refractometer

[thetonyage]

my refractometer measures the salinity in percentiles, from 0-28%. how do i convert that reading to SG?

[philneuss]

Maybe the 28% indication is coincident with 1.028? Im not sure!

[Flying_Hellfish]

Wait..... I was thinking 35ppt, not 35% ..... my bad. But wouldn't 35ppt be 3.5%?

Second edit:

From wikipedia on Saltwater: On average, seawater in the world's oceans has a salinity of about 3.5%

[IAIN -Y-]

Hellfish, I would certainly never consider Wikipedia to have accurate results as even the likes of me (newbie) can edit what is written on there.

Maybe this may help?

http://www.northcoastmarines.com/refract.htm

[Flying_Hellfish]

Originally Posted by IAIN -Y-

Hellfish, I would certainly never consider Wikipedia to have accurate results as even the likes of me (newbie) can edit what is written on there.

Maybe this may help?

http://www.northcoastmarines.com/refract.htm

I'm not saying it is always accurate, but considering I figured 35ppt (1.0264 SG) to be 3.5% and it said the same, I presumed it to be so.

Does this make you feel a little better about it?

The salinity of seawater is usually 35 parts per thousand (also written as o/oo) in most marine areas. This salinity measurement is a total of all the salts that are dissolved in the water. Although 35 parts per thousand is not very concentrated (the same as 3.5 parts per hundred, o/o, or percent) the water in the oceans tastes very salty. The interesting thing about this dissolved salt is that it is always made up of the same types of salts and they are always in the same proportion to each other (even if the salinity is different than average). The majority of the salt is the same as table salt (sodium chloride) but there are other salts as well. The table below shows these proportions:

via http://www.marinebio.net/marinescien...omposition.htm

I don't know the actual conversion for other SG but I believe most are keeping their reef tanks around 1.026 so 3.5% would be a good # to shoot for.

[brycerb]

Doesn't it have two scales, one on each side? Mine does SG and PPT.

[thetonyage]

Originally Posted by brycerb

Doesn't it have two scales, one on each side? Mine does SG and PPT.

No, its an optical one that just has the percent scale

[thetonyage]

Originally Posted by IAIN -Y-

Hellfish, I would certainly never consider Wikipedia to have accurate results as even the likes of me (newbie) can edit what is written on there.

Maybe this may help?

http://www.northcoastmarines.com/refract.htm

RHS-28 is the model similar to what I have

[bertoni]

I'd just buy some of the PinPoint 53 mS solution, and then target whatever it reads. Even if you can find a conversion chart, refractometers often are inaccurate without good calibration. RO/DI water doesn't always work well.

[thetonyage]

bump

[IAIN -Y-]

These 2 might help.

http://66.102.9.132/search?q=cache:8...&ct=clnk&gl=uk

http://www.reefs.org/library/salinity.html

[Flying_Hellfish]

Originally Posted by bertoni

I'd just buy some of the PinPoint 53 mS solution, and then target whatever it reads. Even if you can find a conversion chart, refractometers often are inaccurate without good calibration. RO/DI water doesn't always work well.

This is a very good point. You could calibrate to 10% or something so you have leeway either direction.

I bought this for my refractometer.... http://www.bulkreefsupply.com/store/...ion-fluid.html

[Randy Holmes-Farley]

my refractometer measures the salinity in percentiles, from 0-28%.

Is it really percent (= %), or does it read o/oo, which is ppt.

If it reads in percent, it is somewhat unlikely to be precise enough for reef tank use with the whole scale running from 0 to 28% salt.

[Flying_Hellfish]

Originally Posted by Randy Holmes-Farley

my refractometer measures the salinity in percentiles, from 0-28%.

Is it really percent (= %), or does it read o/oo, which is ppt.

If it reads in percent, it is somewhat unlikely to be precise enough for reef tank use with the whole scale running from 0 to 28% salt.

If he has the RHS28 from the link above then it is %, but if it is PPT that scale doesn't even go high enough.



And for the doubting of the 3.5% This LINK from an article in the "Look Here for Answers" sticky in this forum says this:

Seawater is approximately 96.5% water and 3.5% salt, by weight. When the salinity of seawater is referred to as being 35 ppt (parts per thousand), that is the same as saying 3.5% salt.

I was starting to doubt myself on the conversion. lol

[thetonyage]

Originally Posted by Flying_Hellfish

If he has the RHS28 from the link above then it is %, but if it is PPT that scale doesn't even go high enough.



And for the doubting of the 3.5% This LINK from an article in the "Look Here for Answers" sticky in this forum says this:

Seawater is approximately 96.5% water and 3.5% salt, by weight. When the salinity of seawater is referred to as being 35 ppt (parts per thousand), that is the same as saying 3.5% salt.


I was starting to doubt myself on the conversion. lol

it definitely reads in percent (%). so i want a reading of 3 and a half percent on my refractometer?

[Randy Holmes-Farley]

Well, close to 3.5%,

FWIW, a refractometer that reads fro 0 to 28%, you are going to have very poor resolution in the range of 3.5%.

The link above says the RHS28 has a resolution of 0.2. Assuming you can get that in practice, then you'll be seeing 35 +/- 2 ppt, and that's not especially precise.

It may be adequate, but not optimal.

Be sure to use a seawater standard to calibrate this refractometer as it is a salt refractometer, not a seawater refractometer. So 3.5% is not exactly 35 ppt as made, even if made perfectly.

[Randy Holmes-Farley]

Just to clarify a bit, if that RHS28 refractometer is made perfectly and calibrated to Di water perfectly, it will read 33.3 ppt seawater as being 35 ppt seawater.

i discuss such issues in great detail here:

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


from it:

Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers



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Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is.

It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt).

Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in specific gravity. This figure shows the measured salinity reading for seawater being about 0.0003 lower than it really is.

Regardless of a salt refractometer's scale reading (ppt or specific gravity), aquarists can get around this problem by calibrating this type of refractometer in a seawater standard (see below). Because that type of calibration also gets around important manufacturing errors (slope calibration defects due to the scale being the wrong dimensions), it solves both problems at once.

[Flying_Hellfish]

Originally Posted by Randy Holmes-Farley

Just to clarify a bit, if that RHS28 refractometer is made perfectly and calibrated to Di water perfectly, it will read 33.3 ppt seawater as being 35 ppt seawater.

i discuss such issues in great detail here:

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

Thanks! I always read it was a good idea to calibrate and this explains why. Glad I bought the calibration fluid when I bought my refractometer.

[Randy Holmes-Farley]

Happy Reefing.