Re: What/who defines "unfit for purpose"?


"Anthony R. Gold" <not-for-mail@xxxxxxxxxx> wrote in message
news:qmsga4tmv9kdgrrrrkqn0j863jd7ijcoqi@xxxxxxxxxx
On Sun, 17 Aug 2008 19:48:36 +0100, Betty Swallocks
<redandhairy@xxxxxxxxxxx> wrote:

For kristsake. It is Joe Public who asked not Einstein! The RMS value is
an average value of the sine wave.

The root of the mean square equals the average?


Although you are correct in that the RMS is indeed a type of average, from
an electrical engineering viewpoint, average and RMS don't meant mean same
thing.

For example: the average (or mean) voltage of the sine wave that comes from
a 240 volt 13 amp socket near you is zero. If you take the arithmetic mean
of the voltages at infintesimal time intervals over the whole sine wave, you
get zero because the positive half cycle cancels the negative half cycle.
Since such a measurement is relatively useless, electrical engineers invert
the second half cycle, and if you do the calculation you will find that the
average voltage of the mains refered to above is 216.2 volts. Such a
measurement is still unuseful as it doesn't really tell you what you want to
know.

So what engineers do is to perform the calculation taking the arithmetic
mean of the squares of the voltages at infintesimal time intervals and then
take the square root of the result (known as the 'Root Mean Square' or RMS).
It is not necessary to invert the second half wave in this case. This
calculation gives 240 volts for our mains supply above and is much more
meaningful because a 240 volt RMS AC supply will produce exactly the same
amount of heat from a given electric fire as a 240 volt DC supply.

The type of voltmeters that most people own (A typical £5 jobbie from
Maplins) measures the average voltage. Thus the meter would measure 216.2
volts for the UK mains supply. But the meter is actually calibrated to read
1.11 times higher and will thus indicate 240 volts for this input voltage.
Thus this type of instrument *only reads correctly* when the applied voltage
is a sine wave.


.

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