6 ms·
I was just yesterday changing my oven clock. Interesting to find out why here on HN. I wonder what critical systems (besides my oven) will start misbehaving. Tr
by neals 9y ago
I was just yesterday changing my oven clock. Interesting to find out why here on HN. I wonder what critical systems (besides my oven) will start misbehaving. Traffic lights, maybe?
(edit:Apparently, the emergency broadcast system.)
- detuur 9y agoI assume all mission-critical clocks get their tick from a quartz crystal. Clocks running on mains frequency is a very cheap cost cutting measure.
- Piskvorrr 9y agoIf your clock is mission-critical, it should be rather well kept. Quartz, NTP, GPS, the list goes on and multiplies like rabbits (redundant this redundantly connected to redundant thats and backed up by those...). But indeed, sometimes you don't need sub-second precision, and relying on The Grid is useful.
- blattimwind 9y agoI only have some experience with lab clock sources, but even those have some amount of redundancy by nature. They are generally a very good oscillator (e.g. a very good OCXO) disciplined to a reference with much better long-term deviation (e.g. DCF77 [uses a PM carrier, not the timing signal], GPS, a rubidium source or a miniature caesium source). Short-term (minutes to hours) is governed by the oscillator, and all these sources will enter holdover when the reference does something stupid. More recent ones can tell you the error margin during holdover. Distribution amps can have redundant and cross-checked inputs for independent sources.
- chx 9y agoIf it's mission critical then -- in Europe -- it gets the time from DCF77. It uses an atomic clock to emit a time signal (actually, it uses three and only uses their output when two out of three agrees). It is also compared to further atomic clocks and corrected as necessary. Even many consumer grade clocks will sync to the radio signal once a day and use a quartz otherwise.
- detuur 9y agoI suppose that in the case where the mission itself is critical as well, then indeed it's going to have an atomic clock signal.
- TickleSteve 9y agoA lot of electricity meters can take have the 50Hz mains as a clock-source... Leads to interesting questions such as how the (time-based) metering is affected in this situation.
- Piskvorrr 9y agoSure it is time-based? What I've seen is that the meter sits right on the power line, and can measure the energy consumption quite directly. As for peak/off-peak, this is usually signalled by the provider, using AC as a carrier wave, e.g. https://en.wikipedia.org/wiki/Zellweger_off-peak https://en.wikipedia.org/wiki/Zellweger_off-peak
- TickleSteve 9y agoyes, the meter sits on the power line. Thi sis referring to the clock-source for the metering functionality. In some meters, this is derived from the mains frequency (usually as a fallback tho).
- blattimwind 9y agoOlder traffic lights were controlled by mechanical sequencers: a synchronous motor turns a drum with pins on it (like in a music box) and the pins open or close contacts which drive contactors powering the individual lights. Modern ones are networked SPSes or small industrial PCs.
- isoprophlex 9y agoI have one locally sourced answer to that question: our emergency sirens failed their monthly test run because their internal clocks were out of sync! Source (in Dutch): https://www.nu.nl/binnenland/5164385/ https://www.nu.nl/binnenland/5164385/