4 ms·

Pipette skills improve rapidly if you practice with a microscale.

dekhn 6y ago
that's how we calibrated ours. turns out: most pipettes in the lab were miscalibrated, with 50+% error. Then it turned out our scale wasnt properly calibrated, so we had to replace that too.
dekhn 6y ago
I spent some more time thinking about this and wrote the following. Modern scientists move small amounts of biological materials using a tool called the pipette. Pipettes can work with very small amounts of liquid- down the microliter. When you're running a delicate experiment being able to deliver the precise amount of liquid is critical. Pipettes need to be calibrated. How do you calibrate a device that works with volumes of liquid? Volumes of liquid are hard to measure. Fortunately, water at STP (standard temp and pressure) has a known mass, so you attempt to draw 1 mL, and weigh it. 1 mL of water weighs 1 gram at STP (this is not a coincidence- it's by definition). OK so you're weighing 1 gram of water and adjusting the pipette's calibrator knob so that 1mL on the pipette weighs 1 gram. I guess that means your weighing scale needs to be calibrated, too. Huh. These sorts of scales aren't just "weigh some flour for baking", either. They have to be accurate to the hundredth of a gram, and have walls to avoid fluctuations due to air currents(!!!!) and minor temp changes. The scales are calibrated using calibrated test weights. Oh dear. Calibrated test weights? If you follow the turtles all the way down, you find that there is actually a tracebility chain from your calibrated scale back to one of the defined weights held by NIST, the NIST equivalents in France and Japan (they all share their weights). So you can actually calculate- using those weird rules of error propagation you forget in high school- the error of your scale as a product of the errors in that chain (often, knowing your error bars is more important than knowing the accurate answer). But that's not all. Those defined weights? They're obsolete. Le Grand K (the origin of the kilogram, still kept under lock and key) changed weight over time due to subtle metallurigcal details. The new definition of the standard is created by an obscure machine at NIST, just like the time standards. https://en.wikipedia.org/wiki/Kibble_balance https://en.wikipedia.org/wiki/Kibble_balance is the tool used to do it, and it depends on the NIST time reference. So, turtles all the way down until you get the rubidium fountain.

that's how we calibrated ours. turns out: most pipettes in the lab were miscalibrated, with 50+% error. Then it turned out our scale wasnt properly calibrated, so we had to replace that too.

dekhn 6y ago

I spent some more time thinking about this and wrote the following. Modern scientists move small amounts of biological materials using a tool called the pipette. Pipettes can work with very small amounts of liquid- down the microliter. When you're running a delicate experiment being able to deliver the precise amount of liquid is critical. Pipettes need to be calibrated. How do you calibrate a device that works with volumes of liquid? Volumes of liquid are hard to measure. Fortunately, water at STP (standard temp and pressure) has a known mass, so you attempt to draw 1 mL, and weigh it. 1 mL of water weighs 1 gram at STP (this is not a coincidence- it's by definition). OK so you're weighing 1 gram of water and adjusting the pipette's calibrator knob so that 1mL on the pipette weighs 1 gram. I guess that means your weighing scale needs to be calibrated, too. Huh. These sorts of scales aren't just "weigh some flour for baking", either. They have to be accurate to the hundredth of a gram, and have walls to avoid fluctuations due to air currents(!!!!) and minor temp changes. The scales are calibrated using calibrated test weights. Oh dear. Calibrated test weights? If you follow the turtles all the way down, you find that there is actually a tracebility chain from your calibrated scale back to one of the defined weights held by NIST, the NIST equivalents in France and Japan (they all share their weights). So you can actually calculate- using those weird rules of error propagation you forget in high school- the error of your scale as a product of the errors in that chain (often, knowing your error bars is more important than knowing the accurate answer). But that's not all. Those defined weights? They're obsolete. Le Grand K (the origin of the kilogram, still kept under lock and key) changed weight over time due to subtle metallurigcal details. The new definition of the standard is created by an obscure machine at NIST, just like the time standards. https://en.wikipedia.org/wiki/Kibble_balance https://en.wikipedia.org/wiki/Kibble_balance is the tool used to do it, and it depends on the NIST time reference. So, turtles all the way down until you get the rubidium fountain.