View previous topic | View next topic

Water music.

Page 1 of 1

sally carr
1155613.  Mon Oct 26, 2015 7:58 pm Reply with quote

When I run the hot tap in the kitchen I can go and do something else because I can hear a change in the sound when the water is running hot. Is there a change in the water, the pipes or is it just me?

 
PDR
1155655.  Tue Oct 27, 2015 5:07 am Reply with quote

It's quite possible - hot water is less dense than cold water, and may have a different viscosity (I'd have to look that one up) so if it;s "thrumming" on a sink it could change the energy transfers between the moving water and the stationary sink which create the sound you hear, as well as affecting (if the viscosity changes) the size of droplets which are formed in the splash (which will also affect the sound).

Another thought is that hot water will tend to produce more of a vapour cloud above it, and that will change the sound absorption characteristics of the air around the sink, again changing the spectrum of the sound which escapes.

Human ears can detect very small changes in both sound volume and the sound spectrum characteristics, so even small changes could be audible.

There could be a good Ph.D in it for anyone who studies the phenomenon in sufficient detail to get a proper Handel on the water music.

PDR

 
dr.bob
1155673.  Tue Oct 27, 2015 6:27 am Reply with quote

I would imagine that a part might also be played by the fact that the hot water will cause the copper pipes to expand. This would (slightly) affect the speed and volume of the water passing through them, as well as the aforementioned viscosity within the pipes.

I've certainly noticed a change in pitch of my taps when the hot water starts coming through.

 
suze
1155677.  Tue Oct 27, 2015 6:51 am Reply with quote

PDR wrote:
hot water is less dense than cold water, and may have a different viscosity (I'd have to look that one up)


It does indeed.

The first thing that I didn't know is what the typical temperature of "hot" water is. I discover that there are two schools of thought on this. One health and safety argument - based on minimizing the risk of scalding from hot water - advocates that domestic water heaters should heat hot tap water to 49 C. But another health and safety argument advocates 60 C, on the basis that this is hot enough to kill Legionnaires' bacteria. (source)

Domestic showers are by default set to heat the water to 110 F (43.3 C). That's lower than I'd probably have guessed, but the risk of burns or scalds kicks in at 47 C and the manufacturers of showers are keen to avoid it. (source)


Coming at length back to the point, Wikipedia has a chart showing the viscosity of water at different temperatures. And yes, there's a considerable reduction in viscosity as the water gets hotter.


dr.bob wrote:
I would imagine that a part might also be played by the fact that the hot water will cause the copper pipes to expand. This would (slightly) affect the speed and volume of the water passing through them.


Some older homes will have lead piping rather than copper piping. Since lead is a heavier metal, would I be right to expect it to expand less than copper does? And if so, does this mean that the "Sally Carr Effect" will be less observable in homes with lead piping?

Newer pipework is usually plastic since it's cheaper than copper. How does that expand as compared to lead and copper?

 
Zziggy
1155679.  Tue Oct 27, 2015 7:05 am Reply with quote

suze wrote:
Since lead is a heavier metal, would I be right to expect it to expand less than copper does?

I'm no chemist, so I'm sure somebody will be able to correct/expand on this, but I don't imagine it would depend on weight. Things expand because their molecules have more energy, right? The melting point of lead is about a third the melting point of copper. Hence I would assume that lead atoms bounce about more at a given heat than copper atoms. So I'd assume lead would expand more.

Can you tell it's been a decade since I did any chemistry?

 
PDR
1155684.  Tue Oct 27, 2015 7:36 am Reply with quote

It certainly doesn't depend on density, as a quick empirical check shows.

The highest metallic coefficient of expansion I found was cadmium, at 31 micrstrain/degC and the lowest was tungsten at 4.5 microstrain/degC. Cadmium has a density of 8.7 Tonne/m^3 where Tungsten has a density of 19.3T/m^3. But there are sufficient cases between the two to show that it isn't inversely proportional to density either. It probably relates to crystal latice structure and bonding types or sommink - Efros will know, I'm sure.

The conjecture that it also relates to melting point is an interesting one - perhaps someone with some time could do a quick check or expansion coefficient against melting point to see if there is a correlation? Remember to use the melting point in kelvin rather than celcius, because the artificial 273deg offset will put a fictitious slope on the curve if you don't!

PDR

 
Alfred E Neuman
1155694.  Tue Oct 27, 2015 8:23 am Reply with quote

In my house the explanation is far simpler. The geyser has a header tank which has a ballcock (OK, I could have called it a float valve, but ballcock just rolls off the tongue so much more smoothly). When you open the hot tap, the level in the header drops, and at roughly the same time that the warm water hits the sink, the ballcock opens and the cold water starts (noisily) flowing into the header tank.

 

Page 1 of 1

All times are GMT - 5 Hours


Display posts from previous:   

Search Search Forums

Powered by phpBB © 2001, 2002 phpBB Group