Mark,
 
The company I bought my refractometer from is selling one non-temperature
compensated model for $218 & a cheaper model for $159. The one I have is the
$218-model. (http://www.aquatic-eco.com/products/sr1/sr1.htm)
 
Another company that I frequently buy stuff from is Cole-Parmer
(www.coleparmer.com). They have a temperature compensated salt refractometer
for $290. Even if you think that is expensive, order a copy of their free
catalog. They sell all sorts of stuff from magnifying glasses to dial
calipers.
 
Salinity is defined as the gram of dissolved salts in a kilogram of sea
water, i.e., g/1000 g or ppt (parts per thousand). Because the weight of a
substance is temperature-independent (unlike volume), salinity by itself,
because it is a ratio of two weights, is temperature-independent. However,
when you use a refractometer you don't measure salinity directly; instead you
measure refractive index, which happens to be temperature-dependent.
Therefore, if you use a non-temperature compensated refractometer you have to
make sure that the refractometer itself & the distilled water used to
calibrate it are at the same temperature as the sample. So when I am doing
field work, rather than keep recalibrating the refractometer & wondering if
everything is at the same temperature, I simply collect samples in sealed
tubes (evaporation will change salinity), take them to my lab (i.e., my
basement), wait for everything to equilibrate to the basement temperature,
calibrate refractometer, then measure the salinities of all the samples. To
check the accuracy of my refractometer, I prepared salt (NaCl) solutions of
known concentrations ranging from 5 to 40 ppt. I read these at the beginning
of each session to make sure everything is okay. Last summer I collected
water samples from several locations in the Aegean Sea. Later, when I
measured their salinities via this method, I got values that were within
about 1 ppt of the published values that had been determined chemically.
 
A.