[Elecraft] K3 - REF CAL Question

[ON4WIX]

Let's see who can answer this question first (without looking at a 
thermometer!):

There is one point on the scale of a thermometer where the temp in Celsius 
and Farenheit are equal. Which one?

Have fun,

Glenn ON4WIX
----- Original Message ----- 
From: "Joe Subich, W4TV" <lists at microham-usa.com>
To: "'Lyle Johnson'" <kk7p at wavecable.com>; <w3fpr at arrl.net>
Cc: <elecraft at mailman.qth.net>; <Paul at N4LCD.com>
Sent: Wednesday, June 18, 2008 12:58 AM
Subject: RE: [Elecraft] K3 - REF CAL Question




> F to C is much slower,

Not really ... (F - 30)/2 is generally within 10% and
(F - 32) * 1.1 / 2 is within about 1%.  Both are easy/quick.

Unfortunately, mental math and estimation are skills that have
been lost with the "calculator generation."

73,

   ... Joe, W4TV




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lyle Johnson
> Sent: Tuesday, June 17, 2008 5:14 PM
> To: w3fpr at arrl.net
> Cc: elecraft at mailman.qth.net; Paul at N4LCD.com
> Subject: Re: [Elecraft] K3 - REF CAL Question
>
>
> > You have to do that with your calculator :-)
>
> C to F is fast and easy to do in your head.
>
> The "real" formula is:
>
> F = (1.8 * C) + 32
>
> My mental algorithm is:
>
> double the value,
> subtract 10% (round it off, who cares if it is a degree or two off),
> add 30,
> then add 2 (if you feel the need for the extra precision).
>
> Most of us can double a number in our head pretty easily.
> Round it to
> the nearest 5 and then double it if you have trouble doubling
> 37 or 43
> or 51.  Hey! It's just the "two times" tables, which we all
> learned well
> before our 9th birthday.
>
> Doubling and then taking off 10% of the result is the same as
> multiplying by 1.8 (or 9/5 if you prefer) but a heckuva lot
> easier.   I
> never did learn my "1.8 times" tables...
>
> Again, round it if you trip over something like 74 - 7.4.
> The answer is
> 66.6.  But does it really matter if you decide to subtract 10 and get
> 64?  Or subtract 7 and get 67?
>
> Subtract 30 then 2 rather than 32.  Why?  Because it is easier to not
> deal with the 1's digit until you have to.  If you feel you have to.
>
> I find that I can usually work it out in my head in under a
> second and
> get within a few degrees of the "correct" result.  Near
> enough for most
> needs.  If I'm tired it might take 2 or even 3 seconds to make the
> conversion, especially if I refuse to round things off (being an
> engineer, I sometimes get anal about numbers, which is a good
> indicator
> I'm tired!).
>
> So, for example, let's take 52C. Is the PA too hot?
>
> The real method:
>
> 2 x 52 = 104.
> 104 - 10.4 = 94 (near enough)
> 94 + 30 = 124
> 124 + 2 = 126(if you really care...)
>
> If you'd rounded it:
>
>    2 x 50 = 100
> 100 - 10 = 90
>   90 + 30 = 120
> 120 +  2 = 122.
>
> Still close enough to decide if your finals are cooking!
> (They aren't).
>   Would the 4 degree difference have made you change you mind
> about the
> safety of your PA transistors?  I doubt it.
>
> F to C is much slower, but I rarely have to do that :-)
>
> Enjoy!
>
> Lyle KK7P
>
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