Hello Alan, welcome to the group.
May I ask which bit of kit they are using?
The problem with test strips is they are only really an indication of something, I think people see a colour change and assume that everything is fine but the gaps between the reading scale are too large and the colours from the printer make them very hard to interpret.
When you add that the dots of reagent are all in a line so one chemical colour/reagent runs into another the you really have to question what's being measured. now when you dip them how long, how much of a flick/shake do you give them and do they measure from the minute they hit the surface or some point under the surface assuming they arrive at that point with the 3 or 5 seconds. All to hit and miss so more accurate testing will make looking after your pool easier once you have grasped the basics. The additional cost of the better testers is soon recouped by not running to a pool shop and leaving with arm fulls of things you probably didn't need.
Testing for pH is universally a phenol red reagent but matched with a good colour slide in fine graduations so you can observe what the real pH is likely to be.
Testing for chlorine is an industry standard using a DPD test, this is a requirement for commercial and multi occupancy pools in france for a very good reason, it's proved itself. The only downside with DPD is it can bleach out at shock levels of chlorine.
Other tests for alkalinity, and calcium hardness can be done with titration tests, a tablet count method where the sample changes from one colour to another and the number of tablets is checked against a card which tells you the level, very accurate.
Cyanuric acid, this cannot be tested with dip strips, I have been to pools with owners having 180 and 200ppm where the strips read 0, 30-50ppm (the strips are given to me by the manufacturers but they are rubbish. The conventional test for CYA is a turbidity test where the pool water turns cloudy and you view a dot on a test tube and watch it disappear, at the point it disappears you read the side of the test tube and have the reading. This test is so important to the overall health of the pool.
These test can be purchased individually or as a kit which is invariably cheaper but you must still be able to see colours to use it.
Moving up the scale are the electronic testers. a sample of pool water has a reagent added and then a colour is given which when put into the reader the light source shines and gives a reading digitally so no colour matching errors.
It all comes down to cost and preference the more things you test for the more you generally pay but the clever marketing people also have tricks up their sleeves to catch the unwary. Some manufacturers don't include many reagents so the headline price looks lower against the competition but by the time you have added up the cost of the extra ones you can end up paying more.
Then other companies have pre set parameters so you get annoying "lo" or "hi" when outside of what they think you need but they haven't a setting for different types of pool so how do they know what's right? Vinyl pools need less alkalinity than concrete plaster tiled often lower than some units can measure. sure you can ignore it but then you will use more pH- to control your pool.
To try and make some headway I have looked at the market and made judgements on various companies offerings with cost and features the primary and of course does the company have good after sales should something go wrong. These testers are more than adequate for our use and have been tested over a fair length of time to prove them worthy, if new ones arrive I have a good look at them. One of the dip strip company's latest tester uses individual strips for each test obviously because of interference from one colour to another as I described earlier and the fact the penny finally dropped, as things arrive I let people know but for now I have a good range of testers.