Preferred Numbers and the “Preferred Measurement System”

Charles Renard (1847-1905)

By The Metric Maven

Bulldog Edition

In 1877 the French Engineer Charles Renard was instructed to look into improving captive balloons. These stationary, moored balloons were then in use by the French military, and of great importance. What Renard discovered was that 425 different sizes of cable were being used to moor these balloons. Clearly this large number of cables was not required from the outcome of any Engineering analysis, and were a nightmare to inventory and procure.

Renard determined that for mooring balloons, the most important inherent property of these cables, is their mass per unit length. He was able to develop a mathematical relationship which allowed him to replace the 425 sizes of cable with 17, which covered the same engineering range of requirements.

Renard’s geometric series was a perfect fit for a base 10 decimalized system, as it starts with 10 and ends with 100. The system he had in mind was of course, the metric system. This series produces what are proverbially known in engineering circles as preferred numbers (also called preferred values). Renard’s system was adopted as an international standard, ISO 3, in 1952, and are appropriately referred to as a Renard Series, or R Series. A similar series, the E series, is used to determine the values of electronic resistors, capacitors, inductors and zener diodes.

When metric was introduced into the building industry, a choice of dimensions which could easily be manipulated in one’s head was thought best. Grid lines on drawings are multiples of 100 mm. This is the basic “module” and the center to center of major dimensions are to be multiples of this value denoted as M. Therefore 3M = 300 mm, 6M = 600 mm and 12M = 1200 mm or 1.2 meters. According to Wikipedia:

For example, a multiple of 600 mm (6 M) can always be divided into 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 25, 30, etc. parts, each of which is again an integral number of millimetres.

No decimals! Preferred numbers rock! Why don’t we use metric construction like the Australians again?

But not everyone is so enamored with preferred numbers and the metric system. When I was a youthful Engineer working in Aerospace, I was involved in a number of proposals for large projects. I asked, what to my fellow workers, was an incredibly naive question: “Why aren’t we bidding this in metric?” A copy of the provisional bid “boilerplate” was then shown to me. On one of the first few pages of the proposal, was a small section about metric, it read something like: “The dimensions and system of units will be of the inch-pound system. This is necessitated because of the difficulty of procuring metric fasteners in this country, and because many, many more fasteners and hardware exist and are available in inches than in metric.” I began to realize that this “boilerplate” form had been used from time immemorial as a magical talisman to vanquish any thought of using metric in Aerospace. With each new bid, it was copied like junk DNA. I’m certain a similar document is in use in Aerospace to this day, to keep metric at bay.

At that young age I was definitely naive, because I swallowed the assertion, hook, line and imperial sinker. It seemed that limiting fasteners and other hardware might keep one from creating an essential Engineering design. OMG! for want of a nail, the spacecraft might be lost!  Later I would learn from a salesman what this ploy actually was. It was FUD. In case you haven’t heard of it, this is a salesman’s term for what to do if your companies products are clearly inferior to your competitors. You must instill your customer with Fear, Uncertainty and Doubt (FUD) about the alternative product. If you use a competitors electricity, it will burn all your toast! Your soft water will come out hard! X-rays will come out of your light bulbs! The electrons will spill on the floor, and act like tiny ball bearings and you’ll slip! I hope you have health insurance! You better use Brand X electricity, or suffer the consequences!

The other option often employed to keep customers from choosing a competitor, is to create a proliferation of products who’s only purpose is to be non-interchangeable with any other competitors. I have seen this with RF/Microwave connectors. There are hundreds of them and I used to jokingly refer to each new offering as “connector of the week.” Many of them have Olde English screw threads and metric dimensions—but that is another blog. When a product is chosen by a market place “food fight” there is no guarantee that an optimal solution will be the survivor.  If one chooses a product that is not satisfactory, and is incompatible with competitors, a temptation arises to rationalize its use. “It would cost too much to switch over now, this works good enough.”  The person making the purchase does not want the bad choice to reflect on them, and will do their best to make do. Like the non-adoption of metric in the US, inculcated intellectual inertia to continue using a bad design will often prevail over reform.

The introduction of metric is a perfect opportunity (as Pat Naughtin has pointed out) to introduce much needed reform into the different trades. Pat Naughtin offers a number of examples of useful reform in his lectures, which I will not repeat here. The one example which does stand-out as perfectly in sync with the savings one can obtain by using preferred numbers, occurred in Australia. When metric was introduced into an Australian Ford car plant, the number of fasteners used by Ford were reduced by a factor of four after metric conversion. The implementation of metric threads reduced the hodgepodge of bolts by 88% and nuts by 72%. The number of sheet metal thicknesses in some factories were considerably reduced, which saved on inventory costs, and had no impact on Engineering design options. According to  Kevin Wilks in his book Metrication in Australia (thanks Klystron): When standardizing containers, Australia was able to reduce the number of can sizes, for packing goods sold by mass, from approximately 90 to 30. He goes on:

Another example in wholesale packaging concerned corrugated fiberboard cases for packing fruit. With the establishment of metric packing quantities the opportunity was taken to reduce the variety of shapes and sizes from many hundreds to about 50.

The use of preferred numbers with the metric system is good for business, despite protestations to the contrary. The metric system’s absence in the US requires consumers to pay an unseen externality penalty. Business can ignore metrication because the citizens of the US pick up the tab, but don’t realize it. This unseen cost to consumers exists because of an inefficient measurement system, which powerful segments of the business lobby in the US, have perennially refused to allow government to legislate out of existence, since at least 1921. The cost of extra waste and inefficiency is just passed on to the consumer.

These costs also make American industry more expensive when compared with overseas companies. We need mandatory metrication, and we need it now to reform America, and make it competitive in the 21st Century—before the 22nd arrives.

8 thoughts on “Preferred Numbers and the “Preferred Measurement System”

  1. After reading about willpower and rationality, I still don’t know what makes some people/countries/businesses more willing to bite the bullet and suffer discomfort now for later gain. Isn’t that the essence? Pain now, gain later?

    We can present good theoretical arguments and case studies at all levels (I think of the conversion to euro) but if people don’t want it, they won’t do it.

    My friend sent me this link about irrational behavior:

    I also bargain some personal future gains for current comfort, but perhaps there’s a way to remove the individual desire from collective gains (measurement, currency, the “economy”)?

  2. I had always assumed and thought everyone understood that what really stopped the metrication that almost happened in the 1960’s was protectionism. In the short term it would give an advantage to metric countries that sourced parts.

    Lots of standards are actually protectionist – the no-lead solder boondoggle was about keeping US goods out of Europe – there was no substance to the risk of solder in landfills. ( A case can be made about lead in CRT glass).

    • What stopped the metrication program in the US was small business and the news media. They put fear in the public mind that metrication would increase difficulty with dealing with numbers.

      Nothing they said would change, just the numbers. A 40 mile journey will still be a 40 mile journey, but in metric it becomes 64.373 760 km. A pound of butter will still be a pound, but in metric it will be 0.45359237 kg. A thirty-second inch will still be a thirty-second inch but it will 0.793750 mm in metric.

      It was all going to be a soft conversion nightmare. If numbers changed to rounded metric, the new parts won’t fit with the old. Everything old would have to be discarded at great cost. Someone though forgot to tell all of this to the Chinese. When American factories closed and reopened in China, they had no problems changing sizes to rounded metric to make it easy for them to produce.

  3. “At that young age I was definitely naive, because I swallowed the assertion, hook, line and imperial sinker.”

    Imperial?? As far as i know the US never adopted imperial. The US continued to use the previous English units brought by the colonists. Since the imperial was invented until 1824, what system did the US use previous to this year? They had to use something, what was it called?

    • the correct name of the units in the united states is
      “united states customary sistem”

      • Not system but units. United States Customary Units, or USC. It isn’t a system, it is a random collection of units and includes those metric units in common use in the US.

  4. I was talking about the motivation – not the rhetoric. Most issues become clearer if you follow the money.

    Standards are used to create ‘vendor-lock-in’

    I was looking for a replacement blade for a honeycomb-cutter ( a milling tool we use to trim circuit board leads ) – turns out that there are no end of standards – all worked hard to be incompatible with the others – so they would be the sole source. The trouble is that as work moves to Asia and the demographic wave of baby-boomers causes a further retraction of industry in the US – those tool making companies are disappearing and locked in equipment becomes junk if you can’t buy parts.

    Government erected trade barriers are always a bad thing for the public – and often these barriers are based on ‘standards’ at the request of the industry they protect.

    On the other hand metrication continues:
    1 – I was talking to a young lady about the size of the poles they use for pole dancing – she rattled off size ranges in metric.

    2 – I was working with a young man about 20 and he needed a wire to be a bit longer – said, “about 20mm longer”.

    It is becoming a generational thing. Young guys work on their own cars and can see that a bolt top takes a 14mm socket. They might not recognize a 1/4″ phone jack, but do know an I-phone takes a 3.5mm plug.. There is reason to be optimistic here.

    • This is all because the newer generation is encountering products made in metric countries and often described in metric. They aren’t speaking metric because they want to show independence from their parents or the previous generation. It is because it is much simpler just to use the units the industry uses then try to convert them to inches. Then what? Come up with a strange number that one can’t relate to? Best just to accept the way it is described now, even if you don’t know what it means because you weren’t taught it.

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