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Norm Meck (Koi Club of San Diego)
Disclaimer: Although the most accurate method to obtain the volume
of water in your pond is to use a Water flow meter, in a pinch this
method will estimate the volume. Be sure your salinity meter is calibrated
correctlyand you know which type of meter you have ppt or percent.
Background
"The
amount of salt dissolved in water is termed the salinity and is measured
either as a per cent, in parts-per-thousand (ppt), or in parts-per-million
(ppm) (where 10 ppt = 1% = 10000 ppm). The more common
parts-per-thousand measurement is the weight of the salt in pounds
per thousand pounds of water (about 125 gallons). Pond-keepers often
talk about the pounds of salt per hundred gallons of water. Since
100 gallons of pure water weighs about 800 pounds, one pound of salt
per hundred gallons equates to a salinity of 1.25 ppt (0.125%
or 1250 ppm). (1 ppt = 0.8 pounds per hundred gallons)"
Formula
The difference
between percentage and parts per thousand is that 0.1% equals 1 ppt.
i.e. a ppt reading
is 10 times the percentage reading. (Percent is just another way
of saying parts per hundred).
The simple formula for calculating pond
volume using salinity change:
Known: P
= pounds of salt added
T = actual concentration CHANGE in parts per thousand (or change in % times
10)
(i.e. reading after adding the
salt minus reading before adding salt)
Find: V
= pond volume in gallons
V = (
P / T ) x 120
or pond
volume in gallons equals 120 times the pounds of salt added divided
by the salinity change in parts
per thousand
Why do you multiply by 120?
One part per thousand is one weight of
something (say one pound of salt) mixed with 999 weights of something
else (say 999 pounds of water) giving a total of 1000 parts. (we
normally can round off the 999 to 1000 and still have acceptable
accuracy) So this mixture is 1 part per thousand salt (and 999 parts
per thousand water)
Since one gallon of water weighs 8.33
pounds, and we are looking at the ratio between what is being added
compared to the weight of water; one divided by 8.33 = 0.120048...
Since we are talking parts per thousand, need to shift the decimal
point 3 places to get the 120. i.e. the 120 is 1000 divided by 8.33.
(1000/8.33=120)
Similarly
when medicating a pond and we want a dosage of so many parts per
million, we normally talk about
number of grams of the "medicine" to add. In this case we need to
compare grams with grams of water. Can't use pounds of water here
since we have to use the same weight measure. One gallon of water
weighs 3790 grams. So the simple formula becomes:
Known: V = pond volume in gallons
M= desired
concentration change in parts per million
Find: G = amount to add in grams
G = V x M x 0.00379
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