Measure by weight or volume.

[riesnd]

Hi all,

Most spa chemical containers (perhaps all) specify dosage in ounces.

Is the ounces measurement specified as a volume OR as a wieght?

I have a container of OxySpa (MPS). The label specifies to use 1-1/2 - 2 oz. per 500 gallons. It then also states that 2 tablespoons = approx. 1-1/2 oz.
This doesn't compute if you are talking about volume because 2 tablespoons = 1 oz. (volume).
So in this case it appears 2 oz. is a weight measurement since 2 tablespoons of the product weighs in at 1.6 oz. (on a kitchen scale).
So it appears that the dosage of this particular product is specified as a weight.

Another product I use is Calcium Increaser. The dosage on this container is also specified in ounces but since it is liquid it makes sense that it would be a volume measurement. The ounces gauge on the side of the bottle supports that assumption.

How do you know to go by volume or weight. I'm pretty sure liquid dosages will always be by volume. But with granular it is not clear (at least to me).
Is there a hard and fast rule that granular chemical dosages are specified in ounces of weight and liquid are in ounces of volume?

Thanks for your help.
John

[96SC]

I dose all my chems via volume, dosing isn't an exact science.  Some chems, I am sure, are more dense than others so a tea spoon of one may weigh more/less than another.  Go with volume.

If I'm wrong someone will speak up.

[chem geek]

The dosage quantities for dry chemicals are almost always by weight.  This is because the density of dry chemicals varies and simply tapping a measuring cup can have the contents settle.  As you correctly point out, for liquid chemicals the dosage quantities are usually by volume.

Fortunately, for Dichlor, the density is such that roughly speaking 1 ounce weight is approximately 1 ounce volume.  For non-chlorine shock (MPS), 1 ounce weight is around 3/4 ounces volume (your particular product you measured is a little more dense -- one ounce weight being 1/1.6 = 0.63 ounces volume).  For baking soda (Alkalinity Up), 1 ounce weight is 0.8 ounces volume.  The measured dry bulk density of some common pool/spa chemicals is shown here.

As you've discovered, it's a good idea to at least initially use a kitchen scale to determine the actual density of your specific product you use.  After that, you can dose by volume if that's easier for you.

And remember that there are 2 tablespoons in an ounce of volume, 3 teaspoons in a tablespoon, so 6 teaspoons in an ounce of volume.

Richard

[riesnd]

Liquid by volume (fluid oz) & dry by weight is the dosing approach I have taken in the past but I have always been able to use test strips to verify/adjust the dosage for my particular needs.

For the first time I am using sodium bromide during startup. Since I don't have a way to measure its content in the water, I wanted to make sure I am interpretting the recommended dosage correctly as there is a significant difference in amount when interpretting the recommendation as volume vs. weight. AND back calculating the weight from the ppm recommendation didn't make it obvious, although it did come within 80% of the "by weight" measurement.

Anyway, I'm not looking for perfection, just making sure I am in the ballpark. Alhtough I can't directly verify the result, what you have said gives me some confindence that I am in the ballpark. Thanks for the reply.

John