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Teaspoons to Grams conversion via Chemical density

daizeUK

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18 Jun 2013
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Berkshire
I want to find out what one teaspoon of any particular salt weighs and I'm having difficulties.

First of all, a very polite request to anybody who would like to tell me "it doesn't matter" - I do know that EI is not based on exact measurements, however in this thread I'm looking for a more scientific answer please!

I started by looking up the densities of the usual salts:

Potassium Nitrate KNO3 = 2.109 g/cm3
Monopotassium Phosphate KH2PO4 = 2.34 g/cm3
Potassium Sulphate K2SO4 = 2.66 g/cm3
Magnesium Sulphate Heptahydrate = 1.68 g/cm3

Now to find the volume of a teaspoon. This is where I start running into difficulties. A quick Google gives widely different answers from 3.5 to 5.9 cm3 depending on whether it's an imperial, metric, UK or US teaspoon! I run a quick experiment on my own teaspoon and I figure it's most likely a metric one at about 5 cm3.

So if I multiply the above chemical densities by 5 I should get the weight of each salt in one teaspoon:

Potassium Nitrate KNO3 = 10.5g
Monopotassium Phosphate KH2PO4 = 11.7g
Potassium Sulphate K2SO4 = 13.3g
Magnesium Sulphate Heptahydrate = 8.4g

Now when I empirically test these values by weighing 10 teaspoons of each salt and dividing the result by 10, I get far less than I expected:

Potassium Nitrate KNO3 = <not tested>
Monopotassium Phosphate KH2PO4 = 6.2g
Potassium Sulphate K2SO4 = 6.5g
Magnesium Sulphate Heptahydrate = 5.0g

What's the reason for the huge difference here? Some of these are half the values I would expect - am I doing something wrong?
 

daizeUK

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Thanks Vinkenoog, that was exactly my thought too. However the result with the highest discrepancy was for the potassium sulphate which is a fine-grained, tightly packed salt. If air was a factor shouldn't this result in more of a discrepancy for the magnesium sulphate which has much coarser crystals?
 

ian_m

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Generally EI assumes about 6gr per teaspoon, which what you have found.

Absolutely no point getting any more accurate or even worrying about is as:

- Salts do vary in grain size thus amount per spoon.
- Salts absorb (and lose) water thus weight changes. My MgSO4 gets lumpy as it gets old and absorbs water..I just add the lumps.
- Are your spoons the same size as mine ??? Same size as whoever did the original EI research ?
- Was your teaspoon heaped or level ? Is it same level as last time ? Did you spill salts as you put it in your mixing bottle ?
- What is the water volume of your tank. Mine is 180l ??? Nope. What about deduction for all the substrate I added ???? What about volume of water in canister filter & pipes ???? What about the volume of the filter media (noodles) I have added ?? Probably an error of at least +-10% or even more here.

So use teaspoons, use to same "level" each time you mix and add the required amount each day....Job Done :)
 

daizeUK

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Well, the higher the density, the higher the discrepancy will be when measured as you have; smaller gaps between the grains will then have a larger impact on the total volume to mass ratio.

Thanks for explaining this. I'm still having a hard time getting my head around the idea that the air actually has more impact on tightly packed grains than it does on coarse grains! Common sense tells me that the larger crystals would have more air trapped and therefore a lower effective density.

Generally EI assumes about 6gr per teaspoon, which what you have found.

Absolutely no point getting any more accurate or even worrying about is as:

- Salts do vary in grain size thus amount per spoon.
- Salts absorb (and lose) water thus weight changes. My MgSO4 gets lumpy as it gets old and absorbs water..I just add the lumps.
- Are your spoons the same size as mine ??? Same size as whoever did the original EI research ?
- Was your teaspoon heaped or level ? Is it same level as last time ? Did you spill salts as you put it in your mixing bottle ?
- What is the water volume of your tank. Mine is 180l ??? Nope. What about deduction for all the substrate I added ???? What about volume of water in canister filter & pipes ???? What about the volume of the filter media (noodles) I have added ?? Probably an error of at least +-10% or even more here.

So use teaspoons, use to same "level" each time you mix and add the required amount each day....Job Done :)

Thanks for pointing out some of these pitfalls. I fully understand. My real concern was only why my calculations seemed so terribly wrong, more than 100% variation in some cases and I wasn't sure if the space between granules could be responsible for such a huge error! The mind boggles to think that 50% of a tightly packed substance could be air...
 

EnderUK

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26 Jan 2014
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1,028
Quick way would be to use Yet Another Nutrient Calculator if you want to use grams instead of teaspoons. See the ppm from dry dosing 1 tsp and then switch to what dose to reach target ppm.

But as Ian says you don't have to be accurate. When I measuring to fill up my doage bottle I go to +/-1gram for my 125L and about +/-0.5 for my 28L. I find this easier then counting out 25 teaspoons of salts.
 

daizeUK

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18 Jun 2013
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Berkshire
Thanks! I'd forgotten that Wet's calculator included those options, that's handy! :)

Just to set people's mind at rest, the reason for this question isn't for calculating my own EI dosing. It's to reverse-calculate in tanks who are following some random teaspoon recipe and we want to evaluate that recipe.
 
Joined
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Thanks for pointing out some of these pitfalls. I fully understand. My real concern was only why my calculations seemed so terribly wrong, more than 100% variation in some cases and I wasn't sure if the space between granules could be responsible for such a huge error! The mind boggles to think that 50% of a tightly packed substance could be air...

The error possibly comes from the original density values you've taken. I think it's the bulk density you should be using to start with. There's a table below I found showing them in kg/m3 but you can convert to g/cm3 and it gives different densities than the ones you've used. For example potassium sulphate from the table below has 1.442 g/cm3 bulk density and you are using a value of 2.66 g/cm3 above. If you use 1.442 x 5cm3(spoon)= 7.21g which is not far off your empirical value of 6.5g.
Plus any bulk density value depends on the particular salts you've purchased too so it can be slightly lower or slightly higher.

http://www.powderhandling.com.au/bulk-density-chart
 
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