Refractometer Users


I have a refractometer which is ATC. Do I go by the salinity or the ppt? Do I need to do adjustments if I go by the ppt? What are the advantages of this way of reading and what should the ppt be at?


Active Member
I use the salinity side of it, and I shoot for 1.025. But this also correlates with the 35ppt. So I guess I use both really.
I think that the ppt is just a more accurate method of measurement.


Active Member
WARNING: Long Windy Reply
Temperature does matter when using a refractometer with ATC Automatic Temperature Compensation.
But in most every case ...... only when we calibrate it.
Refractometers used to measure salinity have an optical glass prizm inside.
As light enters the prizm, the light is bent.
This light is bent and projected through to the eyepiece, where we see the upper and lower color line, as it's projected through the scale graduations.
We place a sample of our tankwater on the glass prizm, flip down the light diffusing lid thing, and allow light to shine through the device.
The amount the light is bent, is a direct result of how much "stuff" is in the sample.
The stuff in this case is salt ( and all the other minerals and elements in our saltwater ).
When we look at pure water - the light is not bent much.
When we look at saltwater - the light is bent more.
The whole thing about using these types of refractometers, and getting good accurate results, is by following the calibration procedure.
When we calibrate it - we place a few drops of distilled water on the prizm. Then we MUST allow this distilled water to come to the same temperature of the refractometer. Normally this is 68 degrees F.
The device is built to be calibrated at 68 degrees F.
If you calibrate it with distilled water at any other temperature - then you will introduce error into the calibration procedure.
Basically - you'd be calibrating it to a different temperature than it was designed for. This would give you false readings every time you used it from that point on.
It's the room temperature that the refractometer is kept in, and used in, that is important when you calibrate it with distilled water.
The drops of distilled water will become whatever temperature the unit is. In other words - you place a few drops of distilled water on the prizm that is already at 68 F.
The drops will either warm up to or cool down to 68 F rather quickly.
Then, after a short period of time, you zero the device by turning the screw to align the scale's 1.00 SG and 0.0 ppt to line up with the two color boundry line. We shift the scale when we calibrate it.
As long as you do this calibration at 68 F, then you're good to go.
Afterwards - should the room temperature that you keep and use your refractometer stay within the 50 to 86 degrees F range, then the ATC auto temp compensation will adjust the reading for you. It's the room temperature ( refractometer temperature ) not the tankwater temperature that is temperature compensated and corrected.
The ATC is just another piece of glass ( prizm ) inside the unit that bends the light backdown. It sort of bends the light in the opposite direction as the main prizm. Not very much - just a little.
This feature "corrects" the image as the scale is viewed through the eyepiece. But again .... this auto temp compensation will only correct the reading we see if the refractometer and sample are at a temperature between 50 to 86 F.
Both salinity ppt and specific gravity "readings" are affected by temperature when using a refractometer. They have to be - they are on the same scale.
The salinity of the tankwater is not greatly affected by temperature, within the ranges we keep our aquariums. That tankwater contains so many parts per thousand of salt no matter what the temperature.
It's when we try to measure it, or convert this ppt into the specific gravity scale ...... that's when temperature comes into play.
Salinity in PPT or parts per thousand is just that.
Seawater is normally around 35 parts per thousand.
If you had 3500 pounds of saltwater, and boiled off all the water, you'd be left with 35 pounds of salts.
Specific gravity is not the same at all.
Specific gravity is a comparison of the saltwater to that of pure water.
Pure water having a specific gravity of 1.000
If you take 1 milliliter of pure water ( 1 cubic centimeter or cc ) and weigh it ...... it would weigh exactly 1.000 grams at 68 F.
If you take 1 milliliter of saltwater ( 1 cubic centimeter or cc ) and weigh it ..... it would weigh more than 1.000 grams at 68 F.
Why would it weigh more ? Because there's more stuff in it than just water. Theres' salt in there too, along with the pure water.
So it's going to weigh a little more. It's going to weigh 1.0XX
XX being the weight of the salt in that little cubic centimeter box.
How much more it weighs ? It all depends.
It depends on the actual amount of salt and minerals dissolved in the sample .... and it depends on the temperature. Now we're talking about density. How much stuff is dissolved in the water.
And the density of a given volume of liquid changes as the temperature changes.
If you're using a refractometer with ATC - and calibrate it right - and stay within the 50 to 86 F range, then you need not worry.
If you're using a refractometer without ATC - you still must calibrate it right ....... but you'll need a chart to correct the reading to the temperature of the sample.
If you are using either a floating or swing arm hydrometer - then you MUST know what temperature it has been calibrated at ( designed at ) and what the temperature of your tankwater is at the time you measure.
Then you look at a chart that plots temperature vs specific gravity - and determine the actual salinity in ppt.
Also realize that there are refractometers used to measure other liquids as well .... and some of them are designed to be calibrated/used at temperatures other than 68 F
Sorry for the long reply - this is probably way more than anyone really "needs" to know about a refractometer - but it is pretty interesting how they work.


Thanks for spending the time to provide such a detailed post. The information you provided was VERY helpful! :) :D