Inconsistent test results if repeated

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bkfamily1

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LifeTime Supporter
Jul 8, 2013
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North Canton, OH
If I test FC more than once on the same sample, it seems like my first test is always 1ppm (2 drops) lower than the second and third tests which are identical.

Every time when I finish testing, I rinse the cylinder and stir bar 2 or more times with RO water, shake them off and put them away. Before I run the first test, I rinse the cylinder and stir bar 2 or more times with the sample water and shake them off before filling to the 10ml mark and testing. Between repeat tests I rinse with RO water 2 times then with sample water 2 times then fill to the 10ml mark.

Thoughts?
 
Check for static electricity or other contamination on the dropper tip. I suspect the drop sizes changed somewhere during the first run.

I've done sequential tests and usually find the result to be within one drop, BUT it depends on the FC level. There is a percentage error when measuring volumes in the sample tube so if that is a 5% error then if measuring SLAM levels 20 ppm could readily result in a 1 ppm error which is 2 drops with a 10 ml sample size.

If you are seeing a 1 ppm FC difference at lower FC levels less than 10 ppm, then that's a lot and something isn't right about the testing.
 
Taylor gives a maximum error (i.e. 95% of tests within) for most of their tests of +/- 1 drop or 10% whichever is greater. When you are doing repeated tests using the same reagents, dropper tips, and graduated cylinder, the error will be substantially less, but 5% would not be unusual. Take the saturated cylinder for example. 5% of 10 ml would be only 0.5 ml. Do you think you are measuring more consistently than that? For the drops of titrating reagent, are you squeezing the bottle at a consistent rate so that the size of the drops is better than 5% in volume?

So doing an OCLT with a SLAM could be within 1 ppm up to around 20 ppm FC but if you've got a higher CYA than 50 ppm so a higher FC SLAM level it might be difficult to get right doing it just once. If you repeat the test both the night starting the OCLT and the next morning ending then you'll do better with an average of values. Or if you think you're close you can lower the FC some for doing the OCLT but only if you don't have a clear high overnight chlorine loss.
 
Geeze, I rinse my cylinders out with pool water and let them dry in the sun. I thought I was doing well. I did just buy a gallon of distilled water to verify my pH tester, maybe I can rinse with that?
 
FYI, it may sound like I am doing something special, but that is not really the case. I have an RO system in the kitchen since our well water is nasty. So it is right there at the sink from a separate tap. Otherwise, I would not be doing that. But since I have it right there, I use it for rinsing. Oh, and I am an OCD chemical engineer with lots of time in chem labs!
 
So if you have access to pipettes and other more precise equipment then you are welcome to use it and you'll have more consistent results. That won't fix the absolute accuracy since that is dependent on things you don't control such as the concentration of chemicals in the titrating reagent and the size of the dropper tip, but for OCLT it's relative results that matter so using a more accurate way of measuring sample size would reduce error. This isn't necessary, but since you're a chemical engineer with access to better equipment you can certainly try that if you want to.

I forgot to mention that one other variable is the location for the sample especially if the water is not well mixed with circulation.
 
CG - I have no desire to try to be any more precise, especially if it means setting up any real glassware in the house. My wife would never allow it, and my kids would break it for sure. I was just scratching my head at the consistently inconsistent results. When I mentioned I was a ChE, I was just explaining my compulsive desire to over-analyze things LOL. In some sick way, getting really into the testing thing is taking me back (way back) to my college days in labs. And I will admit that the Speed Stir is stinking awesome. I love pulling the stool (bar stool these days, not a lab stool) up to the sink and getting out the TF100 and putting on my safety glasses (which I now need for the bifocal cheaters in them, not for any chemical safety concerns). I can't help it. It is just in my blood. :geek:
 

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Hi,

I had the exact same problem on my last SLAM - I was doing 3-4 test PER SAMPLE each time and it drove me nuts. I ended up having to buy a 26oz bottle of reagent!

Anyway, this time round (I started another SLAM Monday) I am doing a few things different:

1. relax
2. when testing during the day, I use 5ml sample taken with a medicine syringe, and one drop = 1 FC
3. When maintaining SLAM levels, just keep in in a range - because 1. CYA testing is the least accurate of all tests so there is a possible +/- 10ppm swing , 2. FC testing at high levels is inconsistent. For e.g my CYA is 65-75 , so I just keep SLAM in the 24-30 range
4. for last test of night, and first test of morning, I will do two tests - one with a 10ml sample ,one with 5ml sample. I usually get .5 - 1 ppm discrepancy at high FC levels.
5. You will see a trend over time during SLAM where there is way less FC being eaten , so once I am nearing normal (3-4 fc ) loss per day, I will let it drop over a few days and continue a couple of day tests to make sure there are no spikes, and do overnight test until I am passing at normal levels.
 
chem geek said:
So doing an OCLT with a SLAM could be within 1 ppm up to around 20 ppm FC but if you've got a higher CYA than 50 ppm so a higher FC SLAM level it might be difficult to get right doing it just once. If you repeat the test both the night starting the OCLT and the next morning ending then you'll do better with an average of values. Or if you think you're close you can lower the FC some for doing the OCLT but only if you don't have a clear high overnight chlorine loss.
Click to expand...

So, it seems that for those of us that use SWG, the OCLT can be problematic since target SLAM would be at least 28 (CYA=70). I say problematic since we need to pass an OCLT of 1.0 or less, when in fact the margin of error could be greater than 1.0? Am I understanding this correctly? If so, seems like we need to guess when we think we might be done with SLAM, then lower the FC and do the real OCLT at lower FC levels. If still greater than 1.0, then continue SLAMing with some lost time in between.

This begs the question.....why is such a high CYA recommended for SWG pools? Is it only to save on the SWG cell? If so, I'd rather buy a new cell every couple of years than continue having to SLAM at such high FC levels (that cannot be accurately measured).
 
See this post for some good advice at improving accuracy of FC testing during an OCLT.

If by problematic you mean more difficult, then yes. It's not impossible to do an OCLT at SWG SLAM levels, but it is harder to get right. If one has difficulty but all other criteria are passing and the FC drop from OCLT is within 5%, then you can drop to a lower level to confirm the OCLT below 20 ppm FC where it will be easier to do.

The higher CYA recommendation for SWG pools is because of the usual pH rise in SWG pools from increased aeration from hydrogen gas bubbles and from possible undissolved chlorine gas outgassing. Turning down the SWG % ontime reduces this rate of pH rise. Having a higher CYA level lowers the absolute FC loss per day even at the proportionately higher FC level (possibly due to a CYA or chlorine bound to CYA shielding effect of UV from lower depths). This also has the SWG cells last longer though these days with 10,000 hour lifetimes for cells and being able to get larger cells for not much more expense, this benefit isn't as important.
 
Quote by run53 "... than continue having to SLAM at such high FC levels ..."

What do you mean by "continue having to SLAM"? If you simply keep the SWG set to provide the required chlorine level, you will never have to SLAM.

I think the reason that TFP recommends a high stabilizer level with a SWG is that the SWG creates a small amount of chlorine over a long period of time, as opposed to pouring a lot of chlorine in at a time. High stabilizer allows less chlorine to burn off, so less at any one time is required. JMO
 
chem geek said:
See this post for some good advice at improving accuracy of FC testing during an OCLT.

If by problematic you mean more difficult, then yes. It's not impossible to do an OCLT at SWG SLAM levels, but it is harder to get right. If one has difficulty but all other criteria are passing and the FC drop from OCLT is within 5%, then you can drop to a lower level to confirm the OCLT below 20 ppm FC where it will be easier to do.

The higher CYA recommendation for SWG pools is because of the usual pH rise in SWG pools from increased aeration from hydrogen gas bubbles and from possible undissolved chlorine gas outgassing. Turning down the SWG % ontime reduces this rate of pH rise. Having a higher CYA level lowers the absolute FC loss per day even at the proportionately higher FC level (possibly due to a CYA or chlorine bound to CYA shielding effect of UV from lower depths). This also has the SWG cells last longer though these days with 10,000 hour lifetimes for cells and being able to get larger cells for not much more expense, this benefit isn't as important.
Click to expand...

Yes, I mean more difficult to achieve consistent results.

So, there are 2 reasons to keep CYA higher for SWG pools. First to reduce pH rise. This totally makes sense to me, however, to combat this, why not just manage it with weekly doses of muriatic acid?

The second (SWG cell life) seems like a non-issue.

Is it considered harmful to add muriatic acid on a regular basis?
 
Griswald said:
Quote by run53 "... than continue having to SLAM at such high FC levels ..."

What do you mean by "continue having to SLAM"? If you simply keep the SWG set to provide the required chlorine level, you will never have to SLAM.

. JMO
Click to expand...

In theory, yes, I agree. But stuff happens :-(

It seems I have to SLAM at least once every year for various reasons. One year my SWG cell died and took me a while to figure it out as I thought I had algae consuming my chlorine. This year, I got algae early on and have had a hard time getting rid of it due to using the incorrect Pool Calculator. So operator error is another reason. Bottom line is that for some reason or another, I end up SLAMing once or twice a year. Would be much easier/cheaper at lower CYA levels.
 
You can certainly operate your SWG pool with a lower CYA level if you want to assume you have enough % ontime left since you'll have to raise it to keep up with the increased loss from sunlight. As for acid addition, as long as you are careful about how you add it there's nothing wrong with adding more or adding it more frequently. Just make sure to add it slowly over a return flow and to brush the side and bottom of the pool where you add it. Low pH harms vinyl so you don't want concentrated acid hitting it for any extended period of time.

The higher CYA recommendation came after there were literally hundreds of reports (if not thousands) over the years of SWG pool owners complaining vehemently about the rate of pH rise. Though this was partially mitigated by having lower TA and higher pH targets and for some using 50 ppm Borates, there were still many pools with a pH rise that was too fast and also quite a few pools with undersized SWGs. So the decision was made to have a higher CYA target to address these issues and many SWG manufacturers were doing the same thing (i.e. recommending 60-80 ppm CYA).

Very little of the Recommended Levels are absolute hard limits. Technically speaking, the CSI level near zero for plaster pools is more relevant that just being within the recommended ranges. This is the price of over-simplification to keep things simpler for newbies. Once you have your pool under control and understand the basics, you are certainly welcome to do whatever you want since after all it's your pool, not ours. When deviating from recommendations you just have to know the results or effects of such deviations.
 
Thanks Chem geek for the well thought out and informational response( as usual). Understanding the reasons behind the recommendations allows us non-experts to make better informed decisions abot the right treatment for our pool. Your input as well as several others input is much appreciated.
 
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