Re: Simple explanations. Was: what are the labs for?

[herbgottlieb@juno.com (Herbert H. Gottlieb)]

 Leigh Palmer <@sfu.ca> writes:

> OK, Herb, you get first crack at these:
>
> Pressure balanced mixer valves somehow compensate for a
> reduction in water pressure on one side (either cold or hot)
> by reducing the flow on the other side to keep the shower
> temperature constant. This works for large swings in pressure,
> and also for pressure rises (after the flush tank filler shuts
> off, for instance) with no perceptible hysteresis. There are
> no inputs to the device other than the hot and cold water
> supplies.
>
> How do pressure balanced mixer valves work? Electrical analogs
> gratefully accepted.

The obvious electrical analog is the feedback control circuit such 
as the automatic gain control (AGC) circuit in our home radios.
When the signal amplitude increases part of the signal is fed back to 
the amplifier input as an increased bias voltage which reduces 
the amplifier gain. Similarly, when the signal amplitude decreases,
less bias is applied to the amplifier input so its gain increases.

I have never heard of pressure balanced mixer valves before, 
but I am willing to bet that they work on a similar principle to 
the electric analog discussed in the above paragraph. 

The expensive way to do this would be to include  electronic
pressure transducers in the hot and cold water input pipes. 
When pressure changes are sensed they would signal
electronically controlled valves to open or close in proportion 
to the signal strength.

A less expensive technique is to insert a mechanical pressure-
sensitive drum in the cold and hot water inputs to the 
showerhead. An increase in the water pressure would move the 
drum in a direction that would partially close the pipe diameter 
and restrict the amount of entering water in the opposzite pipe.
A decrease in water pressure would allow the drum 
(probably spring loaded) to move in the 
opposite direction and increase the flow of water.  

The only thing that I can't understand Leigh, is why such a simple
mechanism that couldn't cost more than a dollar or two cost you
an extra $50.

Herb  Gottlieb  from New York City
(Where temperature changes in our unbalanced showers 
are more erratic and challenging than those in Canada)

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