How accurate are pen type salinity meters?

[Gareth elliott]

Added a salinity/temperature meter tomy tools for quick spot checks. Have verified vs my calibrated refractometer they so far they match. Useful or gimmick?

 

[LouieP]

I think the pen is useful unlike the conductivity probe which is not reliable.

 

[wesman42]

It will never be as accurate as something like milwaukee's salinity reader.

 

[Timrpn]

It will never be as accurate as something like milwaukee's salinity reader.

Why is that?