No it doesn't. These devices need regular calibration. Since calibration does not enhance sales it is something that's often overlooked. All that matters is that the results look precise (it cannot measure to 1 ppm CYA, but it will sure try to tell you that) so that customers believe it is accurate. I understand given your background you're probably used to devices being well built and well cared for, but a pool store is not stocking lab-grade equipment and is definitely not holding itself to the same standards of equipment maintenance. It's a sales floor staffed with salespeople working a summer job.
The melamine CYA test is very accurate, despite it's imprecision. Since you are doing it yourself you can trust the results. There is no reason to ever have your water tested at a pool store and it is never recommended here. Do your own testing, trust your own testing. Pool store testing isn't worth the price you pay for it.
I appreciate that pool shops and their testing have a longstanding and well earned reputation for poor test results and even worse advice. I am very much an advocate for doing your own testing. I agree with that absolutely. Nobody cares about your pool as much as you do.
I have, however, a quarter of a century of experience in water testing in an industrial context, a degree in analytical chemistry and a very good understanding of how these instruments work.
I have spent twenty years teaching sales people (mostly engineers) to do water testing. So I also have a very realistic idea of just how many different ways people can mess this sort of stuff up.
This particular instrument is factory calibrated, and comes supplied with a calibration check disc, to make sure that it is giving consistent results. The light sources are LEDs which do not change and degrade over time with anything like the rate that traditional filament sources and bulbs do. It's not normally user calibrated on a day to day basis, but the check disc should be used regularly to make sure that it doesn't need servicing. Generally if it really is out of calibration, all the results will be badly off so it is pretty obvious although probably not to the average pool shop kid. Secondly, as I mentioned there are many reasons why colorimetric tests can be inaccurate, particularly when the pool water is outside the normal range. This is aside from the issues of poor training, sloppy technique, lousy laboratory hygiene, contaminated reagents, expired reagents and so on. So, please don't take this as any suggestion that people shouldn't do their own testing with a good quality and reliable kit.
The CYA test on these instruments is not a colorimetric test. It is a turbidimetric test and there are no colored dyes involved. The reagent (melamine) forms an insoluble species with CYA resulting in turbidity which is measured by measuring the change in the light level passing through to the detector. (This isn't a true turbidity measurement, which measures scattering rather than transmittance). The change in light transmittance through the disc is very reproducibly measured and the instrument is far more sensitive and far more consistent than the human eye. I haven't seen La Motte make any claims about 1ppm resolution for the method. They certainly only report figures to the nearest whole ppm. Having said that I have used other turbidimetric methods which give excellent accuracy and give 1ppm resolution, and other methods that are 2 ppm resolution, so I wouldn't be at all surprised to see that. It is however more precise and more accurate than visual methods particularly at the lower end of the measurement range.
Over three weeks I got three tests at two different shops and they all came back within 1ppm of the mean. Different sample, different operator, different instruments, same result +/- 1ppm. That result perfectly matched what I added to the pool. That is a very consistent result. There was considerably more variability in the other test results and I'm not suggesting people stop doing their own testing. I'm saying that this instrument, for this method is generally reliable and saves a bit of messing around at the start of the season to get your stabiliser levels adjusted when the concentration may be below the level detectable in the home test kits. My local pool shops don't charge for testing as long as you're buying something, if I'm adding stabiliser anyway, I don't mind buying it from them.
