Sybon FG100sa refractometer drifting day to day

[jonathanyhsu]

I have a Sybon Opticon FG100sa refractometer, and am having issues trusting it. I started to test it every morning (approx. same ambient temperature of 21C/70F) using Sybon 35ppt Calibration fluid, and the results vary strongly from day to day.

- One day it will read 35ppt, another it will read 32ppt, and then another morning it goes back to 35ppt.
- The refractometer has ATC and the manual specifies a recommended recalibration time of several months between calibrations.
- All measurements were taken at ambient room temperature of 21C/70F in a garage at approximately the same time in the morning.
- All measurements were allowed to rest for 60 seconds, longer than the instruction manual listed 30 seconds.
- The refractometer was cleaned with RO/DI water using a Hach lab spray bottle between tests and dried with the provided felt cloth.
- The refractometer and standards are stored inside their respective plastic cases, which are placed inside a wooden lab bench shelves (in other words, it's not in sunlight. There may be a cardboard box covering that stuff too).
- The refractometer spec sheet lists accuracy as +/- 1ppt, which would be ~2.857% if 35ppt is the normal measurement. However, my drifting measuerments are 3-4 ppt, outside this spec.

Anyone have any similar experiences? Is my refractometer damaged, or is this kind of experience normal? While the manual says several months between recalibration, is that an experience anyone has had with this (or any other) refractometer? Are there any brands anyone can recommend?

I re-read http://reefkeeping.com/issues/2006-12/rhf/ and realized I'm not 100% sure this is a salinity refractometer as opposed to a brine refractometer (although my instruction manual has advertisements for Sybon aquarium salt mix, and Sybon digital salinity probes for aquarium use). Although I don't think that is causing my issue, if I do have a damaged refractometer, I was going to make the effort to ensure I have a true salinity refractometer this time.

[outssider]

I've noticed my refractometer changes with temperature, not as bad as yours but it does, even with atc. Rather then trying to remember the room temperature last time I used it I just calibrate every time I test. I would guess mine would change 1ppt for every 5-7 degrees room temperature change.

[Rybren]

I had similar issues with mine until I started to store it inside my stand where there is no temperature variation.

[Randy Holmes-Farley]

I re-read http://reefkeeping.com/issues/2006-12/rhf/ and realized I'm not 100% sure this is a salinity refractometer as opposed to a brine refractometer

If you are using a 35 ppt standard, it won't matter.

Maybe the glass part is loose? Is there any way to tighten it down?

I agree it could be a temp issue.

[jonathanyhsu]

Thanks for the ideas; I don't see anything that I can adjust except for this screw that is tightening a metal ring that holds the plastic prism cover. Of course, it may do more than that, but I don't know the internals. The other things that appear adjustable are the rubber covered calibration screw and the adjustable eye piece for focus.

As a side note, it looks like the screw has rust on its surface, so perhaps that's it. I tightened it down about 20-25 degrees and I'll see if that helps with the drifting.

[kurt_n]

A couple thoughts...

1. Are you "smooshing" (technical term) all the air out of the sample when you flip down the plastic cover? So put drop on glass, flip plastic cover down and then press down on plastic cover so it's flat against the glass.

2. Are you rinsing the dropper you're using to put the water sample on the refractometer? If you're using a dropper to take the water sample from your tank, there might be residual water in there that has dried out and left salt behind. That salt then dissolves in your next sample, raising the salinity of what you're measuring. Doesn't take many salt crystals to throw off the reading of a drop or two of water.

[jonathanyhsu]

1. Yes; the plastic cover is textured so air bubbles are very apparent. I can pretty easily ensure there are no air bubbles trapped with the sample by pressing down in a line.

2. The standard I'm using is a squeeze bottle; I just invert it and squeeze a drop out at a time. For samples from my tank I'm using a glass stirring rod and rinsing it off with RO/DI water from the pressure system. I do the same for rinsing off the refractometer as well.

[thegrun]

Have you tightened the ring (bezel) around the adjustment screw? If it is too loose it will not stay in adjustment since simple movement will allow the prism to shift. Don't over tighten or you will not be able to make adjustments.

[jonathanyhsu]

I tried to move the ring/bezel about the adjustment screw, but it was so tight already the ridges/teeth deformed when I tried to use a pair of pliers (and it wouldn't budge). My theory is that ATC is not perfect, that it will reduce the error in reported salinity at higher temperatures, but it does not compensate perfectly. Does anyone know how the ATC works? All I can find is a reference to a metal strip that deforms in proportion to the temperature, but I am not sure if it bends exactly.

Today, a new Vee Gee STX-3 refractometer arrived, along with two new 35ppt standard bottles (AquaCraft, and Pinpoint). At 9am and 22C/72F the STX-3 reported 35ppt as expected with all my standards (even the DIY standard).

I just tested it this afternoon at 27C/80F, and after waiting 2 minutes for ATC to kick in, the same brand-new standards are now reporting 33ppt. While 27C/80F is somewhat close to the operating temperature limits advertised (30C/86F), my only thought is the ATC is a physical feature is not perfect. It's not like the refractometer suddenly jumps in errors when the limit is reached, but rather the errors grow as temperature rises (but not as quickly if I didn't have ATC).

If that's true, then I can accept the fact I will just have to measure my tank in the mornings when everything is cool and effectively at 20C/68F.

[jonathanyhsu]

I called Vee Gee customer support and described the issue; the representative claimed that ATC should not be off by that much and that measurements within the operating temperature range should be within the advertised accuracy of +/- 1ppt. That being said, if true, it sounds like I have an older damaged refractometer (out of warranty) and I received a new damaged refractometer (which Vee Gee offered to replace free of cost).