Did you know an unlubricated refractometer will produce funny numbers?
Your refractometer has a hinged cover that goes over the angled plate of the refractometer. What happens over time is the hinges stiffen up from salt deposits, causing this cover to not sit all the way down on the angled plate. You can not see this by your own eye, but believe me, it's not fully resting on the angled plate. When this occurs, the numbers you see are no longer true. Actually this typically occurs slowly over time, and you most likely don't even realize it.
That little hinged door needs to easily and readily flop down onto the plate. If it is stiff, and doesn't fall freely, you aren't getting an accurate reading of your salinity. You can potentially offset this error by frequently calibrating your refractometer with 1.026 calibration fluid (not RO/DI water). Note that this calibration solution could also be termed 35ppt or 53 mS/cm.
However, the best thing to do is to keep your refractometer hinges well lubricated.
Now I know some of you are thinking I'm a nut case. Well, I am, but your refractometer still needs lubrication....and I'll prove it, especially if you have a stiff-hinged refractometer. What I want you to do is to measure the salinity of your tank. Now follow my lubrication instructions below and re-measure your tank (before re-calibrating). Do you see a difference? Please post your results here so others can see the truth.
So how do I lubricate my refractometer without getting lube all over the place?
Let's first see if you have stiff hinges. Take your refractometer and open the "door" so that it is sticking straigh up:
The door should readily and easily fall back down into position, and not stay in this stiffened position. If the door just stays there, you have really stiff hinges, resulting in funny salinity numbers.
A second test is to hold your refractometer upside down and move it back and forth. The door should readily and easily swing back and forth at all hinge points, with extreme ease with each back and forth motion you make. Again, if not, you've got stiff hinges.
So let's lube your refractometer!
I use WD40, but I would imagine other lubs would work just at well. Now I don't simply squirt WD40 on my refractometer....that would be a mess, and who knows what this lube would do to accurate readings? What I do is very carefully dab small amounts of WD40 directly on the hinge pins only.
So what I do is to first squirt a small amount of WD40 into a very small glass. My wife was insistent on purchasing Waterford Crystal Aperitif glasses....don't ask the price. I'm coming up on 40 years of marriage and we've never used these glasses. They sit in our china cabinet, never used......until now. Don't know what an Aperitif glass is? It's a disgustingly small glass that holds a thimble's worth of liquid that is used to serve an Aperitif or Cordial.....pinkies out.
So I'm figuring I've got to get some use out of these overpriced, never used, small glasses, so I squirt a small amount of WD40 into one of the glasses. Please don't tell my wife.
Now using a toothpick or finishing nail, I dab a small amount of the WD40 on the hinge points and then move the door back and forth to work the lube in. You'll actually feel the door free up and move easily as you are doing this.
Continue to do all hinge points with this lubrication, moving the door back and forth as you work that lub in.
Now when you attempt to put that door straight up in the air, it should immediately fall down into position.
Now for those of you doing my little test above, re-test you tank water and let me know what number you're getting....before and after lubing. Otherwise, once you've lubricated your hinges, you need to re-calibrate your refractometer. Now you are all set to get accurate numbers from your refractometer.
Finishing up, how frequently do I lubricate? I would guess it's about once a year, whenever I notice the door isn't swinging down very freely. This timing could vary widely dependently on frequency of use. I'm only doing water changes twice a month, so lube as needed to keep you refractometer humming perfectly.
EDIT TO ADD:
@WhalesLovePineapple made a video of his experience in lubing his refractometer, and with his permission I've included it below.....Great video. Thank you WhalesLovePineapples for sharing.