dead space is the most under-discussed thing on this board
dead space is the most under-discussed thing on this board, and I am aware this is a minority view on this board.
The whole calculation, once, with real numbers.
You have a 5mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 1.67mg/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 1.67 millilitres, which is 90 units on the barrel. Write "1.67mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
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.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Would rather be corrected in public than confident in private.
best — the order this archive was captured in
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.
Same. I got this wrong once, caught it because the units looked absurd, and have written on every vial since.
Careful — that arithmetic is wrong. 10mg in 3mL is 3.33mg/mL, so 2.5mg draws to 75 units, not what you wrote.
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
Are you accounting for the powder volume, or is that where the confusion is?
if the maths gives you a fraction of a unit, change the concentration
Agreed — units are volume. Once that lands, the rest of the arithmetic is one division.
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.
Bacteriostatic or plain sterile water?
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
a U-100 syringe reads 100 units to the millilitre, every time
Spent 21 weeks confused about this until somebody drew the table for me. 5mg, 2mL, 2.5mg/mL, 0.75mg is 30 units. That is the whole thing.
label the vial with the date as well as the concentration
bacteriostatic water has the preservative, sterile water does not
The arithmetic in full: concentration is mass over volume, so 5mg into 1mL gives 5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 5mg is 5 divided by 5 millilitres, which is 100 units.
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.
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.
twice as much water halves the concentration and doubles the units for the same dose
Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.
- 1Why "more water" keeps confusing people, including me for a month. The vial…8 comments in this branch · started by u/sleep_apnea_sam
- 2The lyophilised cake displaces a small volume, so the true concentration is…6 comments in this branch · started by u/emil_zamora