Noob salinity questions - why so much salt?

[InAtTheDeepEnd]

 

Trying to make twenty five litres of water of a specific gravity of 1.025/ salinity of 35% at 26°c

 

Thought I'd need about 870 grams of salt (25*35).

 

 But 870g only got me to an sg of 1.020. 

I actually needed over a kilo (!!!) Of salt to even be scraping 1.025.  

 

Why?

I'm adding it to RODI water so it's completely pure and dissolving with added aeration and heat and stirring so it's not that not all of the salt has dissolved before being added to the tank. 

 

Either my calculations are off, my scales are off, my refractometer is off, or something else entirely is off (Me, probably - I feel stupid and useless 😒😂😂). 

 

Help? 😅

 

 

(Pics are what was left out of a 2kg bag of instant Ocean salt and the refractometer when I eventually got it to where I was aiming). 

[Murphs_Reef]

Sounds about right too me. I usually mix 35 litre for a 10% water change and it's exactly 3.5 coffee mugs of salt... Never weighed it out to find out what that is in kg.

[M. Tournesol]

Your salt does not contain only salt (sodium chloride). It has over elements in it that are needed to reproduce seawater (potassium, calcium, magnesium, ...).
Therefore, when you are measuring 870 grams, you are not measuring 870 grams of salt but 870 grams of a mix of elements containing salt.

[PeterU]

Have you calibrated your refractometer?

[M. Tournesol]

Just now, PeterU said:

Have you calibrated your refractometer?

It problem is that he calculated the needed quantity of salt considering that it was pure sodium chloride. Thus, the refractometer reading is not erroneous.

[InAtTheDeepEnd]

8 minutes ago, PeterU said:

Have you calibrated your refractometer?

It's a precalibrated one

[Jaren45]

1 minute ago, InAtTheDeepEnd said:

It's a precalibrated one

You should still be calibrating it with either RO/DI or 1.026sg water according to the instructions every week or so. I calibrated mine before every water change just to be safe. Does yours not have a little adjustment screw?

[Clown79]

3 hours ago, InAtTheDeepEnd said:

It's a precalibrated one

All recractometers need calibrating, even pre calibrated ones

 

 

I always checked mine before every water change.

 

Never weighed or measured my salt. I added a bit at a time and checked salinity as i went until it hit 1.025. 

[M. Tournesol]

I do my calibration with my glass densimeter at 1.0255 😉.

If I calibrate mine with RO/DI, it is way out of bound...  

[InAtTheDeepEnd]

lol, so todays job will be calibrating my refractometre then 😂