Hydrometer vs Refractometer

[mike31154]

I have 3 hydrometers and 1 refractometer. One of the 3 hydros is the type you float in water and the other two are swing arm types. All 3 hydros came with used tanks that I purchased. The 'float in water' hydrometer appears to be more accurate than the two swing arm types and reads favourably when compared to the refractometer. In addition the float hydro has a built in thermometer. One thing to remember about hydrometers is they are not temperature compensated and since they are initially calibrated to read at a certain temperature, if you don't compensate, you will get inaccurate readings. Most refractometers are automatically temperature compensated, you just need to wait a few moments for the fluid to get to the same temperature as the refractometer glass.

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. Personally, I calibrate at zero and have even found that it's almost impossible to see a difference at zero whether I use Vernon tap water or distilled water.

Since getting the refracto, I keep my water at around 1.024 on the refracto scale. I don't worry too much about a point or two difference. But I do make sure that when I perform a water change, the new water is as close as possible to the tank water. I check it two or three times to make sure since I tend to change a lot of water every 20 days or so vice smaller changes more frequently. Other than my fish, cuc, I have an anemone and a number of soft corals as well as a hammer and all the livestock seems fine with this level of salinity.

[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