Salinity Issues, refractometers & calibration. Help!

[medicreefer]

I thought I was doing quite well with my refractometer and salinity until I read the Randy Holmes-Farley article about refractometers. As it turns out, I don't think I've been doing so well after all. I'm still having problems/questions that are frustrating me a bit.

First of all, I've discovered that my refractometer is a salt/brine refractometer not a seawater refractometer (as it had been marketed). It has 2 scales, one reading in PPT and the other reading in SG. It has auto temperature correction (ATC) and it clames the ATC range is from 50F to 86F. In the instructions it recommends calibration by "adjustment for null" with distilled water. It also recomends that adjustment for null be carried out in a room with ambient temperature of 68F. I don't get this requirement since the device has the ATC feature. It also says that when working in an environment that is greater than 5F different from this then it should be recalibrated. So then what's the point of the ATC range of 50F-86F?

Anyway, I've been using this refractometer as instructed for about 4 months. I've been calibrating with distilled water and making up salt mixes to 35ppt or so I thought (although apparently this means that if correct the salinity was actually 33.3ppt because it is a salt refractometer not seawater but that's no huge deal).

Yesterday I finally bought the Pinpoint 53.0 solution (with the same refractive index as seawater). After Calibrating my refractometer with this I did a couple of tests. First, the distilled water now reads a fair bit below the 0 line. Second, my supposedly 35ppt tank water now reads at 31 ppt, so my refractometer has been quite "off" when calibrating with the distilled water and I've been running with fairly low salinity (assuming the calibration solution is correct). What's even worse is that my Instant Ocean Hydrometer is even more wrong. It has been reading my tank at a SG of 1.0275 or 37ppt. I didn't think this was terrible with my refractometer reading the water at 35ppt, but now that I know it's only 31ppt that means my hydrometer is way way off. Once I correct my water to a real 35ppt my hydrometer will probably read well over 40ppt. Imagine if I had only ever had the hydrometer? Is it even possible that it's that wrong? I do however some questions about the Pinpoint solution...
- Can it be wrong?
- It says that it has a specific gravity of 1.0264 and salinity of 35ppt at 77F. Does that mean I have to heat up the solution to 77F to properly calibrate my refractometer or is the ambient temperature supposed to be at 77F?... Or does it really matter? I would have thought this shouldn't be an issue with the ATC refractometer but as noted above, even the ATC feature is confusing me.
- Is there any way I can verify the Pinpoint Solution's values?

This is probably all much more simple than I'm making it out to be and I'm hoping somebody can come out and tell me that, with a simple explanation of how to make sure I'm getting the correct salinity measurements.

Thanks for helping out.

[Randy Holmes-Farley]

It also recomends that adjustment for null be carried out in a room with ambient temperature of 68F. I don't get this requirement since the device has the ATC feature.

That is the usual claim, and I agree it makes no sense.

First, the distilled water now reads a fair bit below the 0 line.

That's normal.

Second, my supposedly 35ppt tank water now reads at 31 ppt, so my refractometer has been quite "off" when calibrating with the distilled water and I've been running with fairly low salinity (assuming the calibration solution is correct).

Many people do find that.

What's even worse is that my Instant Ocean Hydrometer is even more wrong. It has been reading my tank at a SG of 1.0275 or 37ppt.

That too is not especially surprising.

I do however some questions about the Pinpoint solution...
- Can it be wrong?

it can only be wrong if it was mismanufacturered, or if someone tampered with it after manufacturing (like buying it, using some, replacing it with RO/DI, and returning it). Both are unlikely, but possible.

- It says that it has a specific gravity of 1.0264 and salinity of 35ppt at 77F. Does that mean I have to heat up the solution to 77F to properly calibrate my refractometer or is the ambient temperature supposed to be at 77F?

No, just use it within the ATC range of the refractometer. You can double check that by heating it, however.

- Is there any way I can verify the Pinpoint Solution's values?

You can make your own if you have a good balance, or check with another type of standard or conductivity meter.