Water meters that send a pulse per volume, and flow and level switches that open or close a contact.
These devices do not send a continuous measurement. They send a pulse or change a contact from open to closed, which a controller counts or reacts to. See Pulse and Contact Inputs for the wiring side.
- A mechanical meter (commonly a multi-jet, turbine, or positive-displacement meter) turns a register as water passes. A pulse transmitter on the register closes a contact each time a set volume passes.
- Reed switch outputs. A magnet on the meter’s register passes a reed switch — two thin metal contacts in a sealed glass tube that close when a magnet is near. Reed switches need no power and act like a simple dry contact, but they are for low pulse rates; they bounce and wear at high speed.
- Hall-effect outputs. A solid-state sensor detects the magnet and switches a transistor. They handle higher pulse rates and do not bounce, but need power and have polarity (they are often open-collector outputs, NPN or PNP).
- Contact-head meters send one pulse per fixed volume, for example 1 pulse per 1 US gallon, 10 gallons, or 100 L — marked on the meter or its register.
The K-factor is how many pulses the meter sends per unit volume (for example pulses per gallon or pulses per liter). Some sources state it the other way round, as volume per pulse — check which the controller expects.
- Volume = pulses ÷ K-factor. Flow rate = pulse rate ÷ K-factor.
- A meter giving 1 pulse per 10 gallons has a K-factor of 0.1 pulses/gal (or 10 gal/pulse).
- Paddle-wheel flow sensors have K-factors that vary widely with pipe size (a small pipe gives many more pulses per liter than a large one) — use the manufacturer’s figure for the pipe the sensor is in.
A wrong K-factor gives a consistent error in the totalized volume — for example make-up or bleed volume, and any chemical feed paced from it.
- A flow switch changes its contact when flow rises above, or falls below, a set point. It says only “flow” or “no flow”, not how much.
- Paddle (vane) switches have a paddle in the flow that deflects and moves a switch.
- Thermal (calorimetric) switches detect flow by how fast the water carries heat away from a heated tip; no moving parts.
- In flow cells, a small float or piston switch often confirms sample flow past the probes.
- Common use: interlocking chemical feed and control so that nothing is fed, and probes are not trusted, when there is no flow past them.
- A float rises and falls with the liquid and operates a switch — commonly a reed switch triggered by a magnet in the float, or a tilt switch in a cable-hung float.
- Common use: low-level alarms on chemical drums and day tanks (to stop a pump running dry), and high-level alarms against overfilling.
- Normally open or normally closed. Choose the contact so a failure (broken wire, stuck float) gives the safe result — typically a closed circuit when healthy, so a broken wire raises an alarm.
- Contact bounce on reed switches and mechanical contacts can count one pulse as several. Controllers usually filter this with a debounce setting or a maximum count rate.
- Pulse rate too high for a reed switch or the controller input, so pulses are missed.
- Electrical noise on long, unshielded pulse cable, particularly alongside power cable, can add false counts.
- Wrong K-factor, units, or pulse volume set in the controller.
- Meter wear and debris. Mechanical water meters under-register as they wear, and at very low flows. Debris and scale jam paddles and rotors.
- Air in the line turns mechanical meters and counts as water.
- Flow switches: a sticking or scaled paddle, the wrong set point for the pipe size, or turbulence that makes the contact chatter near the set point.
- Float switches: floats stuck by crystal build-up or tangled on pipework, floats swamped by agitation or waves, and floats attacked by the chemical.
- Water meters: follow the manufacturer’s orientation (many mechanical meters must be horizontal with the register up) and flow arrow. Give straight pipe upstream as specified, and a strainer where the water carries debris. See Flow.
- Paddle flow switches: mount in a horizontal pipe section with the paddle across the flow, trimmed to the pipe size, and with a few diameters of straight pipe before and after — typically about 5D, per the manufacturer.
- Float switches: mount so the float moves freely, clear of the tank wall, fill streams, mixers, and suction lines. Set the low-level switch high enough that the pump suction stays covered.
- Wiring: use shielded cable for pulse signals on longer runs, keep it away from power cable, and check the polarity for Hall-effect outputs.
- Check materials for floats and switches in chemical service — see Material Compatibility.
¶ Maintenance
- Check the count. Compare the controller’s total against the meter’s register over a period. A difference points to a wrong pulse volume, missed pulses, or noise.
- Test switches by making them operate: stop the flow, or lift and drop the float, and confirm the controller sees the change and responds (for example stopping a pump).
- Clean paddles, floats, and stems of scale and crystal build-up.
- Replace mechanical meters according to local practice and when they visibly under-register; there is no field calibration of the register.
Before pulling a float switch from a chemical tank, or a flow switch or meter from a line, follow Chemical Handling and Isolation. Lines may be under pressure — see Pressurized Systems.
Models documented in this wiki:
None listed yet.