How water meters, flow switches, and level switches signal a controller by opening and closing a contact.
A contact is a switch the controller reads as open or closed. Flow switches, level switches, and external stop or enable signals all work this way.
A dry contact (also called voltage-free or potential-free) is a bare switch with no voltage of its own. The controller’s input supplies a small current, and the contact either lets it flow or not. A dry contact can be wired in either direction and shares no supply with the controller. A wet contact, by contrast, is powered by the device it comes from; check that its voltage matches what the input accepts before connecting it.
Every contact is one of two kinds, named for its state at rest — with no flow, no liquid, or no operator action:
A controller cannot tell an open contact from a broken wire, a loose terminal, or an unplugged switch: all of them read as open. Fail-safe wiring uses that to advantage. Choose and wire the contact so that it is closed when conditions are safe, and set the controller to treat open as “stop” or “not OK”. Then a cut cable, a failed switch, or a lost connection stops the process instead of leaving it running unprotected. For example:
A contact wired the other way — open when safe, closing to raise a fault — looks the same in normal running but silently loses its protection when a wire breaks. Use it only where the device offers no choice, and test it regularly.
A pulse output closes and opens once for each fixed amount of water that passes. The controller counts the pulses and multiplies by the meter’s scaling to get volume, and measures how fast they arrive to get flow rate.
Reed switch meters. A magnet on the meter’s register passes a reed switch, which closes briefly each time. The output is a dry contact: no power needed, either polarity. Reed switches are slow and their contacts bounce — they make and break several times in a few milliseconds each time they close — so the input must filter that out.
Hall-effect and electronic meters. A sensor detects the magnet or rotor electronically and switches a transistor. The output needs a supply, switches cleanly with no bounce, and can run much faster than a reed switch. The transistor output is one of two types:
An NPN output on an input built for PNP, or the reverse, never registers a pulse. Check both the meter’s and the input’s type before wiring, and share the 0 V between the meter’s supply and the input.
K-factor. A meter’s scaling is its K-factor, given one of two ways on the datasheet or the register face:
Enter the value in the form the controller asks for. Entering pulses per gallon where gallons per pulse is expected is a common mistake, and in the example above it makes the total 100 times wrong.
Debouncing and pulse rate. An input filters out bounce by ignoring changes shorter than a set debounce time. That time must be shorter than the pulse at the meter’s highest flow, or real pulses are lost. Work out the highest pulse rate before choosing a meter or a debounce setting:
A reed switch meter is best kept to a low pulse rate. For high flow, choose a coarser K-factor (more gallons per pulse) or an electronic output.
Input types. Industrial 24 V DC digital inputs are classified by IEC 61131-2. The type says what voltage and current the input treats as “on”, and so which sensors it suits:
| Type | On: voltage | On: current | Suited to |
|---|---|---|---|
| Type 1 | 15–30 V | 2–15 mA | Mechanical contacts and three-wire sensors |
| Type 2 | 11–30 V | 6–30 mA | Two-wire proximity sensors that draw more current |
| Type 3 | 11–30 V | 2–15 mA | Most two-wire and three-wire sensors, at lower input power |
All three types read off below 5 V. A two-wire sensor lets a small current through even when off, so connect one only to an input of the type its datasheet specifies.