We all know that an ounce is a unit of weight, right? There are 16 ounces in a pound, 2,000 pounds in a ton and so on. However, in another convention, there are 8 ounces in a cup, 16 ounces in a pint, 32 ounces in a quart, 128 ounces in gallon, etc. In fact, the later ONLY works if we are talking about water or some other liquid with a specific gravity of 1. Although an ounce is a measure of weight, cups, pints, quarts and gallons are units of volume! The term “fluid ounce” is used to overcome this issue. In fact, a “fluid ounce” is a measure of the volume that would be occupied by a quantity of WATER that weighs 1 ounce.
In everyday life, we deal with these differences by convention. Let’s use my favorite laboratory as an example – the kitchen. Most recipes call for quantities of ingredients measured in units that will assure that the proper quantity is used. For example, a recipe may call for a cup (or 8 fluid ounces) of molasses. In fact, a cup of molasses weighs considerably more than 8 ounces (check it out) and its weight may vary depending on the grade or type of molasses. A “cup” of molasses, then, is the right amount to provide the needed flavor, moisture or whatever. In some instances, however, a recipe may specify 8 ounces or a pound of an ingredient like ground meat or a vegetable like celery. In that case you weigh the meat or the vegetable to assure the correct quantity. A cup or pint of ground meat or a vegetable would not provide an accurate measure.
At home, we usually measure ingredients like flour and sugar using cups (a unit of volume). However, cups of two different kinds of flour (let’s say bread flour vs. whole wheat flour) won’t weigh the same. “Sifted” flour doesn’t have the same weight per unit volume as flour that hasn’t been sifted (so make sure you sift that flour if it says to!!). Measuring flour and sugar by the cup is accurate enough for home kitchens. Bakeries and other food manufacturing facilities, however, measure sugar and flour by weight to minimize the inherent differences in the ingredients from different sources and production batches. I was really surprised the first time I saw eggs being measured by the gallon, but, in the food industry, the weight (and volume) of 500 eggs could vary considerably depending on the size of the egg etc.
OK, so you may be saying the old guy is on a tangent. What does all of this have to do with cleaning? Well, a lot actually. Cleaning chemistries vary in specific gravity, viscosity and several other properties. When adding chemistry to a cleaning bath, one should make sure that the measurement is made using the proper units. In some cases, the recommended chemical concentration of a liquid chemistry may be given in “fluid ounces” per unit of water while in others, the unit of measure may be “ounces” per unit of water. Fluid ounces, remember, are measured in cups and quarts while ounces are measured using a scale. The same is true for dry or powdered chemistry. Unless the directions specifically say to measure using a special scoop or some other volumetric means, assume that you are going to use a scale to measure the chemistry.
One other frequently encountered stumbling block related to measurements in the cleaning industry are those cases in which we are asked to mix 1 part of this with 3 parts of that. OK, how do we do that? Is it by weight or by volume? If it is not clear, the process engineer should clarify the intent of the instruction since the difference between by weight and by volume can vary considerably.
This blog has explored only the tip of the iceburg when it comes to anomalies in measurements we encounter in the cleaning industry. Other prime examples can be found in cleanliness specifications, work instructions and a host of other places where they are least expected. Although we usually associate precision and accuracy with good process control, it is important to make sure that everyone is on the same page when it comes to measurements. Remember the Hubble telescope!
– FJF –