Which salinity measurement to believe?

[neiltus]

I have:

Red Sea Refract
Milwaukee Unit
Apex Probe

Today I calibrated my Apex probe (been a few months) with the 53,000 solution (yes a new uncontaminated packet). My tank comes in at 32.1ppt, and that is using the 2.2 temp compensation.

My Refractometer and Milwaukee Unit are both on the table next to the tank, air temp is 77 degrees. I calibrate the Milwaukee Unit using their solution, and it measures the tank at 35ppt.

I then calibrate my Red Sea Refractometer with my RODI, and the tank water is slightly over 33ppt.

Soo....I test my RODI water on the Milwaukee unit and get 0ppt. Good. I calibrate using that RODI water and retest the tank, 35ppt. I test using the 'check solution' that is 1.025 that came with the Milwaukee unit and its 1.025.

Now I know the Milwaukee is quick and dirty and not nearly as precise as a refractometer. Does this unit need to go back to where it came from, because it's throwing 5%+ from the other units?

I also tried it with my QT which the Red Sea unit measured at 32 and change the Milwaukee...35ppt (did not try the probe for obvious reasons.

I am going to repeat this tomorrow while I mix saltwater and see if it just reads 35ppt on half mixed saltwater.

 

[thegrun]

You should calibrate the refractometer with 35ppt solution, not rodi water. Even refractometers that claim to be fine calibrating with rodi water will be more accurate calibrating with a solution close the the value you are going to read. Conductivity values do not necessarily match up with true salinity do to variances in the waters chemistry and conductivity probes are suceptible to electronic interference, so I would take that number with a grain of sand.

 

[neiltus]

You should calibrate the refractometer with 35ppt solution, not rodi water. Even refractometers that claim to be fine calibrating with rodi water will be more accurate calibrating with a solution close the the value you are going to read. Conductivity values do not necessarily match up with true salinity do to variances in the waters chemistry and conductivity probes are suceptible to electronic interference, so I would take that number with a grain of sand.

I have thought of calibrating with 35ppt, as there would be less 'skew' if I was measuring a point close to that, vs as going from 0ppt to 35ppt.

However I was instructed not to.

 

[Percula9]

You can use the calibration fluid from your conductivity meter to calibrate your refractometer.Set to 35ppt.Now your tank should measure correctly.

 

[snorvich]

You should calibrate the refractometer with 35ppt solution, not rodi water. Even refractometers that claim to be fine calibrating with rodi water will be more accurate calibrating with a solution close the the value you are going to read. Conductivity values do not necessarily match up with true salinity do to variances in the waters chemistry and conductivity probes are suceptible to electronic interference, so I would take that number with a grain of sand.

What he said.

 

[neiltus]

Alrighty.

I have an new bottle of Aqua Craft Products Calibration Solution from BRS that says it's 35ppt @77F. I put in my house on the table next to the Milwaukee Ma877 for two hours.

When I use the Milwaukee zero solution or my RODI to zero the unit it reads the check solution at 38ppt (once at 37), and reads my tank water at 35ppt. This is done in a room with only tank lights on.

When I use the check solution to zero my refractometer, it reads my tank at 33ppt. Apex probe is 32.8ppt with 2.2 temp adj.

I think the Milwaukee unit needs to go back to where it originated. Their website says it +-2ppt.