Thread: Measuring Water Volume Using the salt method and TDS meter.

So, I could just skip to the end result but I'm gonna post up my various different attempts just to show you how wildly different results you'll get if you don't do what I now know is crucial.
YOU MUST NOT CALIBRATE YOUR TDS METERS TO 0ppm CALIBRATION FLUID.
You absolutely MUST calibrate your TDS meter to the nearest thing, you can, to the results you are looking for. So in the most common scenario you are likely to be salting your pond to 6000ppm (6ppt) or 0.6% salt. The closest calibration fluid I found was 6.44ppt from Milwaukee. I found this out after getting absolutely gibberish results that would have been impossible for my ponds footprint. Using 6.44ppt meant I knew that my TDS meter was reading accurately at that range (6440ppm).

Anyway, for those that don't know, here is how, I've been lead to understand how, you accurately check your water volume by adding known amounts of salt.
Litres = grams ÷ difference (ppt)

E.g.TDS Reading in pond, before adding salt = 372 ppm (converts to 0.372ppt)
Reading after adding salt = 7500ppm = 7.5 ppt
Difference = 7.128 (7.5 - 0.372)
Salt added = 100kg is 100,000 grams

Pond volume = 100,000 ÷ 7.128 = 14,029 litres (3085 gallons)

How I got there, and ended up with 0.75% salt in the pond, was a long and winding road, LOL. But you can learn from my errors.
Why did I bother:
My pond has tapered sides as it is a fibreglass tub, with straight window apertures on two sides and two different floor levels, one straight and one sloping. Lots of underground return pipes, bottom drain, drum, anoxic tank which also has tapered sides. I did some good maths to get to 2660 gallons but it was always a good estimate of everything I could measure but was always based on averages of those altering measurements.

The issue:
Pond treatments just seemed to do nothing for me if dosing to 2660 gallons.

So my initial TDS reading was actually 301ppm. I added 50kg salt and 24hrs later pond TDS was 3690ppm.
3.690 -0.301 = 3.389
50000 ÷ 3.389 = 14754 litres (3245 gallons)
This seemed high , but I needed to go get more salt, so I could try again and see if those results were repeatable. By the time I had got some more it was 48 hours later and the TDS was now 3510ppm
I added another 25kg salt
24 hours later:
TDS 4740Ppm
4.74- 3.51 = 1.23
25000÷ 1.23 = 20325 litres!
Oh dear, at this point I am smelling a rat. If something is not repeatable then it is not working.

Then 25kg salt
5900ppm
5.94-4.74= 1.16
25000÷1.16 = 21,551 litres

Or if used to the initial TDS and
The total of salt put in:
301ppm
100kg salt
To 5900ppm
5.9-0.301= 5.599
100,000 ÷ 5.599 = 17,860 litres
Which was just not possible to fit that amount of water in the space I've got so I know the TDS meter is at fault

After rudimentary calibration of TDS meter at both 0ppm I then used some 342ppm fluid and it was already way out even at this low end of the scale so I knew that everything so far was questionable:
Now the pond TDS after 100kg salt was reading 6900ppm
It's impossible now to know how accurate the first TDS reading of 301ppm was but we have to stick with it for now and the low number is going to cause the least room for error anyway
6.9-0.301= 6.599
100,000 ÷ 6.599 = 15,153 litres! (3,333 gallons)
Still an impossible volume for my pond and now I know for sure, I have no clue where the pond salt is at and is it safe anymore?
So I got hold of the 6.44ppt fluid and after calibrating TDS meter to 6440 ppm,
(Closest was 6430 as fine adjusting at that end of the scale causes massive swings) We now know that the TDS meter is showing damn close to what it needs to.
That gave the pond a TDS reading of 7500ppm, so:
7.5-0.301= 7.199
100000÷7.199 = 13,890 litres ( 3055 gallons)

Now we finally have a known quantity we can back track to figure out how wrong the initial TDS reading may have been.
The 'known' TDS of the pond has moved from 5900 to 7500 which is a 23.88% difference after TDS calibration so:
301 + 23.88% equals of 301= 372

So if we alter the initial TDS to 372pm
7.5-0.372 equals 7.128
100000÷7.128 =14,029 litres (3085 gallons)

So even with that degree of error, I'm happy that the total pond volume with filtration, bottom drain and return pipes, anoxic tank, and drum is around 3000 gallons. The average of the two more reliable figures is 3070 gallons so we'll go for that.
Phew, time for a pint and to turn up the water changes!