Plain-language definitions of the water treatment, instrumentation, and controller terms used across this wiki, each linked to the page that explains it in full.
- 4–20 mA — An analog signal that carries one reading as a current: 4 mA at the bottom of the range, 20 mA at the top. A current below about 3.6 mA means a fault. See 4–20 mA.
- Actuation — One switching of an output: a valve opening or a pump starting.
- Actuation limit — An alarm raised when a single bleed or dose runs longer than a set time. It does not stop the output. See Actuation Limit (212).
- Alarm — A condition the controller reports, each with a numeric code, a severity, and its own page. See Error Codes.
- Alloy factor — The number a corrosion probe multiplies its reading by for the metal of its electrodes: 1.00 for mild steel, 2.00 for copper. See Choosing the metal.
- Auto — The normal state of an output: its control mode decides when it runs. See Manual Control.
- Biocide — A chemical that kills bacteria, algae, and fungi. Oxidizing biocides (chlorine, bromine) hold a residual; non-oxidizing biocides are fed as periodic slugs. See Biocide Programs.
- Bleed — Draining system water to remove concentrated dissolved solids. Same as blowdown.
- Blowdown — See bleed. In a boiler, surface blowdown removes dissolved solids and bottom blowdown removes sludge. See Boilers.
- Buffer solution — A solution of known pH, used to calibrate a pH probe.
- Calibration — Adjusting how the controller converts a probe’s raw signal, so the reading matches a known standard or a calibrated handheld. One-point calibration corrects an offset; two-point also corrects the slope. See Calibration.
- Carryover — Boiler water, with its dissolved solids, carried out of the boiler with the steam.
- Channel — One reading or one switch on a device: a pH value, a temperature, a relay. A control mode uses one channel. See Devices.
- Chattering — An output switching rapidly on and off, usually because the differential is too small for a noisy reading.
- Check cap — A plug that stands in for a known corrosion rate, used to check and calibrate a corrosion probe. See Calibrating a Pyxis CR-300.
- Cleanliness count — The Pyxis ST-500’s rating of how clean its own optics are. In cooling water, above 3000 means fouled. See Cleanliness check.
- Closed loop — A sealed recirculating system — heating, chilled water, process cooling — that should lose almost no water. See Closed Loops.
- Condensate return — Condensed steam piped back to a boiler’s feedwater. Nearly pure, so it lowers feedwater dissolved solids.
- Conductivity — How readily water conducts electricity, measured in µS/cm. It rises with dissolved solids, so it is used to control bleed and blowdown. See Conductivity.
- Contact — An input that is either open or closed. A dry contact is a plain switch, with no voltage of its own. See Pulse and Contact Inputs.
- Control mode — A rule that decides when an output runs, such as Setpoint Control, Percent Timer, Volume Dosing, or Calendar. See Control Modes.
- Cycles of concentration — How many times the minerals in the make-up water are concentrated in the system: system conductivity ÷ make-up conductivity. See Cycles of Concentration.
- Device — Anything the controller reads from or switches: a probe, a sensor, a relay, a pump. See Devices.
- Differential — How far past the setpoint a reading must return before the output switches back. A bleed with a 2500 µS/cm setpoint and a differential of 50 opens at 2500 and closes at 2450. See Setpoint Control.
- Drift — Fine water droplets carried out of a cooling tower by the air.
- Evaporation — The water that does the cooling in a cooling tower. It leaves the minerals behind. See Cooling Towers.
- Failsafe — A timer or script on a wireless output that turns it off by itself if it loses contact with the controller. See Wireless Devices.
- Feedwater — All the water entering a boiler: make-up plus condensate return.
- Flashing — Hot pressurized water turning to steam as its pressure drops. Steam bubbles around a probe make it read low.
- Flow cell — The fitting that holds probes in a steady sample stream.
- Flow switch — A contact that shows whether water is flowing, used as an interlock so nothing feeds into a stagnant line.
- Hand — An output switched manually: On (Hand) runs it for a set time, Off (Hand) holds it off. See Manual Control.
- HOA — Hand / Off / Auto, the three states of an output.
- Inhibitor — Chemical that prevents scale and corrosion.
- Interlock — A condition that stops a control mode running, whatever the mode itself would decide: no flow, a low tank, an external stop. Interlocks protect the process, not people. See Interlocks.
- IO-Link — A point-to-point digital connection for smart sensors, one sensor per port. See IO-Link.
- K-factor — A water meter’s calibration: pulses per gallon, or gallons per pulse. See Meters and Switches.
- Limit alarm — See actuation limit.
- Lockout — Holding a control mode or output off, for example keeping the bleed shut after a biocide feed so the biocide is not drained away. See Calendar.
- Lockout/tagout (LOTO) — Isolating and locking every energy source before working on equipment. See Isolating Equipment.
- LSI (Langelier Saturation Index) — Whether water is over- (positive) or under-saturated (negative) with calcium carbonate: a scale or corrosion tendency. See Scale and Corrosion.
- Make-up — Fresh water added to replace what a system has lost.
- Modbus address — The number, 1 to 247, that a device on a Modbus bus answers to. No two devices on one bus may share one. See Addresses.
- Modbus RTU — A two-wire RS-485 bus that carries readings and commands to several devices, each at its own address. See Modbus RTU.
- mpy — Mils per year: a corrosion rate, as the thickness of metal lost each year in thousandths of an inch. See Corrosion Rate.
- NPN / PNP — The two ways a sensor output can switch: NPN sinks current to ground, PNP sources it from the supply. See Pulse and Contact Inputs.
- ORP — Oxidation-reduction potential, in mV. It rises with oxidizing biocide activity. See ORP.
- pH — Acidity or alkalinity, on a scale of 0 to 14. See pH.
- Port powered — A Modbus device that takes its supply from the controller’s bus, so the controller can reset it by cutting the supply. See Modbus RTU Devices.
- Pre-bleed — Bleeding a system down before a biocide feed, so conductivity has room to rise during the lockout. See Biocide Programs.
- Profile — The system type chosen at setup — Boiler or Custom. It sets the control mode setup creates. See Starting Up.
- PTSA — A fluorescent tracer added with a treatment product, so the product dose can be measured in parts per billion. See Fluorescence (PTSA).
- Rate limit — A cap on how many times an output may switch in a window of time. See Rate Limit Exhausted (211).
- Relief valve — A valve that opens to release pressure above its setting. Never plug or remove one. See Pressurized Systems.
- Role — A job in a profile — conductivity probe, blowdown valve — that a device is assigned to during setup.
- RSI (Ryznar Stability Index) — A second scale/corrosion index from the same inputs as the LSI: below about 6 scaling, above about 7 corrosive. See Scale and Corrosion.
- Safe state — The position an output goes to when its control mode cannot run safely — normally off.
- Sample & Hold — A sampling mode that flushes a line, closes the valve, and reads the settled sample. Used on boilers where flowing water flashes. See Sampling and Blowdown Control.
- Setpoint — The value at which a control mode switches its output. Rising turns the output on as the reading rises past it; Falling as it falls past it. See Setpoint Control.
- Slug feed — A large dose of chemical fed at once, on a schedule.
- TDS — Total dissolved solids, in ppm. Roughly 0.5–0.7 × conductivity in µS/cm, depending on the water. See TDS and Blowdown.
- Timed Sample — A sampling mode that periodically opens a valve, flushes fresh water past the probe, and reads it. See Sampling and Blowdown Control.
- Toroidal conductivity — Electrodeless conductivity measurement, using two coils around a hole the water passes through. Resists fouling. See Conductivity.
- µS/cm — Microsiemens per centimetre, the unit of conductivity. 1000 µS/cm = 1 mS/cm.
- Volume Dosing — A control mode that feeds a set dose for every set volume of water counted by a meter. See Volume Dosing.
- Water meter — A meter that sends a pulse for each unit of water. On the make-up line, it drives feed in proportion to water use.
- Pulsafeeder, Technical Reference Guide — 9300/9500 Series Cooling Tower Controller, 72-900-29 Rev D, §15 Glossary — several definitions are adapted from it. PDF