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I'm dispensing around 1uL of volume with 1% accuracy. Although I'm confident in my pump, I'd like to be able to routinely confirm that I'm getting the exact volume I'm requesting every time.

How can I reliably measure volume at this small scale? I've tried a precision scale, but the evaporation rate for the fluid is quick and I'm losing non-negligible volumes while waiting for the scale to settle.

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  • $\begingroup$ Weigh the delivery side? $\endgroup$ – Solar Mike Aug 1 '19 at 16:59
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    $\begingroup$ There's a product called the "Pipette Check-it" that might work for you: nextadvance.com/pipette-checkit (disclaimer I've done work for this company as a contractor) $\endgroup$ – Drew Aug 1 '19 at 22:25
  • $\begingroup$ @Drew, the Pipette checkit looks great actually, so close to what I need, but I'd need smaller volumes than 2um and finer resolution. This would work well as a first pass sanity check though. $\endgroup$ – GrantG Aug 5 '19 at 14:30
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This is a challenging measurement. In my opinion best done with a very good balance. Fully shielded from thermals and vibration, obviously. Use a small container with small opening diameter to limit evap to what will hopefully be under 10% of the final measurement (and we will correct for it more later). A clean 2mL glass bottle is good. Fill bottle halfway, with 1mL of a low-surface-tension liquid, like 50% alcohol (does not need to be same the liquid being pumped). Ensure process fluid is degassed. Submerge outlet tube (but do not let it touch the bottle!) and immobilize the tube. Small tube outside diameter better to reduce surface tension force. Building a small fixture and attaching it to the enclosure of the balance is a good idea to protect the delicate tube from vibrations/movement/keep it from touching bottle, and if you're going to need a large number of measurements, also to facilitate the process of changing the bottle. Close the door to balance enclosure and wait a little bit for everything to come to thermal equilibrium. Program the balance to go into a mode that continuously outputs unfiltered data, so that you can see the evaporation rate and background-subtract it. Do not "TARE" the balance at any point, do that in post-processing. Some programming is required to do it right. Pump a little to let the fluid flow also come to thermal equilibrium. stop flow, to read the evaporation rate. then restart the flow or dispense process, and take your measurements. No touching the balance or table at any point. Take note of the ambient temperature.

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    $\begingroup$ Thanks. This is a good technique and more or less what we were setting up to do before management changed everyone's priorities. Classic. Doesn't matter any more though. That was over a year ago, and I've found a new job with a new set of problems. $\endgroup$ – GrantG Jan 4 at 23:24

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