Refractometer vs hydrometer

[oahogset]

I know a refractometer is more accurate, but I still have a few questions...
Why is everybody using SG (specific gravity) instead of salinity, even though they are telling people to use a refractometer? The refractometer is actually measuring how much salt is in the water, not how heavy it is compared to freshwater, so why use SG at all?
Telling someone to do hyposalinity at SG=1.009, but make sure you use a refractometer, kinda sounds like an inaccurate statement to me. Why do you call it hyposalinity when you actually don't say what the salinity is supposed to be? How do I know what the salinity needs to be for hyposalinity? SG=1.009 at what temperature?
Is there any reliable conversion tables for SG/salinity at different temperatures? I have seen a few, but they all vary by 5% give or take.
Why don't people calibrate their hydrometers?
I'm sure some of these questions have been answered somewhere in this forum, but I haven't been able to find them.
Thanks in advance!

 

[salty blues]

You have posed some questions which I am also interested to hear the answers to.

 

[notsonoob]

I don't understand calibrate hydrometers. You can only season it once, or am I missing something?

 

[notsonoob]

specific gravity is just the weight of the liquid compared to pure water. Other materials in your water can affect those readings Salinity changes due to temperature.
You would have to compensate your salinity based off of those temperatures and probalby why the hydrometer leads to inaccuracy.
If I have a temperature compensated refractometer then it will base your salinity off of the actual temperature. Which I would say is a ton more accurate.
If you could ensure that you have just pure salt water with nothing else influencing the needle then you would probably be ok, however, how would the seasoned needle stay calibrated?

 

[geoj]

 

[oahogset]

Originally Posted by NOTSONOOB
I don't understand calibrate hydrometers. You can only season it once, or am I missing something?
Well, there are different ways of doing calibration. You don't necessarily calibrate by adjusting your instrument.
Aren't there calibration solutions out there with known salinity/SG? If your known solution has a SG of 1.025 at a specific temperature, and your hydrometer reads 1.028 at the same temperature, you know to subtract 0.003 from your SG-reading when testing your water. It sure isn't as accurate as using a refractometer, but alot more accurate than not doing any adjustment to the reading.

 

[oahogset]

Originally Posted by NOTSONOOB
specific gravity is just the weight of the liquid compared to pure water. Other materials in your water can affect those readings Salinity changes due to temperature.
I know that. I already stated that in the first post.
Originally Posted by NOTSONOOB
You would have to compensate your salinity based off of those temperatures and probalby why the hydrometer leads to inaccuracy.
If I have a temperature compensated refractometer then it will base your salinity off of the actual temperature. Which I would say is a ton more accurate.
I didn't question if or why the refractometer is more accurate. I am questioning why people say to do hyposalinity at SG=1.009 without specifying the temperature. Or is it understood that the temperature is 20°C (68°F) which is the recalibrating temperature of most refractometers?
Originally Posted by NOTSONOOB
If you could ensure that you have just pure salt water with nothing else influencing the needle then you would probably be ok, however, how would the seasoned needle stay calibrated?
I am still not arguing that a hydrometer is accurate, but don't you think stuff in the water other than salt also affect the reading off a refractometer?
One more question; On a refractometer you can read either salinity or specific gravity. Is the value for the specific gravity at 20°C(68°F) (the refractometer's recalibration temperature)? And is it the SG reading off a refractometer people are talking about when telling you to do hyposalinity at 1.009?