Precision and Uncertainties for Common Lab Equipment

When you record a scientific measurement, the last digit that you record is understood to have some uncertainty, and to be your best estimate. When reading non-electronic devices such as rulers, thermometers, and glassware, the general rule of thumb is to "read between the lines"! This means that you can estimate one more digit or decimal place than the device is marked. But this rule does NOT APPLY to electronic equipment (such as a balance or electronic thermometer) which gives you a direct digital readout. For these digital devices, your teacher will provide you the precision of the instrument.

The following uncertainties apply to careful measurements made by a trained observer:

Length (common metric rulers): (or 0.1 mm )
Mass (electronic balances): always +/- one unit in the last digit. This means that a common centigram balance is +/- 0.01 grams; a milligram balance +/- 0.001 grams.

Volumetric Glassware

  • 10 mL graduated cylinder: (always record to 2 decimal places)
  • 25 mL graduated cylinder: (always record to 1 demical places)
  • 100 mL graduated cylinder: (always record to 1 decimal place)
  • 500 mL graduated cylinder:
  • 50 mL buret: +/- 0.02 mL (always record to 2 decimal places)
  • 10 mL graduated pipet: (always record to 2 decimal places)
  • Fixed volume pipets (glass): +/- of the capacity (Ex: )
Beakers and Flasks: Approximately of the capacity. (But of course, you would never use one of these to measure a precise amount of liquid, would you?)

Thermometer

  • (alcohol or mercury):
  • TI CBL temperature probe:

pH Measurements

  • pH paper: +/- 1 pH unit ( pH paper gives a "quick and dirty" estimate)
  • TI CBL pH probe: +/- 0.1 pH units (even though it reads out to 0.01 ).

pressure

  • TI CBL pressure probe: (even though it may read out to decimal places)