HOW DO I USE THIS REFRACTOMETER!?

[DragonStone]

I have a very expensive, lab quality refractometer that I got for someone who used it for other things. It has the following options for readout:

BRIX%
TA 1990
TA 1971
Oe (GER)
Oe
KMW
Baume

which would be best for measuring salinity and how?? I made up saltwater to manufacturer specifications for 35 PPT and it ended up reading 3.7 Brix...which I see elsewhere is 30 ppt so I feel I am doing something wrong?

 

[Randy Holmes-Farley]

You may not be able to. What is the range of the scale?

Refractometers and Salinity Measurement - REEFEDITION

Title photo by R2R member Rskillz By Randy Holmes-Farley Salinity is one of the most important parameters measured in reef aquaria. It controls not only the salt balance between an organism and its surrounding environment, but also the levels of a host of ions Read more here...

www.reefedition.com

Brix Refractometers

A commonly manufactured type of refractometer is called a Brix refractometer. Its scale usually reads in Brix, or % Brix (percent Brix). These refractometers are used in many industries to measure the concentration of sugar in water such as in the soft drink industry. They can be used to measure seawater’s salinity, but are not always precise enough around the range of seawater’s refractive index to be useful. A resolution of 0.2% Brix is common, and that is borderline acceptable for the reasons detailed below.

Table 4 shows the relationship between seawater salinity, refractive index and % Brix. If a refractometer has a resolution (not accuracy, but resolution, which is the finest amount it can distinguish) of 0.2 % Brix, then that translates to about +/- 1 ppt. So the best resolution would translate to 35 ppt seawater reading 34-36 ppt, which may be adequate for reef aquarists. A Brix refractometer that reads 0 to 10 % Brix with a resolution of 0.1% Brix might be a fine choice for determining seawater salinity in a reef aquarium, (although they are not inexpensive). Some Brix refractometers have a resolution of 0.5 % Brix or even 1% Brix, and they would not be suitable choices.

 

[Randy Holmes-Farley]

Table 4 from above:

 

[DragonStone]

Do you know how this then translates to SG?

The numbers I am getting are not matching - my salt solution says 1.022 but when I measure it, it comes up 3.8 BRIX which is 30 ppt. I added quite a bit more salt and it read...3.8. Is that accurate? What should I be shooting for?

 

[Randy Holmes-Farley]

Do you know how this then translates to SG?

The numbers I am getting are not matching - my salt solution says 1.022 but when I measure it, it comes up 3.8 BRIX which is 30 ppt. I added quite a bit more salt and it read...3.8. Is that accurate? What should I be shooting for?

Sure. A BRIX of 3.8 is 30 ppt, which has a sg of ~1.0226.

As shown in the table above, 35 ppt (an appropriate seawater salinity, sg = 1.0264) is a BRIX of 4.4, and you will want to be able to distinguish that from 4.2 BRIX (sg ~ 1.0249) and 4.6 BRIX (sg ~1.0275). .

 

[DragonStone]

Sure. A BRIX of 3.8 is 30 ppt, which has a sg of ~1.0226.

As shown in the table above, 35 ppt (an appropriate seawater salinity, sg = 1.0264) is a BRIX of 4.4, and you will want to be able to distinguish that from 4.2 BRIX (sg ~ 1.0249) and 4.6 BRIX (sg ~1.0275). .

Okay thank you. Where do I find PPT or BRIX to SG? I have not seen the SG chart anywhere.

 

[TX_REEF]

Okay thank you. Where do I find PPT or BRIX to SG? I have not seen the SG chart anywhere.

 

[DragonStone]

Thank you!! The Brit percentage is by 0.1 with mine

 

[DragonStone]

0.1

 

[Randy Holmes-Farley]

Okay thank you. Where do I find PPT or BRIX to SG? I have not seen the SG chart anywhere.

I created the BRIX to salinity table above using the refractive index of each.

I also have tables that allow conversion of the salinity to specific gravity in articles such as the one below:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

 

[Randy Holmes-Farley]

I do not recommend using that table posted by TX Reef. It is a temperature correction table for a particular hydrometer, and a refractometer has no use for that table. I posted all you need.