The Puffer Forum

Not all are created equally, even two different units of the same brand and model. Too much variance, too much hassle.

And it's not even a cost issue really, because you can find refractometers for very cheap.

Too many pros on the refractometer side and too many cons on the hydrometer side. It's a no brainer.

I personally don't think it's as easy as this. For one thing, price. A basic floating hydrometer is $5, a cheap refractometer at retail is around $40-50, possibly cheapter online. If someone is asking me about setting up a brackish water aquarium for the first time, and they'd sooner stick with measuring cupfuls of salt per gallon, getting them to spend $5 is way easier than $40.

Secondly, a floating hydrometer can be accurate if used properly. Many people don't know how to use them (e.g., they stick them in the tank rather than a glass jar of aquarium water). Some people never understand that they should ignore the meniscus and read only the water level. Others don't clean them properly. Not all hydrometers are calibrated to room temperature, but most are, and aquarium water should be at or close to that calibration temperature to get an accurate reading.

Thirdly, not all fish need this level of accuracy. Brackish water fish certainly don't. Scats, monos, GSPs, etc can all survive massive salinity fluctuations in the wild. To worry about accidentally adding SG 1.0125 water in the aquarium instead of SG 1.0122 is nonsensical. In fact, there's good evidence (i.e., fish spawn readily) that providing salinity fluctuations is good for these species. The only times things like monos and scats have spawned in aquaria is where people have made big (sometimes insane) salinity changes, and for things like bumblebee gobies and glassfish it is absolutely standard practise to change salinity to trigger spawning.

Likewise, if you have hardy marines that live in estuaries (Arothron and Platax spp.) or tidepools (Abudefduf spp. damsels and Micracanthus spp. stripeys), then again, these fish have evolved to tolerate dramatic changes in salinity. They couldn't care less about the accuracy of salinity measurement provided it is within some sort of ball-park range (say, 2-3 hundredths on the SG scale).

Fourthly, I'm also concerned that by assuming the technology is better, people ignore the technique. A badly used or badly maintained refractometer is every bit as inaccurate as any other carelessly used measuring tool. I'd also assume that very cheap refractometers are inherently less reliable or durable or easy to use than the higher-end, more expensive ones. Scientific and professional optical equipment is expensive, and to make consumer-market versions corners are invariably cut. This holds true with telescope and binoculars, for example, and I'd have to assume for refractometers.

So while I agree a good refractometer used properly is a better device for measuring specific gravity than a cheap floating hydrometer, it isn't exactly a "no brainer". There are definite pros to floating hydrometers: specifically their low cost, easy availability, simple maintenance, and sufficient accuracy for brackish/hardy marine applications.

Cheers,

Neale

Phaedrus wrote:Not all are created equally, even two different units of Too many pros on the refractometer side and too many cons on the hydrometer side. It's a no brainer.

Thanks for the rebuttal!

nmonks wrote:than the higher-end, more expensive ones. Scientific and professional optical equipment is expensive, and to make consumer-market versions corners are invariably cut.

Lab grade hand-held refractometers can run up to $1K. Counter top refractometers are usually in the $5-6K range. Both are used on many different liquid types.

We have one of the counter top units in our lab but I've never checked it against my hydrometers.


MP

I'd be very interested to seee how a $12 hydrometer compared to a $6K refractometer. Get back to us on that!

I'd suggest that's comparing apples with oranges. Of course a $5 floating hydrometer like the one I use is going to be less accurate than a lab-quality refractor costing $1000.

A better question is whether a consumer-market refractometer that purports to read to 4 decimal places is as accurate as a scientific-grade instrument that seemingly does the same thing. In other words, I'd expect a lab instrument to read a number like 1.0100 with a margin of error of less than 0.00005 (i.e., 1.00995-1.01005) whereas the consumer model showing the same thing would have a much larger margin of error. Given the price differential is x20 ($50 compared with $1000), then a 20x larger margin of error is not unreasonable. So while the consumer market refractometer may show a value of 1.0100, because the margin is 0.001, the real specific gravity could be anything between 1.0090 and 1.0110.

This is all speculation of course, and I have no idea what the actual tolerances of the two different instruments are, but I have to assume that the lab and the consumer models have very different tolerances given their price differentials.

The moral of the story is that all consumer-end scientific instruments are compromises of accuracy against cost. The question is whether the low cost/moderate accuracy of a hydrometer is good enough for your uses than a higher cost/higher accuracy of a refractometer. I'd argue that for brackish and hardy marines a properly used hydrometer is fine, but for a tank with delicate marines or invertebrates, then a refractometer may well be useful. On the other hand, remember what your maths teacher told you: just because a calculator gives you more decimal places than a slide rule, it doesn't mean that your calculations are any more guranteed to be "right". Use either incorrectly, and you'll get wrong answers.

Cheers,

Neale

Pufferpunk wrote:I'd be very interested to seee how a $12 hydrometer compared to a $6K refractometer.

Check this out:
http://www.reefcentral.com/forums/showt ... genumber=1

Hysterical... reminds me why I'm a useless lab scientist. My head hurt after seeing all those numbers.

I especially like the guy who lists the contents of his "90 Gallon Mixed Reef" as a bunch of pumps and skimmers and gizmos and not one single fish or invert! It's a whole 'nother world of fishkeeping. I'm old school... if the water smells good and the fish look happy, that's all the water testing I do.

Anyway, thanks for the link!

Neale

Yeah, I can't believe all the gadgets folks have to run their reef systems! I'm just getting into the sump/fuge thing myself...

Dunno if I should trust my refractometer now. We had the head chemist from the Shedd Aquarium speak at our last reef club meeting. He swears by the simple swing-arm hydrometers. Said they were actually better than refractometers, but I don't remember why.

It probably doesn't matter which a person uses, so long as they use it properly and maintain it correctly. So if you happen to like your refractor, and I admit they are great fun to play with, then why not stick with that?

Unless you're keeping a reef with inverts (or delicate marines like angels or butterflies), a little error here or there can't possibly matter. I'm sure your GSPs are much more fussed about low nitrates and zero ammonium than they are whether the salinity is 1.015 or 1.020, let alone 1.0185 or 1.0187.

Cheers,

Neale

Ummm... I actually traded in my GSPs for a reef tank. They don't seemed to be bothered by my refractometer either--doing quite well!

Exactly; it's the skill and care that matters more than the equipment used. So while good equipment well certainly help, what really matters are the basics like water changes, the right food, picking suitable species, and so on.

Neale

Pufferpunk wrote:They don't seemed to be bother by my refractometer--doing quite well!

Pufferpunk wrote:Check this out:
http://www.reefcentral.com/forums/showt ... genumber=1

THAT WAS FUNNY!

I'm almost tempted to log in and suggest they check out the ASTM method - (ASTM D1429-03 Standard Test Methods for Specific Gravity of Water and Brine). After all if you're going to measure salinity you should be using the correct method!

It would probably prolong that thread for at least another few weeks.

MP

nmonks wrote:
...The only times things like monos and scats have spawned in aquaria is where people have made big (sometimes insane) salinity changes,
...

Were can I find information about spawning of monos (except M. sebae) and scats. Can you recommend any literature reference? I have not found one report on spawning of these two groups, yet. Older Monos often pair up and clean rocks, but I've never read about them actually laying eggs.

There's nothing published. The last thing I read was a letter sent to me via the PFK magazine, from someone keeping their monos in freshwater. And yes, they were spawning.

Cheers, Neale