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" <[EMAIL PROTECTED]> 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" <[EMAIL PROTECTED]> > To: "'Lyle Johnson'" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]> > Cc: <[email protected]>; <[EMAIL PROTECTED]> > 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: [EMAIL PROTECTED] >> [mailto:[EMAIL PROTECTED] On Behalf Of Lyle Johnson >> Sent: Tuesday, June 17, 2008 5:14 PM >> To: [EMAIL PROTECTED] >> Cc: [email protected]; [EMAIL PROTECTED] >> 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 >> >> _______________________________________________ >> Elecraft mailing list >> Post to: [email protected] >> 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 > > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > 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: 16/06/2008 > 7:20 > > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > 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 > _______________________________________________ Elecraft mailing list Post to: [email protected] 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
