Is 5ml always 5ml?

[reefkeeper]

I have been using my API test kits to test for Ca, Alk, Nitrates, and Phosphates.  Today, I had an expired Salifert test kit and I noticed a 5ml syringe in there to pull in 5ml accurately.  The API kits simply have a test tube with a 5ml line (which I have trusted for two years) that I have to fill, pour out, fill again, pour out too much, and try to get it on the line. But it's a pain getting it exactly on the 5ml line, plus I never know whether it should be the top or bottom of the meniscus.  So, I used the Salifert 5ml syringe to fill the API test tube, and it filled it way above the line!!  What the heck?  I assumed that it was critical to have exactly 5ml of water.  What do you all do?

[Wall_Tank]

How are you reading the line on your test-tube.

You would fill to the bottom of the Meniscus.  http://en.wikipedia.org/wiki/Meniscus

Of course.....can you define how much is way over?

[reefkeeper]

Got it  So, I should fill the test tube so the bottom of the meniscus is on the line.  When I used the Salifert 5ml syringe, the water level was about 1/4" above the line!  Thought?

[Wall_Tank]

I would probably trust the syringe......Are you sure it was a 5 ml Syringe....I have seen a few 6ml syringes in kits.    To be safe, I would probably grab a different syringe and see which one it matches.

Yes, use the bottom of the meniscus.

[lazylivin]

Does your API test tube say 5ml on it or just a line? My guess is you got the wrong test tube. How many ML does it take to fill it to the line?

[reefkeeper]

Hey gang - I decided to expand my testing on this (It's the software engineer in me :-)  Anyhow, I have two different 5ml syringes (from my Salifert test kits) and 6 different test tubes (all with a 5ml line and all from my API test kits).  I filled 5ml in one syringe and put it in a dry test tube, then I repeated with the same syringe in a different test tube.  Finally, I used the other syringe and filled two different test tubes. 

Here are the filled syringes (You can see I pulled them until the bottom of the plastic met the 5ml line.)


Here are the filled test tubes.  You can see they are all filled approximately the same amount above their marked 5ml line.  Note that 2 of my 6 test tubes are slightly shorter than the other 4.  However, with this experiment, I chose 2 tall ones and 2 shorter ones.  They both seem to have the same amount above the line.  I suspect the shorter ones are just from a different manufacturing plant, but they seem consistent.


So, how does one tell which is correct?  I don't have any chemistry beakers anymore.  Can someone else perform the same experiment at home and let's see if we get the same results?  Thanks. -Jeff

[lazylivin]

Ah I think I see what is happening. Their should be an air bubble in your syringe right below the plunger that equals the volume of space in the tip. If you bring water in and out of the syringe while the syringe is under water a couple times it removes that air. Try starting with a dry syringe, bring in the water and leave the bubble present to see if it is closer to the 5 ml line.

[chromiumlux]

Bingo

[Wall_Tank]

The 1st image doesn't show up for me.

[lazylivin]

It was there just a couple minutes ago.

[reefkeeper]

They probably deleted them because they were syringes :-)

[reefkeeper]

OK.  I redid my experiment.  I confirmed that the syringes were dry before I started, then I made sure to pull the water into the syringe only one time.  It left a small bubble in the syringe.  However, when I put the water into the test tube, it looked the same as the photo above.  It was significantly above the line.  Would somene else please try this?  Thanks, Jeff

[kattz]

I use a BD 10ml syringe from a vet hospital.  I purge the air from the syringe by cycling the plunger while the tip is submerged, and pull to the 5ml line.  Right on the line on the side of the API test tube for my tubes when discharged into the tube.

[reefkeeper]

Thanks.  I use 5ml BD syringes.  I don't know which to trust because both BD syringes match each other and all 6 API test tubes match each other.  I don't know how much of a difference in might make, but I assumed the water quantity was critical.  If I have 10% too much water, I would assume my readings are either high or low by 10%.  That seems pretty significant.  Anyone have any ideas?  Maybe it's just time for a Guinness!  Thanks -Jeff

[lazylivin]

It could be that the vials line was painted incorrectly. We need more test subjects to see. Wish I had an API vial but I don't. Sounds like Kattz tested and it came out correctly so we would need his syringe an vial to compare to yours

[reefkeeper]

Great idea Brian.  kattz, where do you live?  If you want, PM me and I can try to stop over or you are welcome to stop by my house.  I'm in Bellbrook.  Thanks -Jeff