Different readings on 2 refractometers with same calibration

corey01 · Dec 9, 2017

So the other day I went to my LFS and purchased a new refractometer with the intent of giving my old one to a friend of mine for Christmas. the old one I had was just a generic one from ebay. and the one I purchased looks almost exactly the same but there is an insert in the case that says its from BRS.

A few minutes ago I sat down and calibrated the new one with the new calibration fluid that came with it. got it to 35ppt or 1.026 no issues. went and measured my tank water its saying 30ppt. thought..umm no?
I grabbed my old refractometer measured the tank with that it said 35ppt no issues.
so I put some of the new calibration fluid on my old refractometer and it also said 35 ppt. so the fluid does not seem to be the issue. I also used my old calibration fluid on both of them and it still says 35ppt.

just went and used it on all my tanks reading between 30 and32ppt on all of them where my other one says they are fine. went and tried it on some ro water also just to see its showing what looks to be -3 steps or so. which I guess if i brought it up to equal RO water the other measurements would fall in line. but your not supposed to calibrate with RO.

anyone have any thoughts? or is it likely a defective meter?

Randy Holmes-Farley · Dec 9, 2017

I don't see how two refractometers can read 35 ppt on the same fluid minutes apart, then get 30 and 35 ppt on a different fluid unless one is so faulty that its calibration falls apart in the minutes between measurements.

If that were the case, it also would not still read 35 ppt on the calibration fluid.

So I'd suggest repeating the calibration of the tw with the same 35 ppt fluid, followed by tank water testing, then reread the calibration fluid with both.

The effect of light color is not this extreme, but calibrate and measure in the same lighting setting.

corey01 · Dec 9, 2017

I dont know. I measured quite a few times. now im getting my old meter dropping a few points to match up closer with the new one but still not perfect. getting about 2ppt difference between them.
im reading the article on meters again as its been a few years. wondering about temp differences perhaps. not sure. I put the calibration bottle in the tank see if it gets up to temp. but according to my thermostat and the tank there's about 8 degrees difference. how long does ATC take to function? I normally wait about 10 seconds or so after putting the sample on the prism to look at it.

Randy Holmes-Farley · Dec 9, 2017

corey01 said:
I dont know. I measured quite a few times. now im getting my old meter dropping a few points to match up closer with the new one but still not perfect. getting about 2ppt difference between them.
im reading the article on meters again as its been a few years. wondering about temp differences perhaps. not sure. I put the calibration bottle in the tank see if it gets up to temp. but according to my thermostat and the tank there's about 8 degrees difference. how long does ATC take to function? I normally wait about 10 seconds or so after putting the sample on the prism to look at it.

ATC functions within a second or two. It is mechanical expansion/contraction of a part of the device.

corey01 · Dec 9, 2017

ok well tried again with the fluid and it worked as well as I expected it would. idk they are both reading within a pretty narrow margin now still a point or two away from each other. I still find it very odd. I guess all my tanks are @ 1.024. do my W/C tomorrow. try to make sure to mix it at 1.026 well see what happens.