[Elecraft] K3 - REF CAL Question

[Stephen Prior]

I'll have a go.  F is degrees Fahrenheit and C is Celcius:

(F-32) x 5/9  =  C, or rearranging,

5F - 160 =  9C

If F = C, we can write

5F  - 160 = 9F 

-160 = 4F, so F = -40

That's woken me up!

73 Stephen G4SJP

On 18/06/2008 07:50, "ON4WIX" <on4wix at skynet.be> wrote:

> Let's see who first answers this one right (without looking at a
> thermometer!):
> 
> There is one point on the scale of a thermometer where the temp in °C equals
> the temp in F.
> Which one is that?
> Feel free to prove mathematically...
> 
> Enjoy!
> 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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