14.3 Chemical Feeders, Controllers, and Probe Care

14.3 Chemical Feeders, Controllers, and Probe Care

Quick Answer: Chemical-system PM is probes, sample lines, feeder hardware, and safeties. Clean, inspect, and replace pH and ORP probes; never let them dry; calibrate with fresh buffers or the manufacturer’s ORP standard. Keep sample-line flow in band and upstream of injection. High cyanuric acid (CYA) and dirty probes both produce false-low ORP. Tubes, check valves, injection scale, erosion-feeder sieves, and empty-tank switches are calendar parts. Carbon dioxide (CO2) and acid systems need leak, securement, and containment PM. A timer that dumps chemical after hours is not a tested failsafe.

Chapter 8 parked probe care here on purpose: a dirty ORP sensor is not a broken peristaltic tube, and a mechanical feeder fault is not a reason to skip wet-chemistry PM. Chapter 7 already introduced feeders and flow interlocks as hardware. This section is 5A: the scheduled work that keeps the chemical loop honest.

Independent teaching uses manufacturer controller manuals as they are written, OSHA chemical-handling sense, and ordinary commercial automation practice. Local codes and the AHJ still govern what you may feed and how you store it. NRPA does not publish a secret national probe-cleaning interval.

Probes: clean, inspect, replace, keep wet, calibrate

pH and ORP probes are electrochemical sensors, usually in a small flow cell on a sample line. They do not last forever, they do not like air, and they do not tell the truth when they are coated with oils, scale, or metals.

Clean and inspect

On the manufacturer’s interval—weekly is a common commercial teaching rhythm, not a hidden AFO statute—remove the probes and clean them with the chemistry the manufacturer names (often a dilute acid soak or a labeled probe cleaner). Do not scrape a glass bulb with a wire brush. Inspect the bulb, the reference junction, the cable, the BNC or threaded connector, and the O-rings. A cracked bulb or a dried, crusted junction is a replace job, not a “calibrate harder” job.

Replacement belongs on the calendar as hours or years of service life, plus any failure to hold calibration. A probe that “still reads a number” can be a three-year-old liar. Keep a spare pH probe and a spare ORP probe in min/max if losing the loop closes the venue.

Never let them dry

The reference side of a pH probe is a wet electrochemical cell. Storage is a cap with the manufacturer’s storage solution (often potassium chloride for pH), not a paper towel on the workbench, not a Monday–Friday dry rest, and not a bucket of muriatic acid “to keep it clean.” If the plant shuts down overnight or for a season, either keep sample flow on a tiny bypass the manufacturer allows, or remove the probes into storage solution. Dried probes drift, respond slowly, and die early. That failure shows up as a controller that cannot hold residual—or that overfeeds because ORP looks artificially low.

Calibration with buffers and standards

pH calibration uses fresh buffer solutions, typically two points (commonly 7 and 4, or 7 and 10, per the controller). Expired, sun-cooked, or shared buffers that everyone dips a dirty probe into are not standards. ORP calibration, where the manufacturer specifies it, uses that manufacturer’s solution or a published ORP standard such as a Zobell-type solution—not a guess from the DPD kit. After calibration, verify against a grab sample: phenol red or electrometric pH from the pool, and DPD free chlorine. A controller that “calibrated OK” but disagrees with a good kit is not finished PM.

Record slope or “calibration passed” on the work order. Closing “probes OK” with no buffer lot and no kit comparison is the same fake close you already rejected on mechanical jobs.

Sample-line flow and false ORP

The controller only sees the water that reaches the flow cell.

• Set sample flow in the manufacturer’s band. Too little flow is a stale sample; the controller chases yesterday’s water and overfeeds. Too much flow can be noisy and can damage some cells.
• Take the sample before sanitizer and pH-adjusting chemical injection. A sample pulled after the injection tee tells the controller the loop is already satisfied while the pool is not.
• Keep the line free of air bubbles, biofilm, and scale. A weekly or monthly tube inspect is cheap PM.
• Confirm the flow switch or rotameter on the sample line still moves. A stuck “flow OK” flag with no water is how feed continues into a dry cell.

False ORP is a PM-and-chemistry trap, not a reason to throw the controller away.

• High CYA depresses ORP for a given free-chlorine residual. The controller may keep calling for feed while DPD already reads high. That is stabilizer chemistry plus a sensor, not proof that the peristaltic pump is lazy. Manage CYA; do not chase ORP by endlessly increasing the chlorine setpoint.
• A dirty or aged ORP probe commonly reads low, so the controller overfeeds chlorine (and can drag pH around with it).
• A dirty pH probe that reads high can drive extra acid and can also disturb the HOCl fraction you think you have.

In practice: Outdoor season, CYA climbed through the 80s while nobody cleaned the ORP probe. The controller showed low ORP all afternoon and ran the feeder until combined chlorine and irritation complaints arrived. The DPD kit had been trying to tell a different story since Tuesday. PM was probe clean plus a CYA plan, not a larger feed tube.

Feeder tubes, check valves, injection, sieves, empty-tank switches

Peristaltic tubes are wear parts. They flatten, crack, and split on a life the manufacturer prints in hours. Put tube replacement on the calendar before the split, and keep the correct tube size in min/max. A tube that has already sprayed hypochlorite onto a motor is corrective.

Check valves at the injection point stop pool water from running backward into the feeder and stop chemicals from sipping into a dead line. They scale shut, especially where calcium hypochlorite, hard water, or a positive CSI leaves mineral at the tee. Monthly inspect-and-soak (compatible cleaner, not a chlorine-and-acid cocktail in one bucket) is PM.

Injection points themselves crust over. A feeder that strokes happily while the pool residual falls is often injecting into a blocked fitting. Pull the injection quill on the calendar; do not wait for the “controller calling, nothing happening” Saturday.

Erosion feeders (trichlor or bromine canisters, and similar) need sieve and basket PM so tablet debris does not plug the outlet, and so you do not run a feeder dry-and-hot. Follow the manufacturer on which tablets belong in which vessel. Never put calcium hypochlorite tablets in a trichlor erosion feeder to “make it stronger.”

The empty-tank or low-level switch is a PM part. Test that it alarms, and that the controller cannot happily “call for feed” into a dry tank all night. A float stuck in the “full” position is how residuals collapse with a green light on the panel.

CO2 and acid systems — safety PM

pH control with CO2 or with acid is still a chemical feed system with injury modes.

CO2 PM

• Cylinders chained or caged upright; regulator and hoses inspected; dated cylinders swapped per supplier rules.
• Leak check (soapy water on fittings is ordinary shop practice) on a posted interval and after every cylinder change.
• Ventilation and any required gas monitoring left in service—not taped over because the alarm chirped during a delivery.
• Injection hardware and check valves on the CO2 solution side treated like any other injection PM.

Acid PM

• Secondary containment dry and intact; labels and safety data sheets current (OSHA Hazard Communication is the broader rule; this is the scheduled walk).
• Tubing compatible with the acid you actually feed. The wrong plastic fails on a calendar you will not like.
• Injection downstream of chlorine injection so you are not mixing concentrated acid and hypochlorite in one pipe.
• Eyewash and PPE on the same mechanical-room walk, because an acid line PM without an eyewash is a safety miss.

Safety PM includes proving the circulation interlock: if the recirculation pump stops, sanitizer and acid/CO2 feed must stop. Chapter 7 introduced that interlock as design. This chapter tests it on a work order.

After-hours timers versus controller failsafes

Some plants still use a wall timer to run a feeder overnight “to catch up.” A timer has no residual feedback. It can overshoot free chlorine, feed into a closed loop with no bathers to mix the water, or keep pumping after someone has locked out the circulating pump. It is a blunt instrument.

A controller failsafe worth the name includes some combination of:

• high and low pH and ORP (or sanitizer) alarms that stop feed,
• a maximum feed time so a stuck relay cannot empty a tank,
• a flow interlock on the main loop and on the sample line,
• a working empty-tank switch.

PM is testing those limits, not disabling them because night staff is tired of beeps. If you use a timer at all, it still sits behind the same interlocks, and it still gets a work order that asks whether residual in the morning was a coincidence or a control.

Symptom table: controller calling, residual falling

When the automation is asking for sanitizer (or acid) and the pool residual still drops, walk delivery and proof-of-flow before you rewrite setpoints. The table is a teaching checklist, not an NRPA fault tree.

The last row is the cousin diagnosis: if the controller is satisfied while DPD falls, believe the kit and PM the sensor. If the controller is hungry while DPD falls, believe the kit and PM the feeder path. Do not silence the failsafe to make the lights stop blinking.

Close chemical PM the same way you close mechanical PM: named person, readings, parts, sign-off. A green controller screen is not a completed work order.