Pressure Sensor                    Submersible Sensor                                    

                                  

Pressure Sensor.   Submersible Sensor. MEMs pressure sensor device                                                     

Image Credit: GE | Kobold | Measurement Specialties

 

Pressure sensors include all sensors, transducers and elements that produce an electrical signal proportional to pressure or changes in pressure. The device reads the changes in pressure, and then relays this data to recorders or switches.

 

How Pressure Sensors Work

Pressure instruments monitor the amount of pressure applied to a part of the process. There are several types of pressure instruments:

  • Sensors-Pressure sensors convert a measured pressure into an electrical output signal. They are typically simple devices that do not include a display or user interface.
    • Elements are the portions of a pressure instrument which are moved or temporarily deformed by the gas or liquid of the system to which the gage is connected. These include the Bourdon tube which is a sealed tube that deflects in response to applied pressure, as well as bellows, capsule elements and diaphragm1 elements.

The basic pressure sensing element can be configured as a C-shaped Bourdon tube (A); a helical Bourdon tube (B); flat diaphragm (C); a convoluted diaphragm (D); a capsule (E); or a set of bellows (F). Image Credit: sensorsmag

  • Transducers- Pressure transducers are pressure-sensing devices. It converts an applied pressure into an electrical signal. The output signal is generated by the primary sensing element and the device maintains the natural characteristics of the sensing technology. A transducer is also a sensor but a transducer always converts the non-electric pressure signal into an electrical signal. Therefore, a transducer is always a sensor but a sensor is not always a transducer. In industry the terms are often interchanged. There are several types of transducers including:
    • Strain gauge
    • Thick film
    • Thin film sensors have an extremely thin layer of material deposited on a substrate by sputtering, chemical vapor deposition, or other technique. This technology incorporates a compact design with good temperature stability. There are a variety of materials used in thin film technology, such as titanium nitride and polysilicon. These gauges are most suitable for long-term use and harsh measurement conditions.

    • Semiconductor strain gauge

Sensor Technology

There are numerous technologies by which pressure transducers and sensors function. Some of the most widely used technologies include,

  • 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.

Diaphragms

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. The common modes of operation are charge mode, which generates a high-impedance charge output; and voltage mode, which uses an amplifier to convert the high-impedance charge into a low-impedance output voltage. The sensors can only be used for varying pressures. They are very rugged but require amplification circuitry and are susceptible to shock and vibration.

 

Piezoelectric Pressure Transducer 

Piezoelectric Pressure Transducer. Image Credit: National Instruments

  • 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. Depending on the type of pressure, the capacitive transducer can be either an absolute, gauge, or differential pressure transducer. 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.

  Design Tip: The electronics for signal conditioning should be located close to the sensing element to prevent errors due to stray capacitance

 

The capacitance of two parallel plates is given by the following equation,
C= µA/d
Where :
µ = dielectric constant of the material between the plates
A = area of the plates
d = spacing between the plates

These pressure transducers are generally very stable, linear and accurate, but are sensitive to high temperatures and are more complicated to setup than most pressure sensors. 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. Four strain gages are typically used in series in a Wheatstone bridge circuit, which is used to make the measurement. When voltage is applied to two opposite corners of the bridge, an electrical output signal is developed proportional to the applied pressure. The output signal is collected at the remaining two corners of the bridge. 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

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.

Piezoresistive semiconductor

Piezoresistive semiconductor pressure sensors incorporate four piezoresistors in the diaphragm.

Image Credit: sensorsmag

 

The output is described by the following equation:
                           Vout/ Vcc = ΔR/R
Where:
Vcc = supply voltage
R = base resistance of the piezoresistor
ΔR = change with applied pressure and it typically ~ 2.5% of the full R.
These are very sensitive devices.

 

Selection Criteria

The GlobalSpec SpecSearch database allows industrial buyers to select pressure sensors by performance specifications, mechanical considerations, electrical specifications, environmental considerations, and special requirements.

 

Performance Specifications

The performance of the sensor is based on several factors intrinsic to the system in which the sensor will be used. These include maximum pressure, pressure reference, engineering units, accuracy required, and pressure conditions.

 

Maximum Pressure

Static pressure is defined as P = F/A; where P is pressure, F is applied force and A is the area of application. This equation can be used on liquid and gas that is not flowing. Pressure in moving fluids can be calculated using the equation P1 = ρVO2/2; where ρ is fluid density, and VO is the fluid velocity. Impact pressure is the pressure a moving fluid exerts parallel to the flow direction. Dynamic pressure measures more "real-life" applications.

Pressure is in all directions in a fluid  

Pressure is in all directions in a fluid.

 Image Credit: schoolforchampions.com

 

Maximum Pressure Range is the maximum allowable pressure at which a system or piece of equipment is designed to operate safely. The extremes of this range should be determined in accordance with the expected pressure range the device must operate within. It is common practice that this value should not exceed 75% of the device's maximum rated range. For example: if the device has a maximum rated range of 100 psi then the working range should not exceed 75 psi.

 

Design Tip: Figure out what the anticipated pressure spikes will be and then pick a transducer rated 25% higher than the highest spike. An additional margin is suggested where "high cycling" may occur.

 

 Pressure Reference

  • Absolute pressure sensors measure the pressure of a system relative to a perfect vacuum. These sensors incorporate sensing elements which are completely evacuated and sealed; the high pressure port is not present and input pressure is applied through the low port. The measurement is done in pounds per square inch absolute.
  • Differential pressure is measured by reading the difference between the inputs of two or more pressure levels. The sensor must have two separate pressure ports; the higher of the two pressures is applied through the high port and the lower through the low port. It is commonly measured in units of pounds per square inch. An example of a differential pressure sensor is filter monitors; when the filter starts to clog the flow resistance and therefore the pressure drop across the filter will increase.
    • Bidirectional sensors are able to measure positive and negative pressure differences i.e. p1>p2 and p1
    • Unidirectional sensors only operate in the positive range i.e. p1> p2 and the highest pressure has to be applied to the pressure port defined as "high pressure"
  • Gauge sensors are the most common type of pressure sensors. The pressure is measured relative to ambient pressure which is the atmospheric pressure at a given location. The average atmospheric pressure at sea level is 1013.25 mbar but changes in weather and altitude directly influence the output of the pressure sensor. In this device, the input pressure is through the high port and the ambient pressure is applied through the open low port.
    • Vacuum sensors are gauge sensors used to measure the pressure lower than the localized atmospheric pressure. A vacuum is a volume of space that is essentially empty of matter. Vacuum sensors are divided into different ranges of low, high and ultra-high vacuum.
    • Sealed gauged sensors measure pressure relative to one atmosphere at sea level (14.7 PSI) regardless of local atmospheric pressure.

The same sensor can be used for all three types of pressure measurement; only the references differ.

 Image Credit: sensorsmag

 

Engineering Units

Pressure is a measure of force per unit area. A variety of units are used depending on the application; a conversion table is below.

1psi    = 51.714 mmHg
          =  2.0359 in.Hg
          =  27.680 in.H2O
          =  6.8946 kPa
1 bar       = 14.504 psi
1 atm.      = 14.696 psi

 

Accuracy 

Accuracy is defined as the difference (error) between the true value and the indicated value expressed as percent of the span. It includes the combined deviations resulting from the method, observer, apparatus and environment. Accuracy is observed in three different areas; static, thermal, and total.

  • Static accuracy is the combined effects of linearity, hysteresis, and repeatability. It is expressed as +/- percentage of full scale output. The static error band is a good measure of the accuracy that can be expected at constant temperature.
    • Linearity is the deviation of a calibration curve from a specified straight line. One way to measure linearity is to use the least squares method, which gives a best fit straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.
    • Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure. Temperatures hysteresis is the sensor's ability to give the same output at a given temperature before and after a temperature cycle.

    Hysteresis  

    Hysteresis is a sensor's ability to give the same output at a given temperature before and after a temperature cycle. Image Credit: sensorsmag

  •  
    • Repeatability is the ability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.

  • Thermal accuracy observes how temperature affects the output. It is expressed as a percentage of full scale output or as a percentage of full scale per degree Celsius, degree Fahrenheit or Kelvin.

  • Total accuracy is the combination of static and thermal accuracy. In cases where the accuracy differs between middle span and the first and last quarters of the scale, the largest % error is reported.
    ASME2 B40.1 and DIN accuracy grades are frequently used:
    Grade 4A (0.1% Full Scale)
    Grade 3A (0.25% Full Scale)
    Grade 2A (0.5% Full Scale)
    Grade 1A (1% Full Scale)
    Grade A (1% middle half, 2% first and last quarters)
    Grade B (2% middle half, 3% first and last quarters)
    Grade C (3% middle half, 4% first and last quarters)
    Grade D (5% Full Scale)

 Pressure Conditions

Industrial buyers should consider the pressure conditions that the sensor will be exposed to and ask the following questions:

  • Over pressure: Will pressure ever exceed the maximum pressure? If so, by how much?
  • Burst pressure: The designed safety limit which should not be exceeded. If this pressure is exceeded it may lead to mechanical breach and permanent loss of pressure containment. Are additional safety features needed?
  • Dynamic loading: Dynamic loads can exceed expected static loads. Is the system experiencing dynamic pressure loading?
  • Fatigue loading: Will the system experience high cycle rates?
  • Vacuum Range is the span of pressures from the lowest vacuum pressure to the highest vacuum pressure (e.g., from 0 to 30 inches of mercury VAC).

Mechanical Considerations

Mechanical conditions of the device determine how the sensor operates within the system. Consideration should be given to the physical constraints of the system, the media into which the sensor will be incorporated, process connectors, and configurations of the system and sensor.

 

Physical Constraints

Physical constraints depend on the system that the sensor will be incorporated in to and should be considered when selecting a pressure sensor.

  • Size
  • Orientation

  • Location

Media

Understanding the media of the system is critical when selecting a pressure sensor. The media environments for the sensor could be:

  • Hydrogen and gases are very compressible, and they completely fill any closed vessels in which they are places.
  • Abrasive or corrosive liquids and gases such as hydrogen sulfide, hydrochloric acid, bleach, bromides, and waste water. Pressure sensors made of Inconel X, phosphor bronze, beryllium copper or stainless steel are the most corrosion resistant materials to be used in the sensor. However, these materials require internal temperature compensation, in the form of a bi-metallic member, to offset the change in deflection of the sensor resulting from a change in temperature.
  • Biological
  • Radioactive systems should include highly sensitive sensors which have explosion proof mechanisms.

The temperature of the media should also be considered when selecting a pressure sensor to ensure the sensor can function in the range of the system.

 

Process Connection 

Pressure port and process connection options generally have male and female options and the standard connection depends on the application.

  • British Standard Pipe (BSP)- Large diameter pressure connectors are needed for lower pressure ranges
  • National Pipe Thread (NPT)- Commonly used in automotive and aerospace industries
  • Unified Fine Thread (UNF)- Commonly used in automotive and aerospace industries
  • Metric Threads- Meet ISO specifications. They are denoted with an M and a number which is the outside diameter in millimeters.
  • Flush connectors- Used to provide a crevice free interface which is ideal for biotechnology, pharmaceutical or food process applications.
  • Diary Pipe Standard- Used with hygienic pressure transmitters.

  • Autoclave Engineers- Used in high pressure applications.

Configuration

Mechanical considerations include several application-driven device configurations.

  • Differential systems measure the pressure difference between two points.
  • Small diameter flow systems allow for flowing liquid or gas to be measured as it moves through the system.
  • Flush diaphragms measure pressure in systems which have either completely flush or semi-flush exposed diaphragms to prevent buildup of material on the diaphragm and facilitate easy cleaning. Exposed diaphragm sensors are useful for measuring viscous fluids or media that are processed in a clean environment.
  • Replaceable diaphragms are easily replaceable within the system to ensure high accuracy.
  • Secondary containment houses the sensor to protect the device from environmental conditions.
  • Explosion proof sensors are used in hazardous conditions.

 

Electrical Specifications

The electrical components of the pressure sensor are extremely important to consider and are specific to the application the sensor will be used in. Such specifications include electrical output, display, connections, signal conditioning and electrical features.

 

Electrical Output

Industrial buyers should consider the electrical output needed for seamless integration into the system controller.

  • Analog- The output voltage is a simple (usually linear) function of the measurement.
    • Pressure sensors generally have an output of mV/V. Most sensors operate from 10 V to 32 V, unregulated supply. The device will also have internal regulators to provide a stabilized input to the electronic circuitry under varying supply voltages.
    • Industrial sensors can have high-level voltage outputs of 0-5 VDC, and 0-10 VDC. The output signal will lose its amplitude and accuracy due to resistance from the cable when transmitting voltages between a few inches and 30ft depending on the level.

 Design Tip: A zero- based output signal, such as 0-5VDC does not offer constant feedback at zero pressure because the controller is unaware if the system is operating or if there is a problem.

  •  
    • HART® Protocol
    • Analog current levels or transmitters such as 4 - 20 mA are suitable for sending signals over long distances.
    • 4-20mA current: is popular for long distances.
  • Frequency: The output signal is encoded via amplitude modulation (AM), frequency modulation (FM), or some other modulation scheme such as sine wave or pulse train; however, the signal is still analog in nature.
  • Digital
    • RS485(MODbus): RS232 and RS485 are serial communication protocols that transmit data one bit at a time.
    • RS232 provides a standard interface between data terminal and data communications equipment.
    • CANbus, J1939, CAN open: connects industrial devices such as limit switches, photoelectric cells, etc. to programmable logic controllers (PLCs) and personal computers (PCs).
    • FOUNDATION Fieldbus a serial, all-digital, two-way communication system that serves as a local area network (LAN) for factory instrumentation and control devices.
    • Special Digital (TTL) devices produce digital outputs other than standard serial or parallel signals. Examples include transistor-transistor logic (TTL) outputs.
  • Combination includes analog and digital outputs 

Display

The display is the interface the user interacts with to observe the pressure sensor reading.

  • Analog Meter-The device has an analog meter or simple visual indicator.
  • Digital- The device has a display for numerical values.
  • Video- The device has a CRT, LCD or other multi-line display.

Connections

Connectors are considered for the electrical termination of the sensor. The use of connectors adds benefits to pressure sensor installation such as easy removal from the system for recalibration or system maintenance.

  • Connector or integral cables connect the sensor to the rest of the system. Integral cables are used for submersed applications such as on pumps or hose down situations.
  • Mating connectors and cable accessories are needed for applications when sealing the sensor is important. Threads are very common for low to medium pressures. National Pipe Threads (NPT) are tapered in nature and require some form of Teflon® tape or putty to seal the thread to a piece of equipment.
  • Connector/cable orientation allows the sensor to be unplugged and the senor un-threaded. High-vibration environments require an inline connector at the end of a length of wire to reduce the loading on the connector pins. Inline connectors increase the life of the sensor. When selecting a cable option, the outer jacket material and inner conductor insulators must be selected to match the application.
  • Wiring codes and pin-outs

Signaling Conditioning

External zero and span potentiometers are used to compensate stray current in the measuring circuit to prevent distortion.
DIN Rail mount or In-line signal conditioning for mV/V units will amplify output signals. These devices are used for applications requiring high level analog outputs and where the pressure transducer is exposed to conditions detrimental to internal signal conditioning or the required pressure transducer configuration will not accommodate an internal amplifier.

 

Electrical Features

  • Wireless sensors allow the information to be transmitted via a wireless signal to the host. Typically, the wireless signal is a radio frequency (RF) signal.

  • Switch sensors change the output to a switch or relay closure to turn the system on or off with changes in pressure.
  • Temperature output devices provide temperature measurement outputs in addition to pressure.
  • Negative pressure output are available with devices that provide differential pressure measurements
  • Alarm indicator devices have a built-in audible or visual alarm to warn operators of changes and/or danger in the system.
  • Frequency response identifies the highest frequency that the sensor will measure without distortion or attenuation. The sensor's frequency response should be 5-10 times the highest frequency component in the pressure signal. Sometimes this feature is given as response time and the relation is:

FB = ½ πτ

Where,

FB = frequency where the response is reduced by 50%
τ = time constant the output rises to 63% of its final value following a step input change.

 

Environmental Considerations

The environment the sensor will operate in should be considered when selecting a pressure sensor. Environmental considerations such as temperature, indoor/outdoor use and use in hazardous locations can affect the accuracy of the sensor.

 

Temperature

Changes in temperature are directly related to changes in pressure.

 

 A plot of the vapor pressure of water versus the water's temperature.

Image Credit: purdue.edu

  • Operating temperature is important to consider. Buyers should be aware of the ambient and media temperatures in the environment of the sensor. If the sensor is not compensated correctly the reading can change drastically.
  • Temperature compensation devices include built-in factors that prevent pressure measurement errors due to temperature changes. A material such as a nickel alloy called Ni Span "C", requires no internal temperature compensation because they are relatively insensitive to temperature.
  • Electromagnetic and radio frequency interference (EMI/RFI) have been identified as environmental conditions that affect the performance of safety-related electrical equipment.

  • Ingress Protection or National Electrical Manufacturing Association rating required. IP protection is used in Europe and follows three parameters; protects the equipment, protects the personal, and protects the equipment against penetration of water with harmful effects. IP does not specify degrees of protection against mechanical damage, explosions, moisture, corrosive vapors or vermin. The NEMA standard for the environments surrounding the electrical equipment tests environmental conditions such as corrosion, rust, icing, oil, and coolants. A full explanation of IP and NEMA standards can be found at Solid Applied Technologies.

Indoor/Outdoor

If the pressure sensor is being used outdoors, the sensor may be sealed or vented depending on the pressure range and the accuracy needed. Environmental exposure can include:

  • Dust/Dirt
  • Temperature extremes
  • Animals and rodent tampering
  • Submersion
    • Fresh water
    • Salt water

    • Depth

Hazardous Locations

If the pressure sensor is going to be used in a hazardous area, the class type and group type must be known in order for the product to comply with NEC or CEC codes in North America.

  • Intrinsically safe

  • Explosion proof

Special Requirements

Some systems may require special calibration and approvals.

 

Calibration

Standard 11-point calibration means the sensor is calibrated to 11 pressure points spanning the full scale range of the pressure sensor. Points such as 0% 20% 40% 60% 80% 100% 80% 60% 40% 20% 0% can be used and going up and down the pressure range will check for hysteresis.
Special calibration with additional calibration points

 

Approvals

Pressure sensors may need special approvals or certifications for operation in certain environments to protect the user and the environment. Specific testing, cleaning procedures and labeling may also need to be implemented for applications.

 

Serviceability

Some additional considerations when installing or budgeting for a sensor in a system are:
How accessible will the sensor be?
How often will it need to be serviced?

 

 

Applications

  • Industrial.

    • Fluid level in a tank: A gauge pressure sensor can be used to measure the pressure at the bottom of a tank. Fluid level can be calculated using the relation:
      h = P/ρg
      Where,
      h= depth below the water surface
      P= pressure
      ρ= water density
      g= acceleration of gravity
    • Fluid flow: Placing an orifice plate in a pipe section results in a pressure drop which can be used to measure flow. This method is commonly used because it does not cause clogging and the pressure drop is small compared to many other flow meters.
      The relation is:
      V0 = √2 (Ps-P0)/ρ
      In some cases, differential pressures of only a few inches of water are measured in the presence of common-mode pressures of thousands of pounds per square inch.
  • Automotive. A wide variety of pressure applications exist in the modern electronically controlled auto. Among the most important are:
    • Manifold absolute pressure (MAP). Many engine control systems use the speed-density approach to intake air mass flow rate measurement. The mass flow rate must be known so that the optimum amount of fuel can be injected.
    • Engine oil pressure. Engine lubrication requires pressures of 10-15 psig.
    • Evaporative purge system leak detection. To reduce emissions, modern fuel systems are not vented to the atmosphere. This means that fumes resulting from temperature-induced pressure changes in the fuel tank are captured in a carbon canister and later recycled through the engine.
    • Tire pressure. Recent development of the "run-flat" tire has prompted development of a remote tire pressure measurement system.
  • Tank level measurement
  • Altitude sensing
  • Pressure verification
  • Differential

Differential pressure application. Image Credit: futek

 

Resources

Brief explanation of a how a strain gauge diaphragm works
Design Essentials: How to select a pressure sensor for a specific application
Fundamentals of Pressure Sensor Technology
IP/NEMA Rating Introduction
Pressure Measurement Glossary
Pressure Transducer Tutorial
Process Connection
Strain Gauges

 

1Diaphragm- A strain gauge diaphragm typically consists of a flat circular piece of uniform elastic material which is manufactured into a variety of different surface areas and thickness' to optimize performance at lower and higher pressure ranges.
2ASME- American Society of Mechanical Engineers

 

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