[Elecraft] K3 - REF CAL Question

[Don Wilhelm]

I will still use F = (1.8 * (C+40)) - 40 -- as I said, I have more 
trouble remembering whether to add or subtract the 32.
Similarly, C = (F+40)/1.8 -40, so it is not so complicated after all.   
Dividing by 1.8 is similarly easy to do in the head - divide by 2 and 
then add 10% of the result - it is not exact, but close enough for 
'government work'.

73,
Don W3FPR

Lyle Johnson wrote:
>> 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
>
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft   
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG. 
> Version: 8.0.100 / Virus Database: 270.3.0/1505 - Release Date: 6/16/2008 7:20 AM
>