Question for both sides:

How critical are you of the way in which metric prefixes are used? Do you insist that to be "proper" one must change to the next prefix on exact multiples of a thousand?

What I have in mind is the way that in many fields it's quite common to skip certain prefixes altogether in common usage.

In measuring capacitance in electronics work, for example, the base unit of the farad is way too big for most situations, the microfarad designation being the most common.

Because the microfarad tends to be used so widely, we most often express larger values in terms of the microfarad rather than switching up to the milli- prefix, e.g. 10,000uF or even 50,000uF.

(Take "u" as meaning the Greek letter mu. I didn't want to cause possible confusion for different character sets.)

When you plug a capacitance value into an equation, it has to be in farads. Otherwise, any multiple of the base unit will do. I personally don't see the problem with any multiple, even centi-, which is not SI strictly speaking.

From my experience of electronic, 50,000uF is certainly an extreme case. If the rest of the design is in uF, there can be little objection to using 50,000uF (in much the same way that we state that the earth's circumference is about 40,000km, not 40Mm).

The rule of thumb that I saw some years ago was to *try* to choose your prefix so that *most* of your numbers lay between 0.1 and 2000. This rule is not sacrosanct, common sense plays a part.

An everyday example that I saw of this was in Italy. There are many hundreds of road tunnels in Italy and the Italians never miss an opportunity to erect a road sign. At the entrance to each tunnel is a sign giving the tunnel's length in metres. The shortest that I saw was 60m and the longest 17,000m.

BTW, I found these signs useful - at least I knew how long I would be in th etunnel.

I'm no scientist and have never even heard of a farad, but I find the way we use large numbers of millimetres in Britain annoying. I don't see the point of using millimetres when centimetres or metres would be more appropriate and give more convenient numbers

One of the reasons that mm are used is that in all but the most accurate of work, no decimal points are required, thereby reducing the scope for error.

But couldn't they convert the final result back to cm or m before presenting it to the non-science public?

Or would that be a violation of some EU directive because cm are not part of the SI system?

<<<But couldn't they convert the final result back to cm or m before presenting it to the non-science public?>>>

They do everywhere else! It is only Britain that can't seem to do things properly when it comes to measurement!

<<<Or would that be a violation of some EU directive because cm are not part of the SI system?>>>

??? I take it you are joking

I quite like the tie up between mph and distance.

In the UK you get 1m and 1/2m signs before turn-offs.

This is handy when you consider a 60mph avg as you approach your turn as you can "imagine" a minute then 1/2 a minute before leaving - ie transfering distance to time.

No doubt the extremists will try and discredit this now through pedantry and a "metric equivelent"!

Bud wrote

<<
But couldn't they convert the final result back to cm or m before presenting it to the non-science public?

Or would that be a violation of some EU directive because cm are not part of the SI system?
>>

It is not a violation of the EU directive. See Section 1.3 of the Annex to the directive. You can see the directive itself by visiting http://europa.eu.int/eur-lex/en/consleg/pdf/1980/en_1980L0181_do_001.pdf

It's nice to see a general consensus on this that common sense and an approach suited to the task in hand should be the guide.

Martin: "The rule of thumb that I saw some years ago was to *try* to choose your prefix so that *most* of your numbers lay between 0.1 and 2000. This rule is not sacrosanct, common sense plays a part."

Interesting. Yes, the 50,000uF example I quoted would certainly be a very large value of capacitance and not widely used (the sort of thing you'd find in a very high-current power supply).

It's quite normal to continue expressing values in microfarads right down to 0.001uF. The nanofarad has gradually crept into use in modern times, but it would be rare to see it on any drawings prior to the 1970s, and not in widespread use until quite recently.

Common usage and convention plays a big part, and we are so conditioned into working with microfarads that I'm sure I cannot be alone in seeing 10nF and automatically "reading" that in my head as 0.01uF.

The pico prefix has figured more prominently, and there always seems to be a gray area where values are expressed in different ways, e.g. 1000pF or 0.001uF.

In many old drawings (prior to about the early-mid 1950s) the picofarad was commonly referred to as a micro-microfarad, written uuF.

Bud,
Point taken about using the base unit when plugging it into equations, but in some cases it's quicker to use an alternate version of the formula with an adjusted constant somewhere to compensate. Other times it's convenient to get the answer as a different multiple of 1000, e.g. with something as basic as Ohm's Law:

Current = Voltage / Resistance

just about everyone soon learns that if you use kilohms you get the answer in milliamps or if you use megohms the current comes out in microamps.

<<<This is handy when you consider a 60mph avg as you approach your turn as you can "imagine" a minute then 1/2 a minute before leaving - ie transfering distance to time.>>>

Very useful indeed for people who have difficulty visualising a mile ;-)

<<<This is handy when you consider a 60mph avg as you approach your turn as you can "imagine" a minute then 1/2 a minute before leaving - ie transfering distance to time.>>>

Very useful indeed for people who have difficulty visualising a mile ;-)

"Very useful indeed for people who have difficulty visualising a mile"

er, what?

you don't need to.

When it says "your exit 1 mile" start saying "one mississippi two mississippi" etc when you get to 60 you'll be at the junction.

Or can't you "visualise" a second?




Or perhaps I just missed out on some humour - I'm having a hard day!

The maxmum speed in most metric countries is 120 km/h. This works out at 1 km every 30seconds. Also very easy to work calculate.

However, where cross-country driving is invovled, I prefer to work at 60 km/h. I mentioned this to some friends a few years ago whe were doing a 115 mile cross-country journey the following day. I converted it to 185km and suggested that it would take a little over 3 hours. THey phoo-poohed the idea. It took 3:10 hours!

"I converted it to 185km"

Did they smile and thank you while patting you on the back?

Steve, they were sceptical when I first made the suggestion. The next day, after they completed their journey, they were so surprised at how closely I had estimated their journey time that the rang me up to congratulate me.

You could also just as easily have said that 120 miles at an average speed of 40 mph would take 3 hours.