Hydrometer vs Refractometer

[midgetwaiter]

Originally Posted by fishoholic

.004 of a difference is normal between a hydrometer and a refractometer. That why refractometers are recommended. A friend of mine's hydrometer was out by .008, that is why hydrometers are not very reliable.

Steven Pro did a test a little while back and compared 10 hydrometers he got from people against an electronic salinity probe and a refractometer. He found them to be both precise and accurate. The reason people prefer refractometers it that they are easier to use but this idea that swing arm type hydrometers are no good is a net myth.

Originally Posted by JDigital

Your refracto should have come with calibration fluid, I got mine at Red Coral too, same one, and mine came with it.

All of the refractometers sold recommend calibration with pure water. It doesn't always work right though. Check this out:

http://reefkeeping.com/issues/2006-12/rhf/index.php#19

Originally Posted by mike31154

I've seen on other boards that some folks recommend calibrating refractometers using a fluid (pinpoint usually) which is at 1.026 specific gravity. The reasoning being that this is where you want your refractometer to be most accurate and if you calibrate at zero, it's not a guarantee that it is accurate at the salinity range we usually check. I asked the question on that board if anyone had calibrated using the 1.026 fluid and then checked the zero with pure water to see if there was any error and never did get a satisfactory response.

It's quite common actually, you're lucky but it could change over time so keep an eye on it. Usually you end up with the refractometer reading about 1.030 but it's a slope calibration issue and it's really about 1.025. I've won 2 bets on this very subject recently.

[mike31154]

m..waiter, I'm not sure I fully understand your response to my quote? What is 'quite common' and how do you end up with 'the refractometer reading about 1.030...?? Bets on what exactly?

[StirCrazy]

Originally Posted by midgetwaiter

Steven Pro did a test a little while back and compared 10 hydrometers he got from people against an electronic salinity probe and a refractometer. He found them to be both precise and accurate. The reason people prefer refractometers it that they are easier to use but this idea that swing arm type hydrometers are no good is a net myth.

actualy it is not a myth at all, over the years there have been several colections doen and the swing arms only have a 25% accuracy rater from new.

also because of the way they are used they have a tendency to go farther out over time.

out of the 6 swing arms I have only 2 were accurate, I threw the rest out and use the 2 for quick checks and store them with ro water to prevent salt from changing the accuracy.

Steve