The Effect of Barometric Pressure on Pipettes

According to the data, air displacement pipettes (which are the type most commonly used in labs) under-delivered by 1 to 30 percent at Mount Washington compared to the data recorded in the control lab. The errors measured were greatest at the lower volumes, and errors increased when the pipette was adjusted to the lowest end of its operating range.

Table 1 shows the data taken with fixed-volume pipettes. Here, the one-microliter pipette under-delivered by 9.8 percent on average. This is compared with the 200-microliter pipette, which under-delivered by 1.3 percent on average.

Table 2 shows the data taken with variable-volume pipettes at their upper- and lower-range target volumes (100% and 10% of the nominal volume, respectively). The data show a dramatic error rate when the variable-volume pipettes were set to dispense less than full-scale volume, and this error also grew as smaller volumes were tested. For example, volume error of over 30 percent was observed when using the two-microliter pipette at the low end of its volume range.

THE CONCLUSION

The data clearly show a consistent shift in delivered volume as a result of lower barometric pressure. In light of this fact, it is important to verify pipettes regularly in the environment in which they are used — especially if the pipettes are serviced and/or calibrated by an outside calibration laboratory located at a different elevation. When pipettes are sent to a calibration lab that is either higher or lower in elevation, even newly calibrated pipettes would likely deliver inaccurately once back in the laboratory, producing inaccurate data.

Fortunately, the data show the magnitude of variation to be relatively consistent, which would indicate that laboratories can determine and rely upon a correction factor. For example, the 10-microliter pipette under-delivered by 2.6 percent and 2.8 percent in replicate tests. Laboratories could compensate for this in one of two ways:

1. Adjust the internal mechanism of the pipette for the specific environmental conditions

2. Adjust the delivery setting. In this case, because the pipette under-delivered by an average of 2.7 percent, setting the pipette to deliver 10.27 microliters would deliver an actual volume of 10 microliters.

Environmental conditions fluctuate within and between laboratories. Regular, on-site verification of liquid handling instrumentation is imperative to account for and avoid the potential for consequential error and to ensure data integrity.