The Metric System

I do not want to dwell on this too much, but I want to note that the Brew Druid uses the metric system for the recipe program and process analysis.  The reason is mostly because fluid flow and heat transfer models are among the things we want to explore, and the mathematical principles of those subjects are just easier to apply with metric units.  For those who may have forgotten or are unfamiliar with the difference, the metric system is based on mass (grams) while the British gravitational system is based on force (pounds).  Mass and force are not the same.  The difference is significant.

I remember in junior high a science teacher explaining the difference between weight (a measure of force) and mass in simple terms.  Weight is the amount gravity pulling on something, mass is the amount of matter in something.  Because gravitational pull grows smaller as you move away from the earth, a rock at high elevation actually weighs less (in pounds) than it does at sea level, ever so slightly albeit.  Mass, however, does not change with elevation.  An x kilogram body measures x kilograms wherever it is; at sea level, on a mountain, in the upper atmosphere or out in space.

To tie those last two paragraphs together, fluid dynamics and heat transfer analysis rely heavily on mass as a material property.  One very simple instance of this is the relation of energy input to stored heat, i.e. as you add energy into matter it heats up.  By definition, one Joule of energy is required to raise 1 kg of water 1 °C.  Since the mass is independent of gravitational pull that relation is universal.  The relationship of weight in pounds to temperature change is not quite as straightforward since the weight is not constant like mass.  Certainly, there are ways to work around this mathematically, but they require weird conversions that make a complex subject more complicated. 

Another complication particular to using the British gravitational system for this kind of work is the importance of pressure changes and gradients.  In the British gravitational system, pressure is measured in pounds per square inch.  The pounds used in this sense are not variable like the pounds used to measure weight.  This creates situations where one needs to differentiate between pounds that are used as a proxy for mass and those used to express pressure.

Bernoulli’s Principle adds yet another layer kettle of fish that I won’t get into.

Since we are committed to metric units for some of the Brew Druid projects, it just makes sense to do it all in metric.  For the benefit of any readers of brew druid content, conversions to more conventional U.S. units will be provided in many cases such as recipes.  Technical articles will largely remain in metric units, I imagine.