Controller Concepts: The Most Important Alarm You Don’t Know About
On better automation devices the time-based alarm, called the “fail-safe alarm”, “overfeed alarm”, “feed alarm” or “timer alarm”, remains the least understood yet most significant warning that operators will ever receive from that modern electronic pool-water controller. This alarm not only describes an unsatisfactory state of affairs, but an inadequate system set-up, poorly chosen or maintained system component, or both. It needs your understanding and your attention. Let’s first work on the understanding.
The “fail-safe” alarm is telling you, loudly and clearly, that you need to take some action or conditions will be degrading fast. It is telling you that if you don’t take that action, whatever unacceptable condition the alarm is indicative of will be getting worse. Often it is getting worse at a rate greater than if the alarm were not even present! You must respond correctly and in a reasonable amount of time, or this annoying and confusing alarm will actually drive the protected variable in the toilet while waiting impatiently for you to “fix” the problem.
This flashing red alarm simply shouts: “I don’t know if you are feeding too much or too little, but I do know you are feeding too long”. And “Oh, by the way, I’m going to shut down your feeder until you respond to me appropriately”.
Both pH and HRR (or ORP) feed outputs are protected by this time-based alarm, with different results if left unattended. The ORP-related failsafe alarm is indicating that arrival at the operator’s chosen set-point did not happen in the time allotted by the alarm’s chosen limit. If during the initial controller set-up you’d picked two hours (a good limit for a busy, outdoor short-course pool at the local high school), and the chlorine pump/feeder is too small for the current, high-load conditions, it will doubtless try to feed steady for more than two hours, and you’ll experience that annoying call. Even if the pump’s output is set too low, the mix is too thin, or you are simply out of chemical, you will surely find yourself alerted by a light or an alarm or, worse, a jingle on your home phone – while you’re watching the big game.
At the same time, the feed system is shut down, soon to make your deficit worse yet! Why shut down? Because you might have had too much, rather than too little, chemical fed into your pool; the system doesn’t know – it’s only measuring time and would not have otherwise shut down the feed. The too-much thing can happen only when the sample reading is not representative of the pool water – like a failed or fooled sample stream for a variety of reasons, or even a bad electrode.
How about pH? The reasons stated above can cause the pH to fail in its attempt to reach set level. In addition, the wrong internal setting (control up rather than control down) in the controller’s pH set-up (did you recently change from soda ash to acid?), or even the wrong chemical choice can do the deed. So the alarm rings? You’d better fix the problem and get things controlled again!
The alarm rings? Attention is what this fail-safe alarm needs, maybe more (and faster) than any other. Listen to that beeper, watch that flashing light, pick up the phone… If your alarm goes un-noticed, you are in a big bowl of kim chee. The cessation of pH feed may drive the water towards a high- or a low-pH alarm, which might take a number hours, but if it’s a sanitizer alarm that kills chemical feed, your pool will migrate to illegal and bacteriologically dangerous conditions in no time at all – especially if you “wait ‘til tomorrow morning” to look into things.
The biggest transgression, common among lazy or uninformed operators, is pushing the reset button every time the failsafe alarm sounds. Yes, it buys you another block of time – two hours in the above example – then you’re back in the Korean fermented cabbage again. The alarm shouts “fix me!”, meaning fill the tank, up the feed, install an additional, parallel pump, correct the controller or fix the sample, not “punch my button”! Some wayward operators actually have soldered the over-ride button closed. Come on…
The biggest concern here at the PPOA Alarm Mismanagement Office is the mis-setting of the failsafe time, almost always too long for the pool at hand. Many controllers are defaulted at four hours, wholly inappropriate for 90% of the pools being controlled! This might be OK for a one-million-gallon, 50-meter lap pool on a busy summer day in San Diego, but for most pools it is ridiculous. If the system is feeding yet hasn’t caught up and shut off from noon to four PM, you weren’t automated during most of that time (see also PrP #13, Feeder Sizing), just simply on but not there yet. Two hours is a good general value, however for the indoor health-club pool a half hour is more appropriate. Hot tub? Even the busy, thirty-foot-wide spa at a Tahoe ski resort is successfully run with a ten-minute failsafe setting. The circulation is running at a 20-minute turnover and the feeders are kick-buns big. The data records shows an average feed cycle of three minutes, with a typical off time about fifteen, even when its busy. Now that’s control.
Proportional? Don’t ask. The engineers in our PPOA Automation Settings Department have nothing good to say about most controllers’ “proportional feed”. The failsafe is usually defeated altogether by the circuitry and, with marginally sized feeders, you’ll see feed all day. Buy two $1.49 switches from Home Depot; they’re just as good as your fancy controller under these conditions. Skip proportional altogether, we advise – unless the body of water is very small, the sample is way behind the pool and the feeders are very big. Even then set the proportional working range at only 5 mV. ORP and .1 pH decade, not the 20 mV and .3 found defaulted on some boxes.
Back to the failsafe, that feed-timer alarm… Honor it. It’s telling you something important. Get to that pool (as soon as you can after the game), hit reset then go find the problem. Your pool and your patrons will thank you.
~kw
