Economies of Scales Yield Fishy Results

Every ergonomist has heard it.  As soon as a push/pull force gauge is pulled out on the production floor, someone invariably compares it to the inexpensive spring scales sold at many sporting goods stores.

“Hey, I could use that on my bass boat!”

“That’s nothing more than a fancy fish scale.”

This is inevitably followed by an introspective discussion of quality versus cost.

“You paid HOW much for that thing!??”

Even as professionals, we are not immune to the reality of budgets.  Good quality, mechanical, push-pull gauges typically sell for $400 and up, and electronic gauges can cost well over $1,000.  This raises a fundamental question: to how many decimal places do we actually need to measure the force when pulling a cart of bricks?  Considering all of the potential sources of measurement error (floor surfaces, condition of the wheels, angle of pull, variation in load, acceleration, etc.) would we be better off providing more ergo teams with basic spring scales, than blowing the budget on one laboratory quality instrument?

Through the years, I have heard colleagues discuss the feasibility of using everything from strain gauges to bathroom scales to measure force in the workplace, but no alternative is mentioned as frequently as fish scales are.

To further the annals of applied science, and economics, a colleague and I conducted a test to evaluate the accuracy of these means of more modest measurement.  Paying a visit to a larger sporting goods store, I purchased one of every kind of fishing scale displayed.  This included manual and electronic models, costing between approximately $10 and $100 each, with capacities up to 100 pounds.

We took these scales to the calibration lab of a large company, where the test weights are all certified by a government agency and handled only with white gloves, and at least one technician had a sense of humor.  The readings on each scale were tested against the calibration weights, in regular increments, while suspended from a fixture.  For comparison, we included a few “professional grade” force gauges from our field kits.

The results were instructive.  Almost all of the scales were accurate within a pound or two at most readings.  This might very well be “close enough” for some workplace measurements.  The cheaper scales, predictably, were less reliable in terms of repeatability of measurements.  The professional gauges were dead on, even though each of them had been used extensively for several years without recalibration.

The key, however, was usability.  All of the fish scales were “pull only”, which meant that they could be used for the cart pulling task described above, but not for tasks such as pushing open doors, etc.  Further, they were reasonably accurate (my definition) when measuring weights suspended below them, and when they had time to stabilize.  This is fine for weighing dead fish, but very different than taking a dynamic, horizontal, pull measurement in the workplace.  Most of the mechanical spring scales fluttered widely when we tried this.  And the electronic fish scales, with the pretty, digital displays that we held so much hope for, needed several seconds of uniform pull to produce a reading.

It’s usually easy to get accurate weight measurements of objects in the workplace from shipping or laboratory scales.  It’s sometimes possible to borrow a decent force gauge from a well equipped maintenance department, or a quality control lab, but that is not something to depend on.  While some of the fish scales tested may work in some situations, none of them gave me enough confidence to recommend them as a general purpose replacement for a good quality, push-pull gauge.  Consider it as a professional investment.

Philip Jacobs, CSP, CPE, is the President of Jacobs Consulting, Ltd., Saint Paul, Minnesota.  He can be reached at jaco0253@umn.edu

This article originally appeared in The Ergonomics Report™ on 2002-07-01.