[Elecraft] K3 - REF CAL Question

[Joe Subich, W4TV]

> 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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