Refractometer and hydrometer

iidylii

iidylii

Active Member
so I know this topic has been beaten to death but I just wanted to hear some more lol...

Thought it also might help some new ppl

I have the atc refractometer and try to keep my ppt at 35ish...

My dad has a hydrometer at the moment...I know that I always hear that your specific gravity reading is going to read lower then it actually is...for example 1.023 might very well be 1.025-1.026 and so on...my question is...is the ppt reading on the hydrometer going to read lower as well or is that going to be accurate? I guess I'm also not sure if the atc unit is also reading false in the ppt? I assume on both units the ppt is correct and the specific gravity reads a little low
 
jay0705

jay0705

Well-Known Member
If its an accurate hydro it should be the same at the avg tank temp. The problem is alot of them just are not accurate
 
iidylii

iidylii

Active Member
Yeah but I thought they were calibrated to read specific gravity at 60 degrees? Like I said a reading of 1.023 at 78 degrees would be more like 1.026 or whatever...maybe i dont know what I'm saying lol
 
bang guy

bang guy

Moderator
You are right that a hydrometer calibrated for 60F will read a slightly lower specific gravity (density). The ppt reading on a hydrometer will not be accurate and is a useless scale. In order to determine the ppt (salinity) with a hydrometer you need three things, the S.G. reading, the water temperature, and a conversion chart.

The reading on an ATC refractometer will give a very accurate ppt (Salinity). ATC = Automatic Temperature Compensation. Placing the water drop and spreading it out on the glass quickly adjusts the water to room temperature.
 
iidylii

iidylii

Active Member
Thanks bang...that's what I was looking for...I told my dad to use the ppt reading on his hydrometer but I will be telling him of my new found knowledge :)
 
bang guy

bang guy

Moderator
In short -

Hydrometers measure water density (Specific gravity eg. 1.025) which varies quite a bit with temperature. They do NOT measure salinity.

Refractometers measure Salinity (PPT eg. 35ppt) which does not vary significantly with temperature. They do NOT measure density.
 
2quills

2quills

Well-Known Member
Yep, you also should account for atmospheric pressure if we're talking about a device that uses the density of water as a means of measurement.

Higher pressure means water occupies less space and becomes more dense.

Someone living in Denver on a nice dry day could see a hydrometer reading lower than that of someone in Florida on a cloudy/rainy day of the same temperature.
 
You must log in or register to reply here.
Top