I realize that many tests have some variation, but when comparing results, especially when looking for trends, reducing some of that variability would be a good thing. Reading sample volume by eye is one source of variability. I have a small pocket scale (200 gm, 0.01 gm readout) that I use to measure the sample. The specific gravity of pool water is very close to 1.00 so mL water is very close to grams water. The procedure is simple: rinse the sample tube with the pool water; dry the tube with a paper towel; put the dry tube on the scale; hit the tare button to zero the scale; then fill the tube to the desired amount (I use an eyedropper for fine adjustments.)
Weighing water samples
- Thread starter DonQ
- Start date
I am in the meh stage. 7.6 or 7.8 on PH? Meh, doe sent matter, all good. FC 6 or 7? Meh, doesn’t matter, above minimum either way. 
- Nov 30, 2019
- 852
- Pool Size
- 8500
- Surface
- Fiberglass
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
For the actual precision required there's just no reason to fret over a meniscus' worth of accuracy.
I am in the meh stage. 7.6 or 7.8 on PH? Meh, doe sent matter, all good. FC 6 or 7? Meh, doesn’t matter, above minimum either way.
I am an engineer by training, a project manager by trade, and type A by nature. There are times when I get a little overboard with how exact I like things. However, when it comes to the pool, I am in the same meh stage as you.
I don't know the exact strength of my chemicals - they have been sitting around for a while
The granularity of the test is 0.5 ppm (for CL), it's never going to get more accurate than that.
I'm not using laboratory clean equipment - even rinsing and drying can leave traces.
Maybe my pool water is homogeneous across the pool, but maybe it isn't.
If my results tell me I have to add something, I'm not measuring that something with a precision balance or graduated cylinder - I'm using a kitchen scale or an old 1 Qt nalgene bottle with half worn off graduations.
I also don't know the exact strength of whatever it is I am adding. I suspect the good people at borax don't ensure each box has no inert materials in with the borax
I also don't know exactly how much pool water I have - its somewhere around 17,000 gal, but it could be 10% off in either direction.
So, yeah, eyeballing the 10 ml in the test cylinder is good enough for me.
@phonedave Yup, don’t need my six sigma training for this! But I was a little anal when started TFP.
But I was a little anal when started TFP.I'm a little surprised at the negativity here. I was an analytical l chemist in my former life and I always tried to reduce unnecessary error if possible. If 'meh' precision is good enough why not just use test strips?
- Jun 1, 2018
- 13,718
- Pool Size
- 26000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
I sometimes use a 5ml syringe- especially if i’m doing an oclt
- i have kids who take meds so i have a bunch & i keep one in my test kit.
- i have kids who take meds so i have a bunch & i keep one in my test kit.
There is precise enough for the application and there is just wild Rear guessing. In my experience test strips fall into the latter.
I use the sample sizer. Easy to use and I feel it just adds a little more consistency to the tests.
Funny, would have thought the analytical chemist would recognize that there's middle ground and using the proper tool for the task is more important than always being insanely precise. No need to break out the electron microscope when a pair of calipers will do. Instead you apparently think that anything less than your way is tantamount to guessing. Well, as Dave pointed out, you're already dealing with a lot of estimates, rounding, and assumptions.
Weigh your samples all you want, I won't stop you. Both of our pools will be sparkly and sanitary.
@DonQ I apologize for the implication of negativity. It wasn’t intended that way.
I do use a sample sizer so it isn’t that I don’t care. More like absolute precision isn’t required in many cases. As long as I am within my target range I am comfortable.
Some use the 25ml sample for FC so each drop is .2 instead of .5. Which is all good.
The nice thing about TFP forum is getting feedback on things. My way/their way may be different to some extent, but in the end we take care of our own pool and do it the way we want.
Take care!
I do use a sample sizer so it isn’t that I don’t care. More like absolute precision isn’t required in many cases. As long as I am within my target range I am comfortable.
Some use the 25ml sample for FC so each drop is .2 instead of .5. Which is all good.
The nice thing about TFP forum is getting feedback on things. My way/their way may be different to some extent, but in the end we take care of our own pool and do it the way we want.
Take care!
I am in the meh stage. 7.6 or 7.8 on PH? Meh, doe sent matter, all good. FC 6 or 7? Meh, doesn’t matter, above minimum either way.
Ha, the longer I have owned a pool, the more this is me. I might test on the weekend. Around Tuesday or Wednesday I pull out the bottles of chlorine and acid and eyeball some in there. Every now and then I do the full round of tests, and everything stays pretty well balanced.
I don't think anyone meant to hurt your feelings, just everyone has their way that works for them 
The accuracy of the test can only be as accurate as the least accurate component. In the case of pH, the color match is really subjective. The titration tests are more accurate but you’re still depending on the precision of the droplet size from a dirt cheap plastic bottle. CYA tests are ridiculously subjective.
I view the results as “too low”, “too high”, or a wide middle range of “ok”; I don’t view the number itself as particularly accurate. If they need adjusting, I use the absolute number to calculate what to add but then use less and retest.
I view the results as “too low”, “too high”, or a wide middle range of “ok”; I don’t view the number itself as particularly accurate. If they need adjusting, I use the absolute number to calculate what to add but then use less and retest.
Because in metrology (the science of measuring, what a fun class that was) you learn that the outcome of an experiment is only as accurate as the least significant figure of its components.
So if you have water volume to 0.1 and your drop test measures to 0.5, increasing the accuracy of the water sample to 0.0001 does nothing to increase the accuracy of the outcome. In order to increase the accuracy of the outcome you have to increase the accuracy of the least accurate component, which is not the volume of water measured.
PoolNewb2020
Well-known member
No reason to go overboard with accuracy or stressing over it. I don't really see any negativity in the replies, just people recognizing that there is very little value added for what seems like a lot of extra effort.
Extra effort, complicated?
No more complicated than eyeballing a meniscus:
1. Put cylinder on scale
2. Turn on scale
3. Fill with sample water.
No more complicated than eyeballing a meniscus:
1. Put cylinder on scale
2. Turn on scale
3. Fill with sample water.
If you already have the scale, then by all means go for it. But I would not suggest somebody buy a scale for the sole purpose of weighing the samples in an attempt to improve the accuracy of their results. Nor would I "use an eye dropper for fine adjustment" in my sample as you stated in your original post. I also don't dry my sample tube between tests to remove any residual moisture that is above the 10 ml (or 25 ml) mark that would throw off a weight. Rinse, dump, and refill.
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