Hi guys:
We're not chemists, but we know enough to be dangerous. Dan, to answer your question simply - Down. Refracts measure the salinity content of seawater using a known refractive index of an element, and the results are normally read in ppt. In a sense, yes it's measuring the density of a sample, but the role temperature has in the results with a refract are not as direct as with a hydrometer. First, with a hydrometer (swing arms we're talking about, but the same should apply to floating ones too) the measuring instrument is submersed in the water, thus temperature has a huge direct affect on the result. Whereas, with a refract, the sample is so small that as soon as it hits the prism plate, the temperatures begin to equalize immediately (or at least within about 30 seconds).
The ATC that refracts can come with are a nice "luxury", but not necessary. The atc is there really to calibrate the refract to the ambient air temp where the measurement is going to be taken, not the sample to the refract. One note about atc's is that they can go bad after a while. Not sure how long of a while, but the strip of metal that the prism sits on will expand and contract (acting like a spring) due to the air temp. Because of it's continual expansion and contraction the "spring" will eventually wear out. That's why sometimes it's better to go with a refract w/o the atc. Just our opin.
The cheap Chinese refracts that everyone buys in the hobby happen to include both the ppt and the specific gravity scales, very convenient, since we're all so used to using the s.g. scale. However, we just learned something curious over on RC, that all Chinese refracts are actually calibrated to measure NaCl, not seawater. However, as we understand it, the difference between the two scales is minimal, and there appears to be a conversion table available. Do a search over on RC for the thread. HTH - Staff
