# Thread: one million parts

1. ## one million parts

Having measured things in ppm for 10 years, I'm wondering: Just how much water is one million parts?

2. ## Re: one million parts

You can't look at water by itself to know how much is 1 million parts. Parts-Per-Million, ppm, when used in pools is a weight-per-weight measurement of concentration. This is easiest to understand in metric units since the density of water in such units is approximately 1 gram per milliliter or 1000 grams per liter. So, 1000 liters of water weighs 1,000,000 grams. 1 gram of substance in 1,000,000 grams of water (solution, technically 1 gram less) is 1 ppm. Adding 1 gram of substance to 1000 liters of water would be 1 ppm. Because of the 1 g/ml density of water, ppm is also described as milligrams per liter, mg/L, since 1 gram in 1000 liters is the same as 1 milligram in 1 liter.

In U.S. customary units, 10,000 gallons of water weighs about 83,300 pounds so one-millionth of this is 1.3 ounces weight. So, 1.3 ounces weight of a substance added to 10,000 gallons is 1 ppm.

Unfortunately, there are some subtleties to the "weight of a substance". For example, all chlorine concentration measurements in water are measured using the equivalent weight of chlorine gas (that would produce the same amount of chlorine in water) as the "substance", regardless of the actual source of chlorine that was used. This makes sense given that all sources of chlorine result in the same effective chlorine substances in the water (assuming other water parameters are held constant). Total Alkalinity (TA) is measured using the weight of calcium carbonate as the substance where the carbonate counts twice in terms of its effect on alkalinity -- that is, it counts twice as much as bicarbonate. Calcium Hardness (CH) is likewise measured using the weight of calcium carbonate. Borates are measured using the weight of the Boron atom. Ammonia is usually measured using the weight of the Nitrogen atom. Cyanuric Acid (CYA) is the most straigtforward since it is measured as the weight of Cyanuric Acid. Total Dissolved Solids (TDS) should technically be measured using the actual weight of dissolved substances in the water, but in practice it is estimated using conductivity and some assumptions of what is in the water based on water chemistry parameters -- calcium is estimated from CH, bicarbonate is estimated from TA, Cyanuric Acid from CYA, and the rest of the conductivity is assumed to be from excess salt, sodium and chloride, such that these all add up with charge balance.

Richard

3. ## Re: one million parts

So 'part' is just used as a relative term as it would be in a recipe with no specific serving size. Thank you for the explanation!

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