please help me build a electronic refractometer

Thread Starter


Joined Apr 24, 2011
Hi my name is Dave and my hobbies include marine fish keeping. I currently have a refractometer (for measuring salt). Currently I manually add a few drops of salty water to the sloped end of what looks like a telescope. I then observe the reading form the other end.

Where the white and blue meet is the current salt level.

So my question is can I build an electronic version, one that can turn the reading onto an electronic number that i can just glance at. Also if it could have some way of turning on a switch if the level gets too low (one that I could set too a certain level).

A way I thought of doing it would be some kind of photo sensitive diode at the reading end and a light at the other end and the amount of resistance would determin the level of salt in the water. I would however think that this would have to be very sensitive and I would imagin as the bulb ages the light levels would alter the results so maybe a laser.

Any help would be much appriciated and if you need any clafication I will try my best. It might be easier to have a usb socket so I can adjust sensitivity etc.



Joined Sep 9, 2010
The refractometer mfgs offer this type of self-reading, however I haven't seen it in a hand-held. A really tough project to DIY. On the other hand, though, consider using a video camera to interface with the existing optics and put the reading onto a screen. It's not hand held anymore, but might be easier to read.

Wouldn't it be easier to just measure conductivity?


Joined Jul 7, 2009
Read up on the Abbe refractometer to know what you're up against. If you want to spend some bucks, there are industrial controls that can do what you want.

Like wayneh, my first thought was measuring the conductivity. I'd start with a cheap digital multimeter and put two probes in the water and read the resistance between them. You should be able to use the refractometer to calibrate the resistance readings. Once you know that works reliably, then it's not a big step to making some control circuitry.


Joined Oct 15, 2009
Wouldn't it be easier to just measure conductivity?
A visual device like he has is called a refractometer. The digital version of it IS basically a conductivity meter.. He actually wants a salinity/conductivity meter. A salinity meter is simply a conductivity meter that has the output scaled to match typical salinity measurements (ppm,etc..). Don't forget temperature compensation... A few degrees will really skew your results if not compensated for..


Joined Sep 9, 2010
The digital version of it IS basically a conductivity meter..
Not really. There really are digital-reading refractometers, and they're used extensively in the sugar syrups businesses, including the wine business. But they're quite expensive ($5K?) and I've never seen a hand-held one.