|
|
Tech Bulletin: Connecting SeaMetrics Flow Sensors to Programable Logic Controllers
There are many applications in which it is desirable to connect one or more SeaMetrics flow sensors to a PLC (programmable logic controller), and the sensors are well suited for this. In many cases the digital pulse of a SeaMetrics sensor can be connected directly, or with a minimum of components.
The SeaMetrics products most commonly chosen for use with a PLC are IP and TX insertion sensors, WT turbine meters, and S-Series low flow sensors. These units, which have three wires, use Hall-effect technology. They need 5-24 Volts DC and up to 8 mA current, and they are current sinking (NPN) devices. These devices can connect directly to the PLC if:
See Figure 1 for connections.
If the PLC input only accepts current sourcing devices, a pull-up resistor must be added. Connection of this resistor is shown in Figure 2. Typically, on a 24 VDC input a 2.2 K Ohm resistor will be effective.
SeaMetrics uses reed switches on two products, the MR and WP meters. While these are infrequently used with PLC’s, the wiring is very straightforward. Reed switches, which have two wires, do not need a power supply to operate. They can be connected in the same way as any other mechanical switch. While they will handle up to 50 mA AC current, in general using a reed switch with 115 VAC input modules is not recommended, both for reasons of safety and switch life.
If the PLC is equipped with a 4-20 mA analog input module, it is
necessary to order the flow sensor with some form of 4-20 mA transmitter.
Two options are the AO55 blind transmitter and FT420 flow computer.
Follow the connection diagrams for these products to connect to the
analog input.
|
Since the three-wire, Hall-effect units are solid state, they do not exhibit switch bounce and can be used at relatively high frequencies. Reed switchs, like most mechanical switches, tend to have considerable bounce and it may be necessary to take precautions against this. Any input module will have certain frequency limitations, and the table on the right gives maximum frequency that a particular flow sensor or meter could be expended to produce.
