Inconsistant refractometer readings

[sgc0286]

HI!

My refractometer keeps giving me inconsistant readings. I checked the sg at around 2pm and it was between 1.024 and 1.025 and I just checked it and it said 1.023. Iam wondering if the difference in lighting would make a difference in the reading. The one earlier in the day was taken in natural light and the later one was not.

 

[ccCapt]

Light has no affect on salinity. The only way salinity can rise is if water evaporates from the tank and leaves a higher salt content and the only way it can drop is if fresh water is added and the salt concentration is lowered.

Do you have calibration fluid to check your refractometer?

 

[sgc0286]

No, I dont have any calibration fluid. I used water from my RODI to calibrate. I know that the fluid will be more accurate, you think that is my problem?

 

[Kurt_Nelson]

The only thing - short of dropping the thing or accidentally moving the calibration screw - that would cause a shift like that during the course of a day is "user error" of some sort. The lighting doesn't change where the little line shows up - it just makes it easier or harder to see the line.

Two things I can think of... one is that you didn't allow the water to sit on the glass long enough to let the temperatures equalize and for the "temperature compensation" aspect of it to work. The other is that you didn't "squish" the plastic plate down on the water. If you put a couple drops of water on the glass and gently lower the plastic plate down, you'll notice the plastic kind of floats on the water. You'll get a different reading with the plastic "floating" versus if you squish the plastic hard down onto the glass to "flatten" out the water. Try it... you'll see what I mean.

 

[sgc0286]

Okay, so is the proper way to gently lower the plate down, or firmly lowering the plate down?​

 

[Kurt_Nelson]

LOL... I knew that was coming!

I don't know, but for me, consistency is the key thing. And in my opinion, the only way that you can do it consistent is to smoosh the plastic plate down hard on the glass.

If you get the calibration fluid (53.0 mS conductivity calibration fluid will read 35.0 ppt) and calibrate it with whatever method (gentle lowering or hard smoosh), and repeat that method when you take readings, it really doesn't matter which method is "correct" since you've calibrated the unit using that method.

 

[sgc0286]

Okay, Thank you!