[lindsay]

After talking to Marcus yesterday,i went and did some digging and came up with this .

http://www.theaquari...r-refractometer

[chriss]

Never had a problem so far, good marketing or am I just cynical?

Chris

[Marcus Watts]

its not so much having a problem with the lower readings, they have just pointed out if you unknowingly mix your salt to a lower concentration you are getting lower mag, calcium and trace elements too, needing more additional dosing to bring the levels in line with what we want in the aquarium.

if the new refractometer was more expensive i'd potentially agree but it's exactly the same price as the old one , and it is 100% accurate when they say basically all refractometers used in the hobby are designed & scaled to indicate brine solution - just the refractive index of pure salt and water - our reef salt is not pure salt but a blend of salt plus additional elements (magnesium, calcium, 80 odd trace elements etc) so therefore has a different (higher) reading on the refractometer scale, leading to an error where we make otr salt solution up too weakly.

I thoroughly read a lot of available information (not just the dd website- one german reefkeeper said this 3 years ago!!!) a month or two ago and totally agree with the dd statement. Over the last few weeks i've used strong 2% water changes (40ppt) twice a week and used salt water to replace some of the evap. This has bought my tank up to 37 - 37.5ppt which is about 1.028 @ 20 degrees (this is a closer to correct reading on a brine refract) . Since then i have not added one gram of magnesium and turned the kalk stirrer right back to an occassional trickle, but that is questionable now as I've seen something very interesting about calcium levels (cheers linds)

one down side -----every single lfs i've walked into has stocks of the old ones on the shelf so are avoiding stocking the new ones yet only one has said they're not out yet !!!

[karnivor]

OK, this might just sound a little too easy, but why cant I go down to my local beach, steal a bottle of sea water an calibrate my refractometer to 35ppt using that?

[Marcus Watts]

karnivor, on Apr 11 2010, 11:28 PM, said:

OK, this might just sound a little too easy, but why cant I go down to my local beach, steal a bottle of sea water an calibrate my refractometer to 35ppt using that?

you could if you know how much mag, calcium etc is in our local seawater , and its actual salinity - trouble is these all vary from ocean to ocean - and to a certain degree from place to place. once you use fistral say as your calibation point then you would be mixing your salt water up to be the same as fistral was the day you took the sample.

if the true scientific makeup of fistral on the day you got the sample was lower in mag and calc, and even salinity, than we like for a reef tank but you calibrate a refractometer to tell it this is a 35ppt solution you will always be out.

no-one needs to buy a new refractometer and I dont think DD are suggesting we do - they have just pointed out that people using an old refract, using 35ppt as there target point are not using a high enough reading to account for the other elements that effect the refractive index of the sample.

if we use a brine refract and make a reef solution to 35ppt it will contain roughly33.5 ppt of actual salt and 1.5ppt of elements - this is not a strong enough solution to get the correct levels of magnesium, calcium and other elements contained in our reef salt.
To get those correct as well a reading of 37 is closer to what we need. - I do think there will always be a degree of inaccuracy because each salt maker puts different amounts of mag/calc/etc in their bucket - It occurs to me that DD salt users will get the most accuracy from this product as it will account for the levels they use in their H20 brand as part of the reading.

Buying their calibrating solution (35ppt) and telling your old refractomer that this is 35 should be all you need to do. Calibrating with ro at '0' apparently leads to increasing inaccuracy the further away from 0 we go up the scale, the calibration has to be close to, or the actual point we want, to be accurate.


I'm only buying a new one because the scale inside is much bigger so easier to see (age I know!! ) and my top cover has been snapped off for ages so i have to balance it on the lense to take a reading.