I worked in biotech for about 12 years, spending part of that time as a process engineer. The workhorse of many biotech and other laboratories is the hand pipettor, used to reproducibly measure and transfer small volumes of liquid from test tube to test tube. The Pipetman style of pipettor, originally and still offered by Gilson of France, consists of a spring-loaded piston plunger connected to a "micrometer" screw calibrated in microliters.
Twist the knob until the desired volume appears in the window, and the piston displacement is adjusted accordingly. The thumb-operated pipettor will then serially aspirate and dispense the desired volume without further adjustment. This design eliminated the need to measure liquid every time it is aspirated using a graduated disposable glass or plastic pipette operated by a vacuum bulb or motorized air pump.
For years, the range of pipettors available was .1 microliters up to 1 milliliter. For volumes of 10mL, a scientist would have to make 10 1mL transfers, or go back to the graduated "serological pipette with motorized air pump attached. I once was auditing a process where 12mL of a reagent was added to a solution using 12 transfers of 1 mL each. I asked the lab technician how she was sure that she made 12 transfers.
After responding that she "just knew," she paused and said, "Uh-oh, did I just add the third or fourth mL?" Eventually Gilson produced a 5mL pipettor, with a slightly different design than the other models. Since the plunger is operated by a thumb, the only ergonomic way to increase the displacement is to significantly increase the piston diameter. The resulting 1-inch-diameter piston required a bell housing, which was split in the middle and joined by threads.
We were ecstatic that we could now get a 5mL pipettor and bought several of them: the Gilson Pipetman Ultra U-5000. Several months later that I noticed lab techs using the old serological pipettes again. When asked why, they told me that they "didn't trust" the 5mL Pipetman because of severe volumetric errors. I tested a few to see if I could reproduce the problem. I was horrified when the volumes were so far off that it was clear to the naked eye, as much as 30 percent to 40 percent error. It didn't take me long to figure out the problem.
The piston "wiper" was an o-ring (as they always are), except in this design the o-ring was sandwiched (literally) between the upper and lower half of the bell housing, such that under-tightening the threads allowed air leakage, and over-tightening compressed the o-ring, forcing it inward and impinging on the piston.
To make things worse, the line between under- and over-tightening was very thin. I tried a temporary fix of tightening the threads just until I got accurate measurements and then placing a dab of cyanoacrylate glue on the joint. I went back later to retest and found that the piston was sticking or loose on some even though the glue dot was still intact. I took all of the Pipetman Ultra pipettors out of the lab, and threw them in a box under my desk. Millions of dollars in high-tech gene expression experiments should not be at the mercy of a poorly made pipettor.
By this time, Rainin (a US firm selling pipettors, now owned by Mettler-Toledo) had come out with their own 10mL pipettor: the P-10mL. I called their technical support line and explained the problem. The tech support guy on the other end confirmed that their design cannot possibly squeeze the o-ring against the piston to a variable extent based on slight differences in thread engagement. Our local sales rep dropped off a loaner P-10mL a few days later. I took it apart, agreed that the design was superior, and got approval to buy them by the end of the day. We haven't had a problem with them yet.
So why the obviously bad design by Gilson? I'm guessing it was a rush to market. The original Gilson (volumes <1mL) is the most common design, still present in labs all over. Now it's even copied by the Chinese. Though there are other designs that I prefer for ergonomics, or with an indexed micrometer, the original Gilson is inexpensive and widely regarded as "tried and true." Too bad the Pipetman Ultra U5000 didn't live up to the reputation.
This entry was submitted by John McDonald and edited by Rob Spiegel.
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