Defining a Pressure Transducer

Why does non-electrical energy need to be converted into an electrical signal?
Once converted into electronic signal, pressure and temperature measurements can be integrated into software programs that enable report generation, the remote or automated monitoring of environmental conditions, or integration into a larger system of variables.

What applications are transducers used for?
Transducers are a core functional component across nearly all major industries, including Oil and Gas, Aerospace, and Automotive. They provide the means for fuel regulation, altitude control, and temperature control, among countless other applications.

Choosing the Right Pressure Transducer

When deciding what type of pressure transducer to use for a specific application, a number of factors need to be considered. Among these are:

Pressure Type: Gage, Absolute, Differential

What is Gauge Pressure?

• “Gauge Pressure” transducers measure the atmospheric conditions locally around the device. For example, comparing (or “referencing”) the pressure within an enclosed tank to ambient atmospheric conditions (~14.7 PSI) outside of the enclosed tank, such as when measuring tire pressure. Measurements will be labeled in PSIG.

What is Absolute Pressure?

• “Absolute Pressure” transducers are functionally si milar to Gauge Pressure devices, but instead of referencing ambient atmospheric conditions, they will reference vacuum (~0 PSI). Absolute Pressure Gauges are useful for applications where certain quantities are unknown or subject to variability, such as when determining the altitude of an airplane. Measurements will be labeled in PSIA.

What is Differential Pressure?

• “Differential Pressure” transducers compare the quantity of two separate medias without using a definitive set reference point (such as atmospheric pressure or vacuum). For example, a transducer may act as a filter monitor by comparing the pressure increase caused by contaminant build-up to the pressure level established upon installation. Measurements will be labeled in PSIG.

• • • • Environment – (e.g. Corrosive, Extreme Temperature, Hazardous Conditions, etc.)
      • Temperature
      • High Accuracy
      • Electrical Output

When is voltage output appropriate (AKA 0-5V, 0-10V, 0-20V)?

• Transducers that output DC voltage are fairly basic devices that are easy to install and troubleshoot while offering high resolution and accuracy. They are appropriate for applications in environments that are free from excessive electromagnetic interference, and where cable length may be kept short (generally less than 50 feet, otherwise the signal degrades and accuracy decreases, but it depends heavily on the wire gauge used when installing the unit: bigger gauge wire = more length before signal degrades. 50 feet is a general limit for 22 AWG). They usually require 4 or 6 wires for operation.

When is current output appropriate (AKA 4–20mA)?

• Transmitters that output current are preferred for applications where long cables (e.g., more than 100 ft) must be used for connections, or in environments where high levels of electromagnetic interference are unavoidable (e.g., high voltage AC motors, pumps, and relays). They require only 2 wires for operation. However, current output does not offer the same level of high resolution as voltage output, and also requires the use of a shunt resistor in order to be readable on data logging equipment.

• • • • Reliability
      • Repeatability
      • Redundancy
      • Energy Consumption
      • Size Constraints
      • Cost

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  Transducer Specifications

  What is “Transducer Accuracy?”

  • • There is no definitive standard for determining transducer accuracy, and so it is important for manufacturer and customer to communicate their accuracy standards and expectations with each other. However, there are definitive characteristics that are required by the International Electrotechnical Commission (IEC) to act as a minimum standard of defining accuracy. They are: Hysteresis, Repeatability (also referred to as “Non-Repeatability”), and Linearity (also referred to as “Non-Linearity”)

  Hysteresis

  • • Hysteresis is defined fairly universally (I say “fairly” because there will ALWAYS be the possibility of exceptions, so it’s important to be aware of that possibility so that any miscommunication between manufacturer and customer are avoided):
    • As pressure is applied to a sensor, this is referred to as “increasing pressure.” When pressure is removed from a sensor, this is referred to as “decreasing pressure.” Hysteresis is determined by measuring output points at predefined intervals during increasing pressure, and comparing them to output points measured at the same predefined intervals during decreasing pressure. The maximum difference found between the increasing pressure points and the decreasing pressure points is the hysteresis value of the transducer, and is mathematically defined as a percentage of the full-scale output of the sensor.

  Repeatability

  • • Repeatability is well-defined as the maximum difference in output when the same pressure is applied, consecutively, under the same conditions and approaching from the same direction.

  Non-linearity

  • • Non-linearity (or simply “Linearity”) refers to the measurement of how straight the transducer output signal is when equally spaced pressure points are applied in an increasing direction. Ideally, transducer output would look like a straight line. In practice, a straight line is impossible to achieve. Non-linearity therefore is the measurement of the deviation between this ideal “straight line” of output and the actual output, commonly referred to as an “output curve.”

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    Strain Gages

    Probably the most common among pressure transducers are the strain gage base
    transducers. Strain gages are bonded into the diaphragm of a pressure transducer,
    then wired into a Wheatstone bridge configuration. When pressure is applied to
    the transducer, it produces a deflection of the diaphragm which creates strain
    on the gage. The physical deformation of the strain gage produces an electrical
    resistance change proportional to the pressure, which is transmitted in an analog
    electrical output signal which can then be read to determine amount of pressure.

    Electrical Output

    Pressure transducers are available in 3 types of electrical output; millivolt (mV), amplified voltage, and 4-20mA.

    Millivolt (mA)

    Among the most economical of pressure transducers, the output of millivolt transducers is technically around 30mV. The actual output is proportional to the pressure transducer input power, also known as excitation. The output changes as the excitation fluctuates. Regulated power supplies are recommended for use with millivolt transducers because of this dependence on the excitation level.

    Voltage Output

    Voltage output transducers provide a much higher output than millivolt transducers because they include *integral signal conditioning. Although model specific, output is typically 0-5Vdc or 0-10Vdc and is not usually a direct function of excitation. Because of this, unregulated power supplies can be sufficient as long as they are within a specified power range. Voltage output transducers are not as susceptible to electrical noise as millivolt transducers because they have a higher-level output, and therefore are more appropriate for use in industrial environments

    4-20mA Output

    Also known as pressure transmitters, a 4-20mA transducer is least affected by electrical noise and resistance in the signal wires, and is therefore best suited for when a signal must be transmitted long distances. 4-20mA transducers are commonly used in applications where the lead wire must be 1,000 feet or more.

    *Most analog signals require some form of preparation before they can be digitized. Signal conditioning is the manipulation of a signal in a way that prepares it for the next stage of processing. Many applications involve environmental or structural measurement, such as temperature and vibration, from sensors. These sensors, in turn, require signal conditioning before a data acquisition device can effectively and accurately measure the signal. For example, thermocouple signals have very small voltage levels that must be amplified before they can be digitized. Other sensors, such as resistance temperature detectors (RTDs), thermistors, strain gages, and accelerometers, require excitation to operate. All of these preparation technologies are forms of signal conditioning.

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