Do you ever get confused with the terms “TC” or “TD”? A pipette is either calibrated to “TC” or “TD” and abbreviations are normally printed on the side or bulb of the pipette.
TC or TD abbreviated for “to contain” and “to deliver” respectively. In a ‘TC’ marked pipette, the contained quantity of the liquid corresponds to the capacity printed on the pipette,
While in ‘TD’ marked pipette, the delivered quantity of liquid corresponds to the capacity printed on the pipette. The drainage holdback error (i.e. the amount of liquid required to wet the inner surface of the pipette and the remaining liquid left at the tip of pipette after it has been used) has already been taken into account during calibration at a standard temperature (which is generally 20°C). We can understand this in a simple following equation:
The volume of TD pipette = Volume of TC pipette + drainage holdback error
The double rings on the upper end of pipette indicate that the pipette is a “blow out” type/TC pipette and should be blown using a rubber bulb. Absence of the ring indicates that pipette is a TD pipette which means the still left liquid in the pipette has already been taken into account during calibration and shall not be expelled by blowing out.
The formal difference between the blow out pipettes “adjusted to deliver” and pipettes “adjusted to contain” may be small (since blowing out the remaining liquid is possible in both cases), but it does make a difference when it comes to the achieving the accurate result of an experiment.
According to International Standards Organization, “ex” is used to indicate that the pipette has been adjusted “to deliver” and the marking “in” is used to indicate that the pipette has been adjusted “to contain”.
TD pipettes are much more common than TC pipette. Most typical graduated pipettes or bulb pipettes are usually calibrated to deliver (TD), whereas capillary pipettes are adjusted to contain (TC).
Further information can be found in ISO 648 Laboratory glassware – Single-volume pipettes and ISO 835 Laboratory glassware – Graduated pipettes.