Sand Glasses

Sand Glasses

We bought a set of sand timers, or hour glasses (although they don't run for an hour), or egg timers (but a 30 minute egg is probably a bit over cooked even for me). I wondered how accurate they were, so I recorded a video and found out:

They are fairly accurate, considering the simplicity involved. I also wondered what happened as the temperature increased so i put one in an oven and timed it. Nothing much happened until it reached 70C, then it slowed down from about a minute to a minute and six seconds. I expected the glass to expand and the timer to run faster so that was a surprise. Why it did this i do not know, but the 'sand' looked a bit stickier, maybe. I also wondered what the 'sand' was and the answer is glass:

One minute timer 'sand'

15 minute timer 'sand'

The 'sand' appears to be small glass beads. The two photos above are at the same scale, as is this one:

The small intervals on the calibration slide are 0.1mm. So the 15 minute sand is about 0.1mm and the 1 minute is about 0.15mm.