10/90 tube graduation error.

[dgs]

Interesting neisel, thanks for that.

I think one of the problems we have here is that we don't seem to have any form of standard to work from, ie something that we know is 10.0mls capacity. The 2x 10ml pipettes I have are grade 2 (how this compares to a grade 1, I don't know)

From my tests the other day the 10ml pipette and the 25ml measuring cylinder seem to check out, so assuming the pipette is closest thing I have to 10mls, I used this to dispense 10mls of IPA into each of my 3 tubes.

I have a tube like the one neisel linked to and this gave 10mls.

The next tube is a conventional shape (no stem) but is taller and slimmer than normal, this also gave 10mls.

The next one is a totally normal shape (like the two in Richards photo) and this one gave 12mls. This is the one I usually use and it turns out it is most probably incorrect around the 10ml mark.

So the question begs how do we dispense 10.0mls of glyc washed oil into a pre-checked tube, are there different grades of syringes?

[dgs]

I obtained some info from the net last night re the accuracy of grade B lab glassware. It turns out that grade B is accurate to 99.8% so I'm sure it is good enough for our purposes. In fact on my pipettes it states the error is a max of 0.1mls.

When using lab measuring devices there is another measurement which states wheather the pipette will contain exactly the stated volume or if it dispenses the stated volume (in which case it will hold greater than the stated volume to allow for liquid retention)

This is stated on the pipette and is either C or IN= contains or D or EX= delivered.

My pipettes have EX so they deliver between 9.9mls to 10.1mls, I'm sure they will be OK for S2 bio but because of the viscosity of glyc pre-washed oil I'm sure the sample retention would be too great to be accurate enough.

I can't find anything about grade A or B syringes so I'll have to have a word with some companys about the accuracy of what they are advertising. There is no point in using just any old 10ml syringe if it is going to deliver 13mls!

[dgs]

Trying to use a little logic with this problem I've come up with the following solution.

If doing a 10/90 on glyc washed oil the most convenient and accurate way is to simply pour the sample into the tube until is is exactly on the 10ml mark, then add the methanol and shake. This way if the tube is inaccurate any error will be small as we are only measuring a small decrease in volume just below the 10ml mark.

When doing a 10/90 after stages 1 or 2 then it is important that as accurate as possible 10.0mls is dispensed into the tube, regardless of what the volume reads. As the inaccuracy on some tubes seems to be at around the 10ml mark then the reading using this method should be accurate enough.

Just one important point here - do we all know how to read the meniscus on the side of the tube?

Biodiesel gives the same shape meniscus as water;

https://www.youtube.com/watch?v=8jxXZhJXvdM