[Discussion] dead space is doing more work than we give it credit for
dead space is doing more work than we give it credit for. Not a hot take, just something I have not seen said plainly here.
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.
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.
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
If two or three other people have done the same thing we might actually learn something. Alone it is an anecdote.
best — the order this archive was captured in
Aim the stream down the wall of the vial rather than into the cake. Reconstitution is a solubility problem, not a mixing problem, and mechanical shear does nothing helpful.
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.
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
write the concentration on the vial in marker, that is the whole post
units are a volume on a U-100 syringe, not a dose
do the arithmetic before you reconstitute, not while holding a syringe
If your calculation produces a fraction of a unit, the concentration is wrong for your dose. Change the water volume until the numbers land on whole units.
Yes. Writing 5mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.
Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.
The whole calculation, once, with real numbers.
You have a 20mg vial. You add 2mL 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 0.5mg you need 0.5 divided by 10 millilitres, which is 5 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.
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.
Show the arithmetic — where did the unit figure come from?
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.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.
Careful — that arithmetic is wrong. 15mg in 3mL is 5mg/mL, so 1.5mg draws to 30 units, not what you wrote.