[Discussion] we are measuring bacteriostatic water at the wrong time and calling it noise
we are measuring bacteriostatic water at the wrong time and calling it noise. It is the sort of thing everyone half-believes and nobody writes down.
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable. Plain sterile water has no preservative.
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
Tell me where this is wrong. That is the useful part of posting it.
best — the order this archive was captured in
The whole calculation, once, with real numbers.
You have a 10mg vial. You add 1mL of bacteriostatic water. Concentration is mass over volume: 10mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 1.5mg you need 1.5 divided by 10 millilitres, which is 15 units on the barrel. Write "10mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
Why "more water" keeps confusing people, including me for a month.
The vial contains a fixed amount of peptide. Water changes nothing about that amount. What it changes is concentration, and concentration is the divisor between the dose you want and the volume you draw.
So doubling the water halves the concentration and doubles the units for the same dose. Nothing is diluted in any sense that matters; you are simply reading a bigger number off the same barrel. Pick the water volume that puts your usual dose on a whole number of units and life gets easier.
Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.
Same. I got this wrong once, caught it because the units looked absurd, and have written on every vial since.
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
Agreed — units are volume. Once that lands, the rest of the arithmetic is one division.