Editing Water clock (section)

==Temperature, water viscosity, and clock accuracy==
When viscosity can be neglected, the outflow rate of the water is governed by [[Torricelli's law]], or more generally, by [[Bernoulli's principle]]. [[Viscosity]] will dominate the outflow rate if the water flows out through a nozzle that is sufficiently long and thin, as given by the [[Hagen–Poiseuille equation]].<ref>{{Harvp|Goodenow|Orr|Ross|2007|p=6}}</ref> Approximately, the flow rate is for such design [[inversely proportional]] to the viscosity, which depends on the [[temperature]]. [[Liquid]]s generally become less viscous as the temperature increases. In the case of water, the viscosity varies by a factor of about seven between zero and 100 degrees Celsius. Thus, a water clock with such a nozzle would run about seven times faster at 100&nbsp;°C than at 0&nbsp;°C. Water is about 25 percent more viscous at 20&nbsp;°C than at 30&nbsp;°C, and a variation in temperature of one degree Celsius, in this "[[room temperature]]" range, produces a change of viscosity of about two percent.<ref>[[CRC Handbook of Chemistry and Physics]], page F-36</ref> Therefore, a water clock with such a nozzle that keeps good time at some given temperature would gain or lose about half an hour per day if it were one degree Celsius warmer or cooler. To make it keep time within one minute per day would require its temperature to be controlled within {{frac|30}}°C (about {{frac|17}}°F). There is no evidence that this was done in antiquity, so ancient water clocks with sufficiently thin and long nozzles (unlike the modern pendulum-controlled one described above) cannot have been reliably accurate by modern standards.  However, while modern timepieces may not be reset for long periods, water clocks were likely reset every day, when refilled, based on a sundial, so the cumulative error would not have been great.{{Citation needed|date=December 2020}}