Subscribe to our newsletters & stay updated
In one of the interludes during an episode of a famous 1960s cartoon show, Bullwinkle J. Moose, appearing in magician’s garb, said, “Hey, Rocky! Watch me pull a rabbit out of my hat.”
To which Rocky would reply, “Agaaaiinnn?”
Bullwinkle continued, undaunted, “Nothing up my sleeve! Presto!”
He then proceeds to pull the growling head of a bear, tiger, lion or rhinoceros from the hat.
Recently, I was rummaging through the lower right drawer of my desk at the office looking for a stapler or some such thing. This drawer is my designated junk drawer. Like Bullwinkle, I am often surprised by what I find. During this particular visit to the junk drawer, hidden way in the back, I noticed the black imitation leather case that held the Water Line Capacity Slide Rule, A.K.A. the Grizzle Stick.
I was unable to find much in the way of information regarding the history of the Grizzle Stick or its inventor, whom I assume from the information on my stick is B. F. Grizzle. Based on the copyright date on the rule, use of the rule started in1944. It was first manufactured in wood. My rule is a later version made of plastic. A web search revealed that Lawrence Engineering Service, once a maker of all manner of slide rules, made this rule for B. F. Grizzle. The Grizzle Stick slide rule performed one simple task, the calculation of friction loss in a section of pipe. Given the pipe diameter, length of pipe and flow, one could determine the head loss.
Many years ago, students in the fire protection engineering curriculum at the University of Maryland were given the Grizzle Stick to assist them in their hydraulics classes. Those courses were ENFP 312 Fire Protection Fluids, which I took in the fall of 1976, or ENFP 310 Fire Protection System Design I, which I took in the spring of 1977. Both courses were taught by Dr. Harry E. Hickey.
You "Gen Xers" and millennials out there may be thinking, “So what’s the big deal about a slide rule that can only do one thing?”
As you may recall from my March 2014 column book review of "Sprinkler Hydraulics" by Harold S. Wass, Jr., the calculation of friction loss primarily involves the Hazen-Williams formula. This involves calculating values taken to the 4.87 and 1.85 powers. Keep in mind there were no desktop computers or Microsoft Excel in the late 70s to simplify the task.
When I entered college as a freshman, I proudly sported a handheld calculator from Texas Instruments. It could add, subtract, multiply and divide. It could also retrieve one number from its memory. To think, all this for a mere $99. For those of us who could not afford a new HP-25 scientific calculator with reverse polish notation—reasonably priced at several hundred dollars each—values for Q1.85 and d4.87 needed to be calculated by hand with the help of logarithmic tables. The Grizzle Stick helped us avoid that hassle.
To find the pressure loss in a length of pipe using the Grizzle Stick, one would line up the pipe diameter (inches) with the length of pipe in feet. Note that above the scale for pipe diameters are tick marks for nominal pipe diameters of schedule 40 black steel pipe. Next, select the flow from a scale on the bottom of the sliding piece and read the corresponding pressure loss that is opposite that flow from the bottom set of scales on the non-sliding piece. Flow scales are provided for units of GPM, cubic feet per second and gallons per day. Pressure losses can be read in pounds per square inch or feet of head. Sorry, no metric units here.
The values for pressure loss were calculated using a Hazen-Williams coefficient (c) of 100. To find the pressure loss for a different coefficient, an additional step was needed. On the bottom of the fixed piece is a scale for “c” values. One would align the desired flow to the desired coefficient and then read the adjusted flow opposite c=100. The adjusted flow was then used to determine the pressure loss.
The fact that the default “c” for the stick is 100 leads me to believe that this was originally designed as a tool for those doing not only fire protection, but plumbing engineering. As you may have guessed, in addition to calculating pressure loss for given pipe diameters, lengths and flows, one can also estimate the size of pipe needed given the flow, pipe length and pressure loss by reversing the calculation steps.
But wait, there’s more! The Grizzle Stick can also be used to determine equivalent lengths of pipe, given the diameter, flow and pressure drop. The Grizzle Stick also provides us with a lesson regarding importance of significant digits when it comes to hydraulic calculations. There is no opportunity for excessive accuracy with this slide rule.
The utility of the Grizzle Stick did not last much beyond those ENFP classes. Shortly thereafter, much cheaper handheld calculators with loads of functions became available, virtually ending the slide rule era for engineers. Collections of hydraulic data like Clyde M. Woods' "Hydraulic Data for Fire Protection Systems," published by the Automatic Sprinkler Corporation of America in 1972, were readily available and could ease the tasks of performing and checking hydraulic calculations.
Bullwinkle’s magician gags were funny because of his surprise at what came out of his hat, once making the excuse, “Wrong Hat!”
My surprise during this recent visit to my junk drawer became the topic of this column.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.