Have you ever had thoughts like these: “Why does my
controller sometimes seem to add too much chemical, while other
times it doesn’t add enough? Why does my controller seem to
be out of control? Is there a better, more predictable way of doing
If so, the answer to all those questions may be PID control.
Understanding that concept, and what it can do for you, can make
all the difference in the control you have over your
Most basically described in swimming pool terms, the letter
“P” in PID stands for proportional. Proportional is the
current error — the difference between the set point and the
actual chemical reading in the water. The letter “I”
stands for integral. Integral is the accumulation of past errors
— a look back to see how often and by how much the difference
between the actual readings and set points were. The letter
“D” stands for derivative. Derivative is the prediction
of future errors — a look forward, predicting the possibility
of differences between set point and actual readings.
Most controllers can be programmed to operate in either an on/off
control mode or proportional control mode. The on/off method of
control is very effective at allowing the controller to reach set
point quickly, but it also may overfeed chemicals due to lag time.
This lag time is the bane of operators with small bodies of water
and/or very irregular loading.
Proportional control mode for most of these controllers means that
it utilizes only the “P” portion of PID as its control
algorithm. The P algorithm will tell the chemical feed device to
turn on and off for multiple specific time periods before the set
point is reached. This so-called
“feed-then-wait-to-see-what-happens” strategy is very
effective for preventing the overfeed of chemicals. The down side
is that under high organic load conditions, the controller will not
be able to keep up, and chlorine levels will be continuously low
and the pH high. Eventually, the controller will go into a feed
overtime alarm and no chemicals will feed at all!
Controller manufacturers recently realized that on/off and P-only
control algorithms possess shortcomings. In response to this,
another control strategy was introduced: enhanced proportional
control. This strategy essentially introduced PI control to the
aquatics industry. The PI algorithm utilizes the P portion of PID
and adds the element of I (integral), which is a look into the past
performance of meeting set point.
Theoretically, PI control helps prevent overfeeds as well as
underfeeds by looking at past feed events. If the controller
determines that set point will not be reached before the time limit
is exceeded, the feed times are automatically adjusted, and pauses
are made shorter and on times longer. This seems like a perfect
solution until a facility experiences multiple periods of heavy
bather loading interspersed with times of inactivity. Of all bather
loading scenarios, this is the hardest type for a controller to
cope with and, unfortunately, it represents typical loading of many
pools. It is highly likely that the controller would begin to
oscillate and randomly cause underfeeds and overfeeds throughout
the day. Sound familiar? This happens because when in PI control,
if the controller recognizes an underfeed situation, the feed rate
ramps up. This ramp-up combined with the lag time in many systems,
will cause overfeed and when the controller recognizes the
overfeed, it turns the feed rate down, causing an underfeed. And so
the oscillations begin. How can these oscillations be prevented?
Enter PID control.
Adding the D (derivative) to the PI algorithm — allowing the
controller to respond with full PID control — the D value
acts as a damper to keep feed oscillations to a minimum. The D
value will anticipate the potential for oscillations and adjust the
feed pause and feed on times accordingly. The controller now is
looking at the full picture — the present (P), the past (I)
and the future (D). The controller can adjust feed events to
properly respond to rapidly varying bather loads and water
One additional control strategy called “adaptive
control” was introduced a few years ago in an attempt to
emulate full PID control. Adaptive control essentially takes a
snapshot of the lag time and uses the information to predict future
events. The problem is that this snapshot is done when the
controller is set up and typically does not represent conditions
with heavy bather loads. Thus, it is not as accurate as a
continuously adjusting PID loop.
The important news for operators is that within the past few
months, aquatic controller manufacturers now are offering
controllers with full PID loop control. In fact, some controllers
even can connect directly to a diaphragm pump, and the controller
will automatically adjust the pump stroke and speed to provide
additional precision to dosing!
PID control in a chemical controller can reduce costs by minimizing
the potential for overfeeds and wasting of chemicals. It also can
minimize operator headaches due to underfeed/overfeed oscillations,
ensuring that pool water meets the required code parameters.
We are in an exciting time when savvy operators who understand the
issues and know solutions are available can make a choice as to
which technologies will help them solve their problems. Goodbye,
under- and overfeeds! Hello, control.