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GOALS:
- Describe the operation of high pressure switches.
- Describe the operation of low pressure switches.
- Discuss differential setting of pressure switches.
- Discuss pressure sensors that convert pressure to current for instrumentation purposes.
Pressure Switches
Pressure switches are found throughout industry in applications where it's necessary to sense the pressure of pneumatic or hydraulic systems. Pressure switches are available that can sense pressure changes of less than 1 psi (pound per square inch) or pressures over 15,000 psi. A diaphragm operated switch can sense small pressure changes at low pressure.
A metal bellows type switch can sense pressures up to 2000 psi. The metal bellows type pressure switch employs a metal bellows that expands with pressure (Fgr. 2). Although this switch can be used to sense a much higher pressure than the diaphragm type, it's not as sensitive in that it takes a greater change in pressure to cause the bellows to expand enough to active a switch. A piston type pressure switch can be used for pressures up to 15,000 psi (Fgr. 3).
Regardless of the method used to sense pressure, all pressure switches activate a set of contacts. The contacts may be either single pole or double pole depending on the application, and will be designed with some type of snap-action mechanism. Contacts cannot be permitted to slowly close or open. This would pro duce a bad connection and cause burning of the contacts as well as low voltage problems to the equipment they control. Some pressure switches are equipped with contacts large enough to connect a motor directly to the power line, and others are intended to control the operation of a relay coil. A line voltage type pressure switch is shown in Fgr. 4. Pressure switches of this type are often used to control the operation of well pumps and air compressors.
Differential Pressure
Differential pressure is the difference in pressure between the cut-in or turn-on pressure and the cut-out or turn-off pressure. Most pressure switches provide a means for setting the pressure differential. In the ex ample shown in Fgr. 5, a line voltage pressure switch controls the motor of a well pump. Typically, a pressure switch of this type would be set to cut in at about 30 psi and cut out at about 50 psi. The 20 pounds of differential pressure is necessary to prevent over working the pump motor. Without differential pres sure, the pump motor would continually turn on and off. This is what happens when a tank becomes water logged. An air space must be maintained in the tank to permit the pressure switch to function. The air space is necessary because air can be compressed, but a liquid cannot. If the tank becomes waterlogged, the pressure switch would turn on and off immediately each time a very small amount of water was removed from the tank.
Pressure switch symbols are shown in Fgr. 6.
Fgr. 1 A diaphragm type pressure switch can sense small pressure changes
at a low pressure.
Fgr. 2 A metal bellows type pressure switch can be used for pressures
up to 2000 psi.
(cont...)
Typical Application
Pressure switches are used in many common industrial applications. A circuit that's used to turn off a motor and turn on a pilot warning light is shown in Fgr 7. In this circuit, a pressure switch is connected to a control relay. If the pressure should become too great, the control relay will open a normally closed contact connected to a motor starter to stop the motor.
A normally open PSCR (pressure switch control relay) contact will close and turn on a pilot light to indicate a high pressure condition. Notice that in this example circuit, the pressure switch needs both normally open and normally closed contacts. This isn't a common contact arrangement for a pressure switch. To solve the problem, the pressure switch controls the action of a control relay. This is a very common practice in industrial control systems.
Fgr. 3 Piston type pressure switches can be used for pressures up to 15,000
psi.
Fgr. 4 Line voltage pressure switch.
Fgr. 5 A line voltage pressure switch controls the operation of a pump
motor.
(cont...)
Pressure Sensors
Pressure switches are not the only pressure sensing de vices that an electrician is likely to encounter on the job, especially in an industrial environment. It is often necessary to know not only if the pressure has reached a certain level, but also to know the amount of pressure. Although sensors of this type are generally considered to be in the instrumentation field, an electrician should be familiar with some of the various types and how they operate.
Pressure sensors are designed to produce an output voltage or current that's dependent on the amount of pressure being sensed. Piezoresistive sensors are very popular because of their small size, reliability, and accuracy (Fgr. 8). These sensors are available in ranges from 0 to 1 psi and 0 to 30 psi. The sensing element is a silicon diaphragm integrated with an integrated circuit chip. The chip contains four implanted piezoresistors connected to form a bridge circuit (Fgr 9). When pressure is applied to the diaphragm, the resistance of piezoresistors changes proportionally to the applied pressure, which changes the balance of the bridge. The voltage across V0 changes in proportion to the applied pressure (V0 _ V4 _ V2 [when referenced to V3]). Typical milli-volt outputs and pressures are shown below:
1 psi = 44 mV 5 psi = 115 mV
15 psi = 225 mV 30 psi = 315 mV
Another type of piezoresistive sensor is shown in Fgr. 10. This particular sensor can be used to sense absolute, gage, or differential pressure. Units are available that can be used to sense vacuum. Sensors of this type can be obtained to sense 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 contains an internal operational amplifier and can provide an output voltage proportional to the pressure. Typical supply voltage for this unit's 8 volts DC. The regulated voltage output for this unit's 1 to 6 volts. Assume for example that the sensor is in tended to sense a pressure range of 0 to 5 psi. At 0 psi, the sensor would produce an output voltage of 1 volt.
At 15 psi, the sensor would produce an output voltage of 6 volts.
Fgr. 6 Pressure switch symbols.
Fgr. 7 High pressure turns off the motor and turns on a warning light
Fgr. 8 Piezoresistive pressure sensor.
Fgr. 9 Piezoresistive bridge.
Fgr. 10 Differential pressure sensor.
Fgr. 11 Pressure to current sensor for low pressures.
Fgr. 12 Pressure to current sensor for high pressure.
Sensors can also be obtained that have a ratio metric output. The term ratio-metric means that the output voltage will be proportional to the supply volt age. Assume that the supply voltage increases by 50% to 12 volts DC. The output voltage would increase by 50% also. The sensor would now produce a voltage of 1.5 volts at 0 psi and 9 volts at 15 psi.
Other sensors can be obtained that produce a current output of 4 to 20mA, instead of a regulated voltage output (Fgr. 11). One type of pressure to current sensor, which can be used to sense pressures as high as 250 psi, is shown in Fgr. 12.
This sensor can also be used as a set point detector to provide a normally open or normally closed output.
Sensors that produce a proportional output current in stead of voltage have fewer problems with induced noise from surrounding magnetic fields and with volt age drops due to long wire runs.
A flow-through pressure sensor is shown in Fgr. 13. This type of sensor can be placed in line with an existing system. In-line pressure sensors make it easy to add a pressure sensor to an existing system.
Another device that's basically a pressure sensor is the force sensor (Fgr. 14). This sensor uses silicon piezoresistive elements to determine the amount of pressure to the sensing element.
Fgr. 13 Flow-through pressure sensor.
Fgr. 14 Force sensor.
QUIZ:
1. What type of pressure switch is generally used to sense small changes in low pressure systems?
2. A pressure switch is set to cut in at a pressure of 375 psi and cut out at 450 psi. What is the pressure differential for this switch?
3. A pressure switch is to be installed on a system with pressures that can range from1500 psi to 1800 psi. What type of pressure switch should be used?
4. A pressure switch is to be installed in a circuit that requires it to have three normally open contacts and one normally closed contact. The switch actually has one normally open contact. What must be done to permit this pressure switch to operate in this circuit?
5. What is a piezoresistor?
6. Refer to the circuit shown in Fgr. 7. If the pressure should become high enough for the pres sure switch to close and stop the motor, is it possible to restart the motor before the pressure drops to a safe level?
7. Refer to the circuit shown in Fgr. 7. Assume that the motor is running and an overload occurs, causing the OL contact to open and disconnect coil M to stop the motor. What effect does the opening of the overload contact have on the pressure switch circuit?