[APD] Re: how do solenoids work?

["Bill Wichers" <billw at waveform_net>]

>Forgive the silly question, but does anyone know exactly how solenoids
>"turn off" the flow of CO2? As the valve on the bottle remains open,
would >there be a danger of the pressure damaging the regulator? I'd like
to turn >off the CO2 at night. Would the resulting rise in pH (from about
6.8 to >7.1) hurt the fish?

A typical solenoid valve has either a small moving plug inside or a small
moving piston. The solenoid itself is just an electromagnet (coil of wire
basically) that, when energized, pulls the plug or piston in such a way as
to CLOSE the valve (in a 'normally open' valve), or to OPEN the valve (in
a 'normally closed' valve). So the important thing is:

Normally OPEN valve = gas flow is OFF when the solenoid is ENERGIZED
(electrically), gas flows through the valve when the solenoid is NOT
energized.

Normally CLOSED valve = gas flow is ON when the solenoid is ENERGIZED, gas
flow is stopped when the valve is NOT energized.

There are also SPDT valves, which have three ports and switch the flow
from one outlet to the other when energized. These are not generally
useful for aquaria applications unless you plug one of the outlet ports.

Typically the solenoid valve comes AFTER the regulator so it should not be
exposed to really high pressures. Many gas solenoid valves are able to
handle several hundred PSI without problems -- even the little cheap ones
I got off ebay some time back, so even if your regulator output is a bit
high the valve should be able to cope with it. At the 900 PSI or so of the
tank BEFORE the regulator many valves will fail though.

To summarize, in a typical aquarium CO2 setup, the controller energizes
the solenoid valve when it sees a too-high pH (equivalent to "needing more
CO2"), and this energized valve allows CO2 to flow into the tank. The
controller de-energizes the solenoid valve to stop the flow of CO2 once
the pH setpoint has been reached. A teensy bit over simplified, but close
enough... Control theory is actually pretty complex and I'm not sure if
the pH controllers on the hobbiest market go beyond the
on-at-setpoint/off-at-setpoint type of control anyway.

>rant: My expensive Harris regulator dumped last weekend, nearly killing
the >fish. This CO2 thing is very frustrating and somewhat scary, but the
>results are so wonderful, I want to continue using it. I'm hoping a
>solenoid will >help make for a safer setup. Oh well. $$$$$
>
>Amy

A controller and solenoid valve *should* be able to prevent a dump. A
needle valve will also help to limit the effects of a dump. Being careful
to never let the tank get to low will also limit the chances of a dump
(and you must *weigh* the tank to get an accurate idea of the remaining
CO2, the pressure gauges don't really help until you are essentially out
of CO2 already).

The most trouble-free setup should be:
tank -> regulator -> solenoid valve -> needle valve -> diffuser

You need a pH controller to electrically actuate the solenoid valve. The
controller can then shut the CO2 flow off when the set pH has been reached
in the tank, regardless of the state of the tank pressure (and thus even
during a 'dump'). A timer can also be used to control the solenoid valve,
but this won't provide the "intelligence" that a controller would.

    -Bill

*****************************
Waveform Technology
UNIX Systems Administrator


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