 mL. Record the initial reading.

Direct comparison to reference standards ( telephone/ online)

Totalizer method - use a signal generator - when using the stopwatch's start/ stop button to open and close the counter's gate, this method partically eliminates human reaction time and therefore has a lower measurement uncertainty than the direct comparisonn method