How to terminate the function that runs every n seconds

[Mark Lawrence]

On 14/01/2015 17:37, Grant Edwards wrote:
> On 2015-01-14, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
>> On 14/01/2015 16:33, Dave Angel wrote:
>>
>>> Note that neither Timer nor sleep makes any promises about how
>>> accurately it matches the requested time.
>>
>> Reminds me of working on Telematics S200/300/4000/5000 telecomms kit in
>> the early 90s where the timers were mains based, so a one hour timer
>> would go off at about one hour, 30 seconds.
>
> I don't understand.  Power line frequencies are _very_ accurate and
> have been relied upon for timekeeping since the 1930s.  We're talking
> a few hundred PPM over a 24 hour period.  A 30 second error over a one
> hour period seems _really_ high.
>

 From 
http://www2.nationalgrid.com/uk/services/balancing-services/frequency-response/ 
"National Grid has a licence obligation to control frequency within the 
limits specified in the 'Electricity Supply Regulations', i.e. ±1% of 
nominal system frequency (50.00Hz) save in abnormal or exceptional 
circumstances.".  I wouldn't describe ±1% as very accurate and certainly 
not within a few hundred ppm.  I'm assuming that this kind of limitation 
applies around the world, so could you be getting confused with some 
other more accurate frequency control?

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence