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- To: Aquatic Plants Digest <Aquatic-Plants at actwin_com>
- Subject: End-of-tank-dump comment
- From: Wright Huntley <jwwiii at pacbell_net>
- Date: Sun, 06 Apr 2003 06:07:56 -0700
- User-agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.0.2) Gecko/20030208 Netscape/7.02
Recent posts reveal a profound lack of understanding of what causes the
subject problem, and how to fix it. Fix it with a pressure regulator *and*
a throttling valve to limit flow rate. Spend more time on system safety
than on whether a particular brand of regulator has a bigger low-end error
A fluid in a delivery system has two important properties -- pressure and
flow rate. They are not only not the same, they can be quite independent
from each other until some other system properties come into play.
Pressure is the force on the walls of the container, expressed in units of
force per unit area. For example, here in the US, where archaic units are
still popular, it is pounds per square inch (psi). It is Newtons per
square cm in the rest of the world, except France, where it is poorly
understood due to the language problem. Pressure is the property that
makes the soft silicone tubing balloon up and pop off of barbed fittings,
for example.[Always twist them secure with wire twist ties like you use on
Rate of flow is the volume being delivered per unit of time. Here in
aquatic plants, we use the very imprecise "bubbles per minute" as the
expression for how fast the CO2 molecules are being delivered to the
Put a hose into a normal-depth tank, and it will take a pressure of 1 or 2
psi to drive the gas down to the bottom where it can bubble out of the
hose. Incredibly tiny increases in pressure can cause the flow rate to go
from a bubble once in a while to an explosive burst of gas. There is
nothing but friction in the hose to restrict the flow rate, so it goes wild.
Add a simple valve between the *pressure* regulator and the hose outlet,
and the bubble rate can be turned back down. If the pressure regulator is
now set a little higher, the bubble rate increases a little bit, but not
catastrophically. As pressure is set higher and higher, a point sometimes
is reached where the bubble rate hardly increases at all. The valve has,
through internal turbulence, started to act as a true flow-rate regulator,
not that you really care.
There are now three pressure areas. The internal tank pressure can be from
about 800 psi down to almost zero as the liquid boils off and the gas is
used up. The regulated pressure can go from your initial setting to just a
few percent higher as the regulator loses control near the last of the
gas, then on down to near zero. The outlet (and final) pressure is still
set by the water depth where the gas is delivered.
If your adjustable choke valve is operating in a pretty turbulent-flow
region, the bubble rate will hardly change as the tank empties. If it is
not, flow will increase a few percent, but certainly will not allow the
tank to "dump."
"Dumping" came from an early attempt to make the pressure regulator work
in a more favorable region by using a reverse-flow-check valve with a
spring load that took about 10 psi to open. Unlike a regular throttling
valve, once open it let any flow rate through that wanted to go, as long
as there was a 10 pound pressure drop across it to hold it open. It was
nearly as unregulated for bubble rate as the example of the open hose,
above. The unregulated flow was just moved up by 10 psi (which *was* a
more stable operating region for most pressure regulators).
It was hard to set accurately, and required constant fiddling until
stability was finally achieved. Then the dreaded day would come when one
forgets to refill an emptying tank and the last of the gas is blasted into
your aquarium as the regulator goes through its normal little bump when
the tank and regulated pressure get very close to each other.
Note that I never once said "needle valve" in the above. Any old valve
will work that lets you set the bubble rate where you want it. Needle
valves have lots of turns and are a bit easier to adjust. You can waste
big bucks on fancy industrial valves, or you can go to home depot and buy
a $2 tubing valve like you might use to turn off your refrigerator ice maker.
If a soap check reveals that it does not leak around the stem when set and
running, and it lets you set flow where it needs to be, then that is all
you need. The single-stage (usually two guage) pressure regulator gets CO2
pressure down for safe distribution levels for hoses and fittings. Flow
rate is set entirely by the throttling valve, after that. One pressure
regulator can feed dozens of valves, BTW, each set to a different flow rate.
A reverse-flow check valve is a big waste, as reverse flow of water into
the tank is, in practice, impossible. If you *must* have one, get the
simple little ones with a neoprene flap that take very little pressure to
open them. Never, ever use the spring-loaded kind like the Eheim one that
caused many of us grief.
Bottom line -- here is what you need to never dump a tank:
Tank --> PSI setting regulator --> throttling valve --> outlet device
(Reactor, emitter, filter inlet, etc.).
Every tank dump ever known happened without that throttling valve to
restrict flow changes with small pressure changes. Don't ever leave it out
of your system. It is as important as the faucet you use to set your
garden sprinkler, and is performing exactly the same function.
Wright Huntley -- 209 521-0557 -- 731 Loletta Ave, Modesto CA 95351
Tax season to-do list:
1. Sharpen pencils.
2. Mark calendar.
Then attend WCW XII, on April 26th and 27th and unwind: