When I hear the term “luck of the draw,” I can’t help but imagine an unfortunate soul sitting at a craps or roulette table in a casino, head in hands and slot machines ratcheting in the background. It might be what the server says while handing the patron a free drink to help console that he just lost his shirt, “Well buddy, I guess it’s just the luck of the draw.” Once we’ve become sober to reality, most of us know there is indeed some probability that we’re not going to get rich at a casino. This probability is based on statistics, the collection, analysis, interpretation or explanation and presentation of data. In plumbing engineering, our mathematical model is the Hunters Curve. Published in 1940, Roy B. Hunter wrote the house rules to make sure the odds were stacked in our favor and plumbing systems would be adequately sized. He did so in a way we should all appreciate, and he passed on a responsibility to us that we should take to heart.

The “Hunter’s Curves” that we hear so much about are part of a report that were “carried on by the National Bureau of Standards, now known as the National Institute of Standards & Technology (NIST), overseen by the Federal Department of Commerce. The report itself is known as Building Materials & Structure Report BMS65, “Methods of Estimating Loads in Plumbing Systems.” At the time of its publication, you could own the report for 10 cents, the cost of playing a riverboat slot. The Plumbing Manual itself, BMS66, was double the cost at 20 cents. You can download either of those documents for free now from the NIST website. As I look through these two BMS reports, it appears that a lot of information has remained the same. Does this imply that the science used to create these charts and tables was so spot on that they just got it right the first time? Or, should we be re-evaluating the inputs that were used to create some of the information we use?

As I read through reports like BMS65 from days of yore, what strikes me most is the level of mathematics presented by the authors to describe their methods. Even though I’m not intimately familiar with the formulas for probability and statistics that were used, I have a level of confidence that a scientific method was performed. A method that involved information gathering, analysis, interpretation and presentation of data.

The second thing that strikes me is a level of humility in the report. It’s almost an admittance that plumbing system modelling is not an exact science. The BMS65 report, dated August 1940, includes 13 sections. Sections V and VI dedicate multiple pages to “Character of Flow in Plumbing Systems” and a “Statement of the Problem.” In these sections, Hunter describes his methodology for modelling plumbing systems and the challenges of creating a model with various fixtures operating independently but at the same time, simultaneously, with each fixture type having its own flow characteristics. I call this a level of humility because it shows that Hunter spelled out his thinking and leaves it open to further investigation and development. To me, this is the sentiment I see promoted in organizations like NFPA who encourage professionals to join Standards Committees, or ASPE that encourages participation in their research foundation or codes development. 

Opposite of that is the type of thinking where engineers and designers will say, “That’s the way it has always been done,” or when asked why something is specified a certain way, the engineer will state, “Because I said so.” I’ve found that teaching others gives us an opportunity to review what we were taught and question “why?” Just recently, I was teaching another engineer to upsize the vent pipe as you terminated it through the roof. I went to go find the code reference that requires this, and you know what? I couldn’t find it. This is an example of something I was taught and had always done, but couldn’t prove to myself that it was a code requirement. I decided to be open to a new idea — a vent termination does not always have to be upsized. This may sound a little odd, but it actually takes some humility to admit to myself that I had been overdesigning a system element for so many years. 

Hunter and the engineers and scientists of his era gave us a gift when they did their work. They explained their methods and how they came to their conclusions. They baked statements into the report such as “the numerical values… represent the author’s judgement… and are not to be regarded as standard values.” They passed on a responsibility to all of us to re-evaluate the data we use and to test out their methods. 

Plumbing engineering uses data, statistics and probability to design systems. Unlike gambling at a casino, the statistics we use have not been pre-determined. Neither are they static. The inputs change and need shepherding if we are going to be the best we can be. How many of our younger engineers realize that water closets used to flush 3.5 gallons for each use? To quote a famous Greek gambler, “The house doesn’t beat the players. It just gives us the opportunity to beat ourselves.”

As we approach the 80th anniversary of Hunter’s work, I encourage you to read through BMS65, “Methods of Estimating Loads in Plumbing Systems.” Appreciate the work our predecessors accomplished and realize what we do is not the luck of the draw. What we do is not based on some alternate mathematics or science. We can all continue to do the work of great science; collect data, analyze, share and discuss.