How did you figure out the lack of accuracy?
I’ll have to make sure I keep the reading next time I ship off an ICP sample
Continuous lower salinity in my ICP vs what I measured with the Hydrometer when I took the sample - I do intentionally measure Salinity, Alkalinity, CA, NO3, and PO4 at the time of ICP sample taking to compare the accuracy of my home tests and establish a calibration factor in the future.
There are two concerns a devil's advocate could raise: a) Did you measure correctly? and b) How do you know the Fauna Marin ICP is measuring salinity correctly?
Re a): I started using a dedicated Hydrometer glass cylinder and tried out various conversion apps - since I care about Salinity in PSU and need to convert SG to PSU - to make sure I avoided as many as possible testing errors, properly consider temp adjustments, etc. All pretty much yielded similar results (though there are some differences). Also, lots of folks in the German club I am in use the TM and other Hydrometers, and it is widely known that they very often deviate by +1 to -1.5 PSU, but sometimes, rarely though, you get one that is spot on. It's okay to apply a calibration factor to your own future measurements once it's established, but mine is -1.3 PSU, and it bothers me how inaccurate it is.
Re b): I do not know if they measure correctly. But the conductivity meter they use is a pretty nice device, and they, more than others, advocate dialing in salinity first before anything else, and put in a lot of effort to get reasonably accurate results in all their ICP tests - if not for others, then for managing their own coral farm.
Overall, is a salinity deviation in the +1/-1 PSU range an issue? Probably not, but it is for me, hence the effort I put in and hopefully the refractometer helps putting this perennial issue to bed.