conductivity probe not matching manual tests

[reeferandy]

I have two conductivity probes, one in my main sump and the other in my 100 gallon water mixing vessel which I use for water changes every 2 weeks. Both probes read a salinity of 37ppm after I properly mix new salt in the large vessel. But when I use other means of measurement including a very high precision refractometer and an electronic optical refractometer both read 33ppm. Both manual devices were calibrated using RO water and the Apex probes were calibrated according to Neptune instructions. The readings from both Apex probes and both other devices are very consistent. But why is there a constant 4 point difference? Which do I believe? Just as an aside there is a difference between the pH probe and the Sallifert pH test kit as well. I have been told by staff at Neptune that the probe is far more accurate. But it is always reading slightly below my desired pH for example 7.9 (there is predictable variation over 24 hours) while the test kit reads 8.2. Again, in your opinion, which do I believe? IMO it probably doesn't matter because my SPS coral is thriving and looks amazing but I want to better understand the technology and why there is a very consistent difference between probes and test kits/devices. Thanks,
Andy

[zombie]

Did you have temp compensation enabled and calibrate them in room temp fluid? If so that would explain all of the difference right there if your house temp is 5F less than tank temp at time of calibration.

You might be an engineer if...You have no life and can prove it mathematically.

[reeferandy]

I thought of that but the basement room that the sump is in, because of all my equipment, happens to be at almost the same temperature as the tank-water i.e. 77F. Also, for the conductivity probe there is no "automatic temp compensation" only a manual one which I am not quite sure how that is useful. Furthermore my electronic optical refractometer measures the temperature of the sample and compensates for this automatically. The manual refractometer does not, however as I stated the room temp and water temp are about the same right now.

[zombie]

What do the manual ones read if you put refractometer 35ppt solution on them? (You can make this with table salt and ro water if you dont have some on hand. Search randy holmes refractometer calibration on Google for the recipe)

You might be an engineer if...You have no life and can prove it mathematically.

[reeferandy]

I never thought of trying a standardized solution. Great idea! I will try that.