If you get a smooth Normal Distribution with a low standard deviation and the tests are just as likely to be over as under, then you can assume that the mean is close to the actual number.
This also helps eliminate outliers where some error has created a reading that is way off.
The stated Accuracy, Precision, Resolution etc. for any given meter, drop test, test strip etc. are based on the assumption of the test being done exactly right with the correct procedure, meter calibrated properly, reagents in good condition etc.
There are many sources of error that can affect the test beyond the capability of the test.
The only time you can get exact numbers is when you can count something.
If you have a small pile of change, you can count the different coins and give an exact amount for each coin like 5 quarters, 3 dimes etc.
Even then, you can have uncertainty because people can make mistakes in counting.
When you measure something like length, weight, salinity, temperature etc. you cannot get to an exact number because there will always be some uncertainty in the measurement.
The main questions are how much certainty is required to make a decision and how do you get to that certainty?
For example, if you are a Pool Company that is going to install a SWG for a customer and you send a service tech out to measure the salinity so you know how much salt to bring and add and they come back and say that the salinity is 2,145 ppm.
If you rely on this number and add salt, and the number was really 3,500 ppm, then you will be over the limit and now you have to do a partial drain and refill.
So, you need a way to specify the level of certainty for any reading and you need to know how certain you want to be before taking action.
A digital Meter might show an exact number to 1 ppm, but you need to understand that this is not as accurate as it seems.
Using PPT should be as useful as a reading in PPM based on the accuracy and resolution of the test.
If we assume a +/- 100 ppm accuracy, then that is 0.1 ppt.
So, if a salinity meter shows 3.2 ppt, then you know that the level is 3,100 ppm to 3,300 ppm to maybe 97% certainty.
Giving a reading of 3,238 ppm is not useful and it can even inspire a level of confidence that is not warranted.
If you really need confidence in the reading, you should use multiple independent readings like a K-1766 and a calibrated salt meter.
If the readings are close, then you can have more confidence.
If the readings are far apart, then you know that one or both have to be significantly off.
I do not like relying on any single test for salinity especially for critical things like adding a lot of salt.
