Can't get consistent FC results

[run53]

No matter how much I read here about getting consistent 10ml samples, holding the dropper bottle vertical, removing static, dropping slowly, constantly swirling, using the proper amount of powder, etc.......I am just not able to get consistent FC readings. From the same sample, if I do multiple tests, I get about a 1 ppm deviation in readings with an FC of about 12.0. That seems pretty bad.

I don't know, I guess 1ppm is not that critical unless I'm doing an OCLT. But my lack of consistency at relatively low FC levels, leaves me no confidence in my ability to perform an accurate OCLT at higher FC levels.

From everything I've read, I seem to be having trouble with 2 of the recommendations.....

1) Vertical dropper bottle: When I keep it vertical, the first several drops just flow out on their own by gravity, without even squeezing, so I have no control over the rate. At first they come out very fast and then after several drops, I need to squeeze a little so I am able to retain drop control. I seem to be able to get more consistent results by NOT holding the dropper in a vertical position as I seem to have more control. While this may be more consistent, I'm not sure it is accurate.

2) constant swirling while dropping: I drop with my right hand and I am not coordinated enough swirl with my left hand while dropping. Is constant swirling critical or can I drop 5 drops then swirl?

 

[Griswald]

Speedstir...takes all of that away.

 

[run53]

Speedstir...takes all of that away.

Thanks!
Just saw a video on that thing. Looks cool and I think I will get one as it should solve issue #2 for me.

However, how does that solve the first issue..... vertical dropper problem I am having?

 

[Patrick_B]

Fix the first issue by holding the bottle upright and then squeeze out some air...a fair amount at least. While holding the squeeze, turn it upside down as if you would to test and let the air back in. This always helps me, but I have no idea why.

It is ok to drop in a few drops and swirl, but as you get close to the end point (you should have an idea where you are anyway) go back to one at a time. You don't want to over shoot the end point and lose track of the result. I can't agree more that a speed stir is the best option here.

 

[run53]

OK, thanks for that info on the dropper. I will try that next time.

 

[BassTrix]

Recently, I was experiencing inconsistent FC readings. I was near the bottom of my DPD powder and the powder had clumped together at the very bottom. I was breaking up the clumps to get it back to a powdery state and using it. I decided to get out a new vial of DPD to compare. My FC readings went right back to being consistent.

As for the drops, I have the K2006 kit which uses 3/4oz reagent bottles. I refill from 2oz bottles. On one occasion, I got the drip nozzles mixed up between the 3/4 and 2oz...the result was what you describe. I noticed the nozzle felt loose, so I swapped them and the drip problem went away. It's possible your drip nozzle is a little loose and letting some air pass. You may try popping out the nozzle and adding 2-3 wraps of teflon tape (used on pipe fittings).

 

[run53]

Recently, I was experiencing inconsistent FC readings. I was near the bottom of my DPD powder and the powder had clumped together at the very bottom. I was breaking up the clumps to get it back to a powdery state and using it. I decided to get out a new vial of DPD to compare. My FC readings went right back to being consistent.

As for the drops, I have the K2006 kit which uses 3/4oz reagent bottles. I refill from 2oz bottles. On one occasion, I got the drip nozzles mixed up between the 3/4 and 2oz...the result was what you describe. I noticed the nozzle felt loose, so I swapped them and the drip problem went away. It's possible your drip nozzle is a little loose and letting some air pass. You may try popping out the nozzle and adding 2-3 wraps of teflon tape (used on pipe fittings).

I tried that , but didn't make a difference. Always the first batch of drops come out very fast without squeezing. It was already a tight fit, so I didn't try the teflon.

 

[run53]

Fix the first issue by holding the bottle upright and then squeeze out some air...a fair amount at least. While holding the squeeze, turn it upside down as if you would to test and let the air back in. This always helps me, but I have no idea why.

.

Tried that but still got gravity drops at the beginning without squeezing the bottle.

I took 3 readings this morning

TC=9.5
TC=11.5
TC=11.5

The first one was off by 2.0

I'm guessing the (2) 11.5 readings are correct but who knows. I think the drop size is causing my inconsistencies. I was more consistent (but maybe less accurate) when I was holding the bottle at an angle where the drops could not come out as easily as when vertical.

 

[pabeader]

easy, hold the bottle away from the vial for the first few drops.

 

[run53]

seemed like a good suggestion. So I tried it, but the results were even more inconsistent.

1st reading FC=9.5
2nd reading FC=11.0
3rd reading FC=10.0

All from same water sample.