Pressure switches are used in a variety of applications where the pressure of pneumatic or hydraulic systems must be sensed. Pressure switches are available that can detect pressure changes as little as 1 psi (pound per square inch) and as large as 15,000 psi.
PRESSURE SWITCHES
At low pressure, diaphragm-operated switch may detect minor pressure changes (Figure 2–1).
Figure 2-1 A pressure switch with a diaphragm can detect
minor pressure changes at low pressure.
A metal bellows switch can detect pressures of up to 2000 psi. The metal bellows type pressure switch makes use of a metal bellows that expands in response to pressure (Figure 2–2).
Figure 2-2 A metal bellows style pressure switch can
withstand pressures of up to 2000 psi.
Although this switch can detect considerably higher pressures than the diaphragm type, it is not as sensitive since it requires a bigger change in pressure to force the bellows to expand sufficiently to activate a switch. For pressures of up to 15,000 psi, a piston style pressure switch can be employed (Figure 2–3).
Figure 2-3 Piston pressure switches can withstand pressures of up to 15,000 psi.
Regardless of how pressure is sensed, all pressure switches activate a set of contacts. Depending on the application, the contacts may be single pole or double pole, and they will have a snap-action mechanism. Contacts cannot be allowed to shut or open slowly.
control. This would result in a faulty connection, scorching of the contacts, and low voltage difficulties with the device they control. Some pressure switches have contacts big enough to connect a motor to the power line directly, while others are designed to regulate the functioning of a relay coil. Figure 2-4 depicts a line voltage type pressure switch.
Figure 2-4: Pressure switch with a line voltage.
This sort of pressure switch is frequently used to regulate the operation of well pumps and air compressors (Figure 2–5).
Figure 2-5 The functioning of a pump motor is
controlled by a line voltage pressure switch.
Differential Pressure
The difference in pressure between the cut-in or turn-on pressure and the cut-out or turn-off pressure is referred to as differential pressure. Most pressure switches allow you to adjust the pressure difference. A line voltage pressure switch controls the motor of a well pump in the example depicted in Figure 8-5. A pressure switch of this sort is often adjusted to cut in at around 30 psi and cut out at around 50 psi. The 20 pounds of differential pressure is required to keep the pump motor from overworking. The pump motor would constantly switch on and off if there was no differential pressure.
When a tank becomes wet, this is what occurs. To allow the pressure switch to work, an air gap must be maintained in the tank. The air gap is required because air can be squeezed but liquids cannot. If the tank became waterlogged, the pressure switch would instantly turn on and off whenever a very tiny amount of water was removed from the tank. Figure 2-6 depicts pressure switch symbols.
Figure 2–6 Shows Pressure switch symbols
Typical Application
Pressure switches are utilized in a wide range of conventional industrial applications. Figure 2-7 shows a circuit for turning off a motor and turning on a pilot caution light. A pressure switch is linked to a control relay in this circuit. If the pressure becomes too high, the control relay will open a usually closed contact attached to a motor starter to halt the motor.
A typically open PSCR (pressure switch control relay) contact will shut and turn on a pilot light to signal a high pressure condition. Take note that the pressure switch in this example circuit requires both usually open and normally closed contacts.
Figure 2-7 High pressure shuts off the motor and illuminates a warning light.
This is not a common contact configuration for a pressure switch. To remedy the problem, the pressure switch regulates the activity of a control relay. This is a typical approach in industrial control systems.
Pressure Sensors
Pressure switches are not the only pressure detecting devices that an electrician may come across on the job, particularly in an industrial setting. It is frequently vital to know not only whether or not the pressure has reached a given level, but also the quantity of pressure. Although these sensors are normally regarded to be in the instrumentation sector, an electrician should be aware with some of the different types and how they work.
Pressure sensors are designed to generate an output voltage or current that is proportional to the amount of pressure felt. Because of their compact size, dependability, and precision, piezoresistive sensors are particularly common (Figure 2–8).
Figure 2–8 Shows Piezoresistive pressure sensor.
These sensors are offered in two pressure ranges: 0 to 1 psi and 0 to 30 psi. A silicon diaphragm combined with an integrated circuit chip serves as the sensing element. Four implanted piezoresistors are coupled to form a bridge circuit on the chip (Figure 2-9).
Figure 2–9 Shows Piezoresistive bridge.
When pressure is applied to the diaphragm, the resistance of the piezoresistors varies according to the pressure, causing the bridge’s balance to shift. The voltage across V0 varies according to the applied pressure (V0 V4 V2 [when compared to V3]).
The following are some typical millivolt outputs and pressures:
1 psi = 44 mV
5 psi = 115 mV
15 psi = 225 mV
30 psi = 315 mV
Figure 2-10 depicts another sort of piezoresistive sensor. This specific sensor may detect absolute, gauge, or differential pressure. There are vacuum-sensing devices available. This sort of sensor can measure pressure ranges of 0 to 1, 0 to 2, 0 to 5, 0 to 15, 0 to 30, and 0 to 15. (vacuum).
The sensor has an integrated operational amplifier and can produce a voltage proportional to the pressure. This unit’s typical supply voltage is 8 volts DC. This unit’s controlled voltage output ranges from 1 to 6 volts.
Assume the sensor is designed to detect pressures ranging from 0 to 5 psi. The sensor would give a 1 volt output voltage at 0 pressure.
The sensor would provide a 6 volt output value at 15 psi.
Figure 2–10 Shows Differential pressure sensor.
Sensors with a ratiometric output are also available. The word ratiometric refers to the fact that the output voltage is proportionate to the supplied voltage.
Assume the supply voltage is increased by half to 12 volts DC. The output voltage would likewise rise by 50%. The sensor would now output 1.5 volts at 0 pressure and 9 volts at 15 psi.
Other sensors that provide a current output of 4 to 20 milliamperes rather than a controlled voltage output can be acquired (Figure 2–11).
Figure 2–11 Shows Pressure to current sensor for low pressures.
Figure 2-12 depicts one type of pressure to current sensor that can detect pressures as high as 250 psi.
Figure 2–12 Shows Pressure to current sensor for high pressure.
This sensor may also be utilized to give a typically open or normally closed output as a set point detector.
Sensors that provide a proportional output current rather than a voltage have less issues with generated noise from nearby magnetic fields and voltage dips caused by lengthy wire lengths.
Figure 2-13 depicts a flow-through pressure sensor. This sort of sensor may be integrated into an existing system. In-line pressure sensors enable adding a pressure sensor to an existing system simple.
Figure 2–13 Shows Flow-through pressure sensor.
The force sensor is another gadget that functions similarly to a pressure sensor (Figure 2–14). To detect the amount of pressure applied to the sensing device, this sensor employs silicon piezoresistive components.
Figure 2–14 Shows Force sensor.
