DIY hydrometer mark 2

[tomfrh]

I made another wine bottle hydrometer for measuring salt.

The scale is 600mm long and runs from 0 to 8500ppm in 500ppm increments. It’s accurate to within half a division or better, ie +/-250ppm, so it’s as good as titration testing.

Theres a correction table on the side of it, which corrects for temperature and non-salt TDS. Eg at 10 degrees C you subtract 2500 ppm from the reading on the scale to get the correct salt reading. It’s amazing how much difference temperature makes.

Hopefully I get many years out of this thing.

View attachment 85211

 

[Shirker]

You going to post the build instructions? I don't often have many empty wine bottles on hand but I can empty a whisky bottle if pressed to do so.

 

[jeremy1979]

Very curious how that thing works.


12000 Gal Pebble tech, Hasa liquid chlorinator.

 

[ashtonfitzgerald]

Great idea! Yes, temperature makes a big difference with hydrometers. I don't run a swg, but am curious about the SG of my pool water. Maybe I'll dig up ,one from the home brew equipment and have a look[emoji4]

 

[tomfrh]

Very curious how that thing works.


12000 Gal Pebble tech, Hasa liquid chlorinator.

you put it the pool and read the number on the scale, ie how much of the stick is poking out of the water. Same as those lines on the side of big ships. The more salt the higher it floats.

It also floats higher in colder water, which confounds the reading, so you also have to add a correction factor based on the temperature. Eg if it’s 10 degrees C you subtract 2500ppm from the number you read.

 

[Katodude]

Do you have instructions on how to build this. I have enough empty wine bottles in my recycling bin now to make whole bunch of these.

Very cool BTW.

 

[scdaren]

So you are measuring the density of the water, correct?

Do other dissolved solids, like CH also affect density, to the point that it could affect the salt reading? Or is the affect so small compared to salt (or temp for that matter) that it's negligible?

- - - Updated - - -

Just re-read the original post and saw you have a correction table for TDS...

 

[tomfrh]

So you are measuring the density of the water, correct?

Do other dissolved solids, like CH also affect density, to the point that it could affect the salt reading? Or is the affect so small compared to salt (or temp for that matter) that it's negligible?

- - - Updated - - -

Just re-read the original post and saw you have a correction table for TDS...

yes hydrometers work by measuring density. They float higher in denser fluids.

In a pool the main factors affecting density are salt and temperature, and to a lesser extent other dissolved solids.

the scale is TDS at 20 degrees C

The table then corrects for the temperature and other dissolved solids, leaving you with the salt level. In my pool the non-salt TDS is about 600, so this is built into the table.

 

[tomfrh]

Do you have instructions on how to build this. I have enough empty wine bottles in my recycling bin now to make whole bunch of these.

Very cool BTW.

This is how I did it:

Wash and dry a 750ml screw cap wine bottle.

glue a 600mm x 4mm diameter galvanised steel rod (ie an old irrigation spike) to the top of the wine bottle cap and let it set.

progressively add lead fragments and resin binder into the bottle until it floats at an approximately correct level.

once resin has cured calibrate with plain water and salt solutions at 20 degrees, and set out the scale from those known points. The length of the scale divisions are best calculated from buoyancy principles.

That’s how I did it, but I f you understand the concept of a hydrometer you can make them from all sort of things.


https://en.m.wikipedia.org/wiki/Hydrometer

alternatively you can buy a hydrometer! I couldn’t find a good one round here or online, so decided to just make this one...

 

[tomfrh]

View attachment 85491

This is how you use in practice.

see it’s floating at the 7000 ppm line

less the 2500ppm correction factor for 12 degrees C water = 4500ppm salt

 

[Shirker]

How did you calibrate the graduations? The density of water by temperatures is not linear. Is it "linear enough" over such a short range?

 

[tomfrh]

How did you calibrate the graduations? The density of water by temperatures is not linear. Is it "linear enough" over such a short range?

The marked graduations are linear based on the effect of 1000ppm change in salt from a starting point of 20 degrees C water and 4500ppm salt level. The effect of additional salt on density is not exactly non linear as you move away from 20C and 4500ppm but it's close enough.

The change in density due to temperature (which is very non linear) is subtracted from the reading.