Salinity and refractomiters.

[nicholasb]

I have be using a refractomiter, for three years with out any problem. The screw to a just it got rusty, so I invested in a new Read Sea refractomiter.
I fully understand the difference between salinity (weight of salt per volume of water), and S.G (the density which can change with temperature).
When testing R.O.D.I water to calibrate the refractomiter, 0 PPt and 0 S.G are not on the same line. My tank water is 35PPt at 20.c (68f) with the PPT set at 0 with R.O.D.I water. This gives me an S.G of 1.025. If I calibrate refractomiter to read 0 on the S.G scale, I get 1.026/7 S.G and 37 PPT.
Does any one know why 0 PPt and 0 S.G are not on the same line.
Looking at picture of the chart seen through the view finder on the net you can see that 0 PPt and 0 S.G don't match up.
Many thanks.

[jason2459]

1.000 = 0ppt ro/di
1.0264 = 35ppt seawater.

Do not calibrate to 0. Use 35ppt solution from someplace like pinpoint.

[jason2459]

Some good reading on this

http://reefkeeping.com/issues/2006-12/rhf/

http://www.reefkeeping.com/issues/2004-06/rhf/

[nicholasb]

This does not explain the problem with the Red sea refaractomiter.:(

[jason2459]

This does not explain the problem with the Red sea refaractomiter.:(
Because 0 does not equal 0.

As a side note I have had two cheap brine refractometers. A DnD h2Ocean supposedly true sea water one. And a red sea and vee gee true sea water refractometers. The red sea reminded me of a $30 cheap brine Chinese knock off refractometer. I am not impressed one bit with it and it needed frequent calibration. Of all of them the vee gee was easiest to read, best quality, and most consistent in not needing recalibration that often at all. Well worth every penny as it was also the most expensive besides my Milwaukee digital which is nice but prefer my veegee.

[disc1]

The only temperature where 0ppt and 1.000 SG line up together is 4 degrees C. Anywhere above or below that temperature the density of pure water is below 1.0.

[bertoni]

We'd need to know the specifics of what the Red Sea refractometer is designed to answer your question. Salinity now is defined in terms of conductivity and temperature, and SG is dependent on temperature, as has been noted, so it's hard to be sure exactly what the scales are supposed to read.

[woodnaquanut]

The only temperature where 0ppt and 1.000 SG line up together is 4 degrees C. Anywhere above or below that temperature the density of pure water is below 1.0.

Best explanation I've read! Thanks disc1.

I always figured it was cheap refractometer with cheap scales. I calibrate to SG and ignore the ppt scale.

[jason2459]

Best explanation I've read! Thanks disc1.

I always figured it was cheap refractometer with cheap scales. I calibrate to SG and ignore the ppt scale.
Yeah I guess it was better then just 0 doesn't equal 0. lol