Refractometer help

[happyhourhero]

i have always (about two years) taken rodi water out of my storage containers at room temp and calibrated my refractometer to 0 and used that.

Here recently, I bought some 35ppt calibration solution and have calibrated to that. It told me my tank SG is a couple of points low. I mixed some new salt water in a 5 gallon bucket and used 3 full cups of reef crystals where I usually do 2.5. I checked this before I did my wc and it came in at 1.025 but it should have been higher than that.

I tested some rodi and it was a couple points below zero.

Is the fact that I am doing everything at room temp, which I have always done, causing the mismatch? Should I just go back to calibrating to 0 with my rodi like I have always done? My tank is happy now and I don't want to mess things up over this.

Thoughts?

 

[Brew12]

i have always (about two years) taken rodi water out of my storage containers at room temp and calibrated my refractometer to 0 and used that.

Here recently, I bought some 35ppt calibration solution and have calibrated to that. It told me my tank SG is a couple of points low. I mixed some new salt water in a 5 gallon bucket and used 3 full cups of reef crystals where I usually do 2.5. I checked this before I did my wc and it came in at 1.025 but it should have been higher than that.

I tested some rodi and it was a couple points below zero.

Is the fact that I am doing everything at room temp, which I have always done, causing the mismatch? Should I just go back to calibrating to 0 with my rodi like I have always done? My tank is happy now and I don't want to mess things up over this.

Thoughts?

The temperature of the water doesn't matter when using the refractometer. The 2 drops of water you add will quickly come to the same temperature as the refractometer as soon as you put them on. The temperature compensation (if yours has it) is all about the temperature of the refractometer itself.

Normally it is more accurate to calibrate your refractometer using a calibration close to the range you are trying to maintain. In your case, I'm wondering if the calibration fluid is off. I use reef crystals and a 1/2 cup per gallon always gets me close to 1.025. I would recommend taking your refractometer and the calibration fluid to your LFS and testing it against their equipment.

 

[JamesP]

You should calibrate at the range you are testing. So use the 35ppt solution and calibrate every time you pull it from the box for a series of measurements. This is important as calibration slips while in storage. This is true for any calibration. For example you calibrate ph with 10 and 7 solution for tank water which is around 8, but 4 and 7 for calcium reactors which run in a lower range. Calibrating with ro is only useful for testing near 0ppt. Add salt until it measures right and that is the number to write down for measurements. Forget everything you did before as it was not calibrated correctly so those numbers cannot be used for calculating things now.

 

[Alfrareef]

I had similar surprise when calibrate with calibration solution.
Now I calibrate with a solution from time to time...

 

[skim]

What Refractometer is it? Is it designed for testing of Sea Water or just Salinity.

The temperature of the water doesn't matter when using the refractometer.

Actually the ATC is only good for couple of degrees maybe max of 4 or 5 and that is from the calibration point of the Meter, which could be 16 degrees or 20 degrees on Salinity units. On the newer ones like from Red Sea its set point is 25 degrees and when the algorithms ( equation to set a base ) this temp and it factors in other salts that are in Sea Water. Red Sea recommends using RODI for their units to calibrate unit. In the older units I believe it would be a good idea to use a base Salinity to calibrate, but even then they do say on bottles that 1.025 at 25 C so to get the most accurate calibration you want the Solution and Meter to be at 25 C or as close as possible. Now Calibrating with RO/DI should be no issue with temp as it is pure so Zero is Zero.