which refractometer to get?

[craqerjaq]

My Red Sea hydrometer is a piece of crap and since a lot of people are recommending a refractometer...where exactly do I get one.

If someone can give me a link to a site that sells refractometers...people are saying they are around 40 bucks but I'm finding a huge price range and really don't know what to look for.

[Ninong]

Hi craqerjaq,




They come in a wide price range but all you really need is the basic $39.99 model. It works just fine.

I bought mine from this guy for $42.99. It works fine!



P.S. -- Read Randy's article: Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

[craqerjaq]

that will do...thanks

[craqerjaq]

it came today and I have to say these things are pretty cool...didn't really get how it worked till I tried it...and I understand why people recommend them.

[Ninong]

Okay, now that you have it, here's some new info to absorb. What you have is a saltwater refractometer, not a seawater refractometer.

What this means is that if the refractometer reads zero with distilled water, then when it reads 35 ppt salinity, you will really have only 33.3 ppt salinity. This is not a problem. My goal was to get a reading of 36 ppt on the refractometer scale knowing that that meant that my actual salinity was somewhere around 34.3 ppt.

You can recalibrate your saltwater refractometer using calibration fluids so that it will read seawater correctly but that's really unnecessary and it looks like a very big PITA, based on Randy's explanation in the following article:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

Read through that article to get an understanding of how a refractometer works, but if you're like me, you will get bored with the lengthy explanation on how to recalibrate it. All you have to do is keep it calibrated with distilled water so that distilled water reads zero. And then realize that whatever the reading is in ppt, your true ppt is about 1.7 lower.

Virtually all hobby refractometers are salt refractometers, not seawater refractometers. Most hobbyists using refractometers don't even know this. Their salinity is lower than they think it is.

And now that you have a refractometer, get used to the idea of discussing salinity in ppt (parts per thousand), not specific gravity. You will also see the abbreviation PSU (practical salinity units) or simply S=. Both mean exactly the same as ppt. 35 ppt can be expressed as 35 PSU or S=35. That last one, S=35, is the one most often used in the literature. Of course S could equal more or less than 35. I'm just using that as an example since S=35 is the average salinity over coral reefs.

It's better to avoid using specific gravity because many people are confused about the correct specific gravity equivalent for NSW of 35 ppt. You will often see it expressed as 1.025 SG but it should be 1.0264 SG. If you only take your readings from the ppt scale and only report in ppt, then you will never have to be concerned about people misunderstanding what you're talking about. People are confused by the saltwater specific gravity scale that shows up on saltwater refractometers. They forget about the adjustment. And the adjustment for specific gravity between saltwater and seawater is not the same as the adjustment for salinity between saltwater and seawater.

Just stick to salinity readings and realize that if you want salinity of 35 ppt, then you would have to read 36.7 ppt on your new refractometer. If your reading is 35 ppt, then your true salinity is 33.3 ppt. Or just go with 36 ppt knowing that it will be above 34 ppt but less than 35 ppt.

[craqerjaq]

As far as calibrating the refractometer...is using bottled spring water or whatever it is the same as distilled for setting the device? Before I started changing the density it was reading 1.022 on the refractometer...I'm currently working on raising the SG, it's now reading 1.026. so I'm a little low then?

[Ninong]

As far as calibrating the refractometer...is using bottled spring water or whatever it is the same as distilled for setting the device?

Spring water may (probably does) contain minerals. That's what gives it its taste. Distilled water (or R.O./D.I. water) has no taste at all.

For that reason, you should never use spring water (or tapwater) for making up your saltwater or for freshwater top-off. You should clean off your refractometer afer each use (preferably by rinsing with R.O./D.I. water and wiping with a clean cloth). If you have R.O./D.I. water, then use that to check your refractometer's calibration.

However, when it comes to calibrating a refractometer, it really doesn't make a lot of difference because the TDS numbers we're talking about are truly insignificant when compared to the TDS numbers for seawater at 35 ppt. Therefore, it would be better to use distilled water, but if you don't have any distilled water, go ahead and use spring water or even tapwater.

You should NOT have to calibrate your brand new refractometer. It comes calibrated from the factory. IMO, it would be better to NOT attempt to recalibrate it to make it read like a seawater refractometer unless you happen to have the appropriate specific gravity calibration fluids on hand. Very few people do.

If you don't have distilled water or R.O./D.I. water, then use a couple of drops of spring water just to convince yourself that your new saltwater refractometer is indeed properly calibrated from the factory. It should read zero. If it does, and I'm 99% sure it will, then you're good to go. Just remember, when your new refractometer reads 35 ppt, you actually have only 33.3 ppt salinity.

So, if you want 35 ppt salinity, bring your reading up to somewhere between 36-37 ppt on the salinity scale of your refractometer. Actually 36.7 ppt to be exact but refractometers aren't easy to read to that degree of precision. Just make sure it reads at least 36 ppt and less than 37 ppt and you will be in good shape. And stop referring to specific gravity.

Before I started changing the density it was reading 1.022 on the refractometer...I'm currently working on raising the SG, it's now reading 1.026. so I'm a little low then?

Please read just the salinity scale -- that's the ppt scale -- and ignore the specific gravity scale. Since you have a saltwater (NaCl) refractometer and NOT a true seawater refractometer, when your refractometer reads 36.7 ppt NaCl, you will have 35 ppt seawater. I liked to stop at 36 ppt on my refractometer because I wanted my aquarium's salinity to be between 34 and 35 ppt but I didn't want to go above 35 ppt.

[craqerjaq]

Refractometer

that is the one I got specifically...I didn't have to adjust it when I got it...I justtested it out a little with different water samples I had available...one being bottled water and the other the tank water. I like the thing but this is one that reads higher than actual, right?

[Ninong]

I like the thing but this is one that reads higher than actual, right?

Yes, virtually all hobby refractometers are NaCl refractometers, not seawater refractometers. When yours reads 35 ppt salinity, you really have 33.3 ppt salinity. If it reads 36 ppt, you really have around 34.3 ppt.

[Laurie]

I've found that it's important to hold your refractometer at just the right angle in order to get an accurate reading. I thought my refractometer was reading too high too - and I couldn't adjust it down to 0. Then I realized I wasn't holding it in a true horizontal position. When I leveled it I got an accurate reading.