does temp affect salinity readings?
- Thread starter jacksonpt
- Start date
lordofthereef
Member
the temp does affwct the s.g. of saltwater.
broomer5
Active Member
Yep - temperature does affect how the hydrometer will read specific gravity, but the salinity does not change.
Take a 1 gallon jug full of freshwater at 50 degrees F.
Take another 1 gallon jug full of freshwater at 80 degrees F.
Add 1/2 cup of Instant Ocean to each jug.
Now measure this with a hydrometer that reads in SPECIFIC GRAVITY.
The reading will be different.
The AMOUNT of salt is the same in each jug - so the SALINITY of each jug is the same. You put 1/2 cup into each jug.
SALINITY and SPECIFC GRAVITY are not the same.
SALINITY is a measurement of stuff ( salt ) in the water in parts per thousand ppt.
SPECIFIC GRAVITY is not the same.
This is because specific gravity COMPARES the DENSITY of the SALTWATER to the DENSITY of FRESHWATER.
You must find out what TEMPERATURE your instrument ( hydrometer ) is CALIBRATED to first.
Take your reading.
Do the correction for specific gravity vs temperature from a chart.
Take a 1 gallon jug full of freshwater at 50 degrees F.
Take another 1 gallon jug full of freshwater at 80 degrees F.
Add 1/2 cup of Instant Ocean to each jug.
Now measure this with a hydrometer that reads in SPECIFIC GRAVITY.
The reading will be different.
The AMOUNT of salt is the same in each jug - so the SALINITY of each jug is the same. You put 1/2 cup into each jug.
SALINITY and SPECIFC GRAVITY are not the same.
SALINITY is a measurement of stuff ( salt ) in the water in parts per thousand ppt.
SPECIFIC GRAVITY is not the same.
This is because specific gravity COMPARES the DENSITY of the SALTWATER to the DENSITY of FRESHWATER.
You must find out what TEMPERATURE your instrument ( hydrometer ) is CALIBRATED to first.
Take your reading.
Do the correction for specific gravity vs temperature from a chart.
jacksonpt
Active Member
Dear god Broomer... if I didn't know better, I might have to ask if you have a life outside of your tanks. 
One more question for ya... if water temp affects SG but not salinity, than how can the same hydrometer measure both on the same, fixed scale? My hydrometer has a scale, on one side it measures salinity and on the other it measures SG. As my fresh saltwater warmed up before I added it to my tank, the reading on the hydrometer changed. Now it makes sense (according to what you said) that the SG would change, but shouldn't the salinity stay the same? But according to my hydrometer, SG and salinity changed at the same rate. Does that mean that my (probably el-cheapo) hydrometer really only measures SG?
One more question for ya... if water temp affects SG but not salinity, than how can the same hydrometer measure both on the same, fixed scale? My hydrometer has a scale, on one side it measures salinity and on the other it measures SG. As my fresh saltwater warmed up before I added it to my tank, the reading on the hydrometer changed. Now it makes sense (according to what you said) that the SG would change, but shouldn't the salinity stay the same? But according to my hydrometer, SG and salinity changed at the same rate. Does that mean that my (probably el-cheapo) hydrometer really only measures SG?
broomer5
Active Member
jacksonpt
When we use a hydrometer to measure specific gravity - we do it so we can infer what the salinity will be at the calibrated temperature of the hydrometer.
Inferring is an indirect way to measure something.
A hydrometer is an indirect way to measure salinity.
The dual scales - reading in both ppt salinity AND specific gravity are only accurate on THAT particular hydrometer AT the temperature it was calibrated for.
Pure water has a specific gravity of 1.000 at 68 degrees F ( 20 C ).
The reason this is ....... if you take a 1 cubic centimeter of pure water ( 1 ml ) and warm it to 68 degrees F, it will weigh exactly 1 gram. Picture a small glass tank of water that is 1 cm x 1 cm x 1 cm. This little box is a cubic centimeter.
Fill it with 68F water - it weighs 1 gram ( the water does not the box - the box is theoretical )
So people decided to call this value 1.000 specifically for water.
It's the weight of 1 ml of water.
It weighs 1 gram.
It takes up 1 cubic centimeter of space.
The entire metric system's units of length and weight were derived from the how water behaves.
Water expands and contracts based upon it's temperature.
The temperature it freezes they designated this as 0.0 C
The temperature is boils they designated this as 100.00 C
Regarding other liquids - any other liquid will either weigh more or less - for the same cubic centimeter of space.
If it is lighter than water - it will have a lower density in the cubic centimeter of space. It's specific gravity will be less than 1.000
If it is heavier than water - it will have a higher density in the cubic centimeter of space. It's specific gravity will be more than 1.000
To answer your question - yes the dual scales would be accurate in theory - if the sample of saltwater you are measuring is at the exact temperature the hydrometer was calibrated at. Any temperature other than that - and it's a washout.
Say your hydrometer was calibrated for 60 degrees F.
You mix up saltwater at 60 degrees F.
You measure it with the hydrometer at 60 degrees F.
The specific gravity reads 1.026
The salinity reads 35 ppt
All is cool, accurate and easy.
But we don't run our tanks a 60 F
So you must take the specific gravity of the saltwater reading - and go to a salinity vs specific gravity vs temperature table - and look up the answer.
Most good saltwater hobby books have these tables in them.
Most good hydrometers come with a table as well, but not always.
When we use a hydrometer to measure specific gravity - we do it so we can infer what the salinity will be at the calibrated temperature of the hydrometer.
Inferring is an indirect way to measure something.
A hydrometer is an indirect way to measure salinity.
The dual scales - reading in both ppt salinity AND specific gravity are only accurate on THAT particular hydrometer AT the temperature it was calibrated for.
Pure water has a specific gravity of 1.000 at 68 degrees F ( 20 C ).
The reason this is ....... if you take a 1 cubic centimeter of pure water ( 1 ml ) and warm it to 68 degrees F, it will weigh exactly 1 gram. Picture a small glass tank of water that is 1 cm x 1 cm x 1 cm. This little box is a cubic centimeter.
Fill it with 68F water - it weighs 1 gram ( the water does not the box - the box is theoretical )
So people decided to call this value 1.000 specifically for water.
It's the weight of 1 ml of water.
It weighs 1 gram.
It takes up 1 cubic centimeter of space.
The entire metric system's units of length and weight were derived from the how water behaves.
Water expands and contracts based upon it's temperature.
The temperature it freezes they designated this as 0.0 C
The temperature is boils they designated this as 100.00 C
Regarding other liquids - any other liquid will either weigh more or less - for the same cubic centimeter of space.
If it is lighter than water - it will have a lower density in the cubic centimeter of space. It's specific gravity will be less than 1.000
If it is heavier than water - it will have a higher density in the cubic centimeter of space. It's specific gravity will be more than 1.000
To answer your question - yes the dual scales would be accurate in theory - if the sample of saltwater you are measuring is at the exact temperature the hydrometer was calibrated at. Any temperature other than that - and it's a washout.
Say your hydrometer was calibrated for 60 degrees F.
You mix up saltwater at 60 degrees F.
You measure it with the hydrometer at 60 degrees F.
The specific gravity reads 1.026
The salinity reads 35 ppt
All is cool, accurate and easy.
But we don't run our tanks a 60 F
So you must take the specific gravity of the saltwater reading - and go to a salinity vs specific gravity vs temperature table - and look up the answer.
Most good saltwater hobby books have these tables in them.
Most good hydrometers come with a table as well, but not always.
hairtrigger
Active Member
Whoa... that's heavy stuff. I don't trust my hydrometer anymore. Thanks having me second guess myself now Broomer. Show off.
broomer5
Active Member
HairTrigger
I didn't know this 3 years ago.
It took a long time to get it straight in my head - and sometimes I don't even know if I understand it right. I need to either "see" it or get it into another form for me to better understand.
I do know this - my inexpensive swing-arm meter is a p.o.c.
I bought a floating glass hydrometer next - but it's hard to read. The scale is not wide enough where we run our tanks. Tiny little green band that means it's okay.
I don't want to know if it's okay - I wanted to know what the specific gravity is LOL.
Another piece of equipment that stays put in the fish closet.
I finally bought a refractometer when the money was available.
It is very repeatable - and I do trust what it tells me.
It's a treat to use.
I didn't know this 3 years ago.
It took a long time to get it straight in my head - and sometimes I don't even know if I understand it right. I need to either "see" it or get it into another form for me to better understand.
I do know this - my inexpensive swing-arm meter is a p.o.c.
I bought a floating glass hydrometer next - but it's hard to read. The scale is not wide enough where we run our tanks. Tiny little green band that means it's okay.
I don't want to know if it's okay - I wanted to know what the specific gravity is LOL.
Another piece of equipment that stays put in the fish closet.
I finally bought a refractometer when the money was available.
It is very repeatable - and I do trust what it tells me.
It's a treat to use.