Vacuum sensors are used to measure vacuum or sub-atmospheric pressures. Vacuum means pressure below atmospheric. Since true vacuum is never attained, the measurement is in respect to a near absence of gas pressure. Vacuum can be measured using a conventional pressure sensor; however they typically do not resolve extremely low concentrations of gas due to poor signal- to-noise ratio. Vacuum sensors rely on physical properties of gaseous molecules that are related to the number of such molecules per volume of space.

Vacuum Sensor Designs

Sensors that work in the vacuum range use some kind of physical displacement or material property change in order to make a measurement. Medium to high vacuum sensors use properties of the environment, such as thermal conductivity and ionization, to make a measurement. 

Low Vacuums

Low vacuums can be measured using mechanical means such as those listed below.

Piston technology uses a sealed piston/Cylinder to measure changes in pressure. Mechanical deflection uses an elastic or flexible element to mechanically deflect with a change in pressure, for example a diaphragm, Bourdon tube, or bellows.

 Diaphragm Pressure Sensor.  Image Credit: machinedesign.com

Piezoelectric pressure sensors measure dynamic and quasi-static pressures. The bi-directional transducers consist of metalized quartz or ceramic materials which have naturally occurring electrical properties. They are capable of converting stress into an electric potential and vice versa.

MicroElectroMechanical systems (MEMS)are typically micro systems manufactured by silicon surface micromachining for use in very small industrial or biological systems.

Vibrating elements (silicon resonance) use a vibrating element technology, such as silicon resonance.

Variable capacitance pressure instruments use the capacitance change results from the movement of a diaphragm element to measure pressure. The device uses a thin diaphragm as one plate of a capacitor. The applied pressure causes the diaphragm to deflect and the capacitance to change. The deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit. Capacitive absolute pressure sensors with a vacuum between the plates are ideal for preventing error by keeping the dielectric constant of the material constant.

Strain gauges (strain-sensitive variable resistors) are bonded to parts of the structure that deform as the pressure changes. Strain gauges are rugged, accurate, and stable, they can operate in severe shock and vibration environments as well as in a variety of pressure media. Strain gauge pressure transducers come in several different varieties: the bonded strain gauge, the sputtered strain gauge, and the semiconductor strain gauge.

 Strain gauge pressure transducer. Image credit: openticle.com

Semiconductor piezoresistive sensors are based on semiconductor technology. The change in resistance is not only because of a change in the length and width (as it is with strain gage) but because of a shift of electrical charges within the resistor. There are four piezoresistors within the diagram area on the sensor connected to an element bridge. When the diaphragm is deflected, two resistors are subjected to tangential stress and two to radial stress. 

Medium - High Vacuums

At medium and high vacuums it is more accurate to measure vacuum with thermal and molecular devices.

Thermal conductivity - The thermal conductivity of gas is measured using a Pirani gauge. It is a simple device that contains a heated plate and measures the amount of heat lost by the plate. The amount of heat lost depends on the gas pressure. There are several designs of the Pirani gauge. One design includes using two plates with different temperatures. The amount of power spent for heating is the measure of gas pressure. Another design uses a single plate to measure the thermal conductivity of gas by heat loss to the surrounding area. The gauge in the image below uses a thermal balance technique by dividing the sensing chamber into two sections and filling one with gas at a reference pressure and the other is connected to the vacuum that is being measured. Each of the chambers is the same size, shape, and construction and contains a single heated plate. The temperature measure is done with a thermocouple.

Pirani Vacuum Gauge. Image credit: Handbook of Modern Sensors

Ionization gauge Vacuum gauges which use ions are similar to vacuum tubes. The relationship between the ion current and the filament is a nearly linear function of molecular density (pressure). The operating principle is the same as a vacuum tube gauge. However, the plate is substituted by the wire surrounded by a grid, while the cathode filament is outside. There are two types available: hot cathode and cold cathode. The main difference between the two types is their method of electron production. Cold cathode devices draw the electrons from the electrode surface by a high potential field.

Additional Pressure Readings

Many vacuum sensors can perform additional pressure readings such as absolute, differential, gauge, compound, and sealed pressure.

• Absolute pressure is a pressure measurement relative to a perfect vacuum.
• Differential pressure is the difference between two input pressures.
• Gauge pressure is the pressure measured above the local atmospheric pressure. It is the most common pressure measurement.
• Positive and negative (vacuum) pressures can be measured using a compound vacuum sensor.
• Sealed gauge pressure is relative to one atmosphere at sea level (14.7 psi), regardless of local atmospheric pressure.

Read How to Select Pressure Sensors for more information on pressure readings.

Performance Specifications

Vacuum range is the most important specification to consider when selecting vacuum sensors.

 Vacuum range is the span of pressures from the lowest vacuum pressure to the highest vacuum pressure. When reading the chart below, it is important to keep in mind that high- vacuum is a lower pressure than low vacuum.

Pressure range and degree of vacuum. Table credit: National Physical Laboratory

Operating temperature is another important element to consider when selecting a vacuum sensor. Operating temperature is the full-required range of ambient operating temperature. Temperature and pressure are directly related to each other. If the temperature of the operating environment increases the pressure in the system will increase. In order to prevent equipment damage, it is important to know the extreme temperature ranges of the area.

 Temperature and Pressure Relationship. Image Credit: Science-faciles.com

Features

Vacuum sensors provide features such as:

• TTL-compatible switches are compatible with transistor-transistor logic.
• Built-in audible or visual alarms that signal when the switch or sensor has been turned on or off. This is important when the vacuum pressure of a system needs to be closely monitored.
• Temperature measurement outputs allow the user to observe the temperature of the system and adjust temperature and/or vacuum level as needed.
• Temperature compensation includes built-in factors that prevent pressure measurement errors due to temperature changes.
• Negative pressure outputs are available only with vacuum sensors that provide differential pressure measurements.

Applications

Applications for vacuum sensors include

• Chemical processing
• Freeze drying
• Helium leak
• Detection
• Sterilization
• Lamp, lighting, and laser products
• Cathode ray tubes (CRT)
• Electron microscopes
• High energy physics
• Optical, functional, and plasma-enhanced deposition
• Gas delivery manifoldsMechanical vacuum pumps
• Mass spectrometers
• Metallurgical processes

Resources

High Pressure & Vacuum

Fraden, Jacob. Handbook of Modern Sensors: Physics, Designs, and Applications. New York: Springer, 2010. Print.