Pressure switches and pressure transmitters
Mechanical and electronic pressure switches, pressure transmitters and transducers: components that measure pressure, switch at a set value or send a signal to the PLC.
A pressure gauge shows the pressure; a pressure switch or transmitter runs the machine according to it. In our power units and automation projects we select and commission the components that stop the pump, charge the accumulator or limit the clamping force of a press, and we make the same selection for your machine.
Types
Piston pressure switches
Used in high pressure hydraulic circuits; the piston withstands pressure spikes and the range goes up to several hundred bar.
Diaphragm pressure switches
Work precisely at low pressure; used on air, water and low pressure oil lines.
Electronic pressure switches
Switches with a display, multiple set points and adjustable hysteresis; besides the switching output they can also give an analogue signal.
Pressure transmitters
Convert pressure into a standard signal such as 4-20 mA or 0-10 V; a PLC, display or recorder reads this signal.
Vacuum switches
Used on vacuum lines to check whether the suction cup is holding the part.
How to select
The first question is what the signal will do. If opening or closing a contact at a set pressure is enough, a mechanical pressure switch is sufficient and economical; if the pressure value has to be monitored continuously, recorded or used for closed loop control, a transmitter is needed.
On a pressure switch, hysteresis, the difference between the switch-on and switch-off pressure, matters as much as the setting range. In a unit that charges an accumulator this difference determines how often the pump cuts in; if it is set too narrow, the pump keeps starting and stopping.
For a transmitter, the measuring range, output signal, electrical connection and process connection are determined together. On hydraulic lines with pressure spikes, the measuring range should be selected to cover the spike peaks as well.
Pressure equivalent of a 4-20 mA transmitter signal
| Signal | 0-250 bar transmitter | 0-400 bar transmitter | Percentage of range |
|---|---|---|---|
| 4 mA | 0.0 bar | 0 bar | 0% |
| 8 mA | 62.5 bar | 100 bar | 25% |
| 12 mA | 125.0 bar | 200 bar | 50% |
| 16 mA | 187.5 bar | 300 bar | 75% |
| 20 mA | 250.0 bar | 400 bar | 100% |
Calculation: pressure = (signal − 4 mA) ÷ 16 mA × measuring range. Because 4 mA corresponds to zero pressure, a broken cable can be told apart: if 0 mA is read, the fault is in the connection, not the pressure.
Frequently asked questions
What is the difference between a pressure switch and a pressure transmitter?
A pressure switch opens or closes a contact at the set value; a transmitter converts pressure into a continuous electrical signal, so the value can be read at any moment.
What is hysteresis in a pressure switch?
It is the difference between the pressure at which the switch cuts in and the pressure at which it cuts out. It prevents the pump from starting and stopping more often than necessary.
Can you add a pressure switch to our existing power unit?
Yes. We identify a suitable connection point in the circuit, select the pressure switch or transmitter and commission it together with the electrical connection.
How do I select the measuring range of a pressure transmitter?
The working pressure should not exceed about two thirds of the range, and the spike peaks should not exceed the full range. A range larger than necessary reduces measuring accuracy.
4-20 mA or 0-10 V?
Over long cable runs and in electrically noisy environments 4-20 mA is safer; a current signal is not affected by cable resistance, and a broken cable shows up immediately as 0 mA. Over short distances 0-10 V is also sufficient.
Why does my pressure switch keep switching on and off?
The hysteresis may be set too narrow, or the switch may be connected at a point with pressure spikes. Moving the switch to a calm point or fitting a snubber in between solves the problem in most cases.
Stock, sizes and part numbers
We select and supply the products in this group according to the application. Send us your requirement, the size and the operating conditions on WhatsApp and we will recommend the right product.