A liquid class is only as trustworthy as the measurement behind it, and the eye is not a measurement. To know a transfer is accurate you need a number, and there are three quantitative ways to get one. Each has a regime where it shines and a regime where it struggles, so the skill is matching the method to the transfer.
Gravimetric: weigh it
Gravimetric measurement puts the dispensed liquid on an analytical balance and converts mass to volume using density. It uses the real liquid with no dyes, covers a wide volume range, and gives immediate feedback, which makes it the workhorse for tuning a class. Its limits: you must know the density, temperature affects it, and a balance weighs one channel at a time, so it cannot verify a whole 96-channel or 384-channel head at once. At very low volumes the balance resolution becomes the bottleneck.
Photometric: read a dye
Photometric measurement adds a dye and reads absorbance in a plate reader, using Beer's Law to relate absorbance to volume through a standard curve. Because each well reads independently, it can verify every channel of a multi-probe head at once, and it is the method that works for liquid-filled systems where weighing would miss a dilution effect. The trade-offs are choosing a dye that does not change the behavior of the liquid, a path length that gets unreliable below about 10 microliters, and the extra step of building a standard curve.
Fluorometric: excite a dye
Fluorometric measurement is the more sensitive cousin of photometric. Instead of subtracting transmitted light, it excites a dye and reads the light it emits, which is well suited to low volumes and does not depend on well geometry. It can use opaque plates, since emitted light is read directly. The costs are much the same as photometric, with more sensitive and more expensive dyes and readers, and the same care over dye choice.
Choosing between them
- Tuning a class across a wide range: gravimetric, for direct feedback with no dye.
- Verifying a whole multi-channel head at once: photometric, one read per well.
- Liquid-filled systems where a dilution must be caught: photometric.
- Very low volumes: fluorometric, for its sensitivity.
Pick the measurement that fits the transfer. A balance is honest but serial; a plate reader is parallel but needs a dye and a curve.