WF Pressure Sensor Terminology and Basics
Table of Contents
Absolute Pressure:
Pressure measured relative to a vacuum. Usually expressed in pounds per square inch absolute (psia).
Altitude Pressure Transducer:
Air pressure sensors used to determine altitude from a pressure altitude profile.
Auto-referencing:
A technique that eliminates errors by sampling one or more reference pressures and then correcting the output signal function.
Atmospheric Pressure Transducer:
An absolute pressure sensor that measures local ambient pressure.
Basic Output:
The raw output of a pressure sensors that is neither amplified nor compensated for by calibration.
Best Straight Line (BSL):
The best straight line selected so that the true sensor response curve contains three points of equal maximum deviation.
Burst Pressure:
The best straight line selected so that the true sensor response curve contains three points of equal maximum deviation.
Common Mode Error:
An error that is independent of the primary input variable (input pressure). For WF Sensors , all offset errors are common-mode errors.
Common Mode Pressure:
Pressure that can be applied to both sides of the diaphragm simultaneously.
Compensated Output:
The output of a pressure sensors that is compensated for one or more performance parameters, such as offset, span, temperature effects on offset and span, nonlinearity, position sensitivity, front-to-back linearity, and preheat offset.
Compensation of the performance parameters may be achieved by design or calibration. Various compensations can be realized by design, for example, all WF-Package products are inherently compensated for front-to-back linearity and position sensitivity errors. Some examples of parameters that are compensated for through calibration are offset, span and the effect of temperature on offset and span.
Differential Pressure:
The difference in pressure measured between two pressure sources. Usually expressed in pounds per square inch difference (psid). When one source is a perfect vacuum, the pressure difference is called the absolute pressure. When one source is the local environment, the pressure is called gauge pressure.
Differential Pressure Transducer:
Device that measures the difference in pressure between two pressure sources connected through a pipe to its input.
DIGITAL OUTPUT:
The output of a pressure sensors, expressed as a discrete (quantized) quantity, usually communicated using a standard interface such as I2C or SPI. The benefits of digital output signals are a direct interface to the host microprocessor (uP), improved noise immunity, and a smaller component count.
Error Band:
The deviation of a sensor’s response from its BSL, defined by the lines on either side of its BSL, including the maximum deviation measured for a given normal mode or common mode error.
Full scale:
The algebraic difference between endpoints. One endpoint is the actual offset voltage and the other endpoint is the upper limit of the range.
Full Scale Offset:
The offset of the sensor output voltage sensitivity to pressure over a specified temperature range. This corresponds to the temperature coefficient of sensitivity. The characteristics of this transfer curve are very close to the second order equation for a basic piezoresistive sensing element.
For millivolt output devices, this is compensated by passive resistance; for amplified output devices, it is ASIC-compensated, using a second-order curve fitted to the data acquired by each component over a specified pressure and temperature range. The characteristics of this transfer curve do not change with any other conditions.
Gauge Pressure:
Pressure measured relative to ambient pressure (psig).
Pressure Hysteresis:
Pressure hysteresis is measured as the maximum difference between outputs at reference conditions before and after a pressure cycle.
Interchangeability:
The band of error defined by the maximum signal deviation when a sensors is replaced by any other transducer of the same type with the same pressure input and temperature range.
Linearity:
The maximum deviation of the measured output from the “best straight line” at constant temperature (25°C), determined by three points (bias pressure, full scale pressure and one-half full scale pressure), where Y = measured value per device.
Linearity, Hysteresis Error:
Error in the response of the output voltage to pressure over the entire operating pressure range relative to the ideal output voltage response; deviation of the output signal from the first order transfer curve response to pressure. This error is a function of pressure rather than temperature.
This error is calculated by measuring the pressure at three pressure points; zero pressure. Full scale pressure and one-half full scale pressure and is calculated from the “best straight line” curve fitted to the measured data.
Minimum/Maximum Values:
are the specification guarantee limits. These limits are usually between the test limits and the specification limits using a protective tape for 100% testing.
Most Probable Error:
The error band obtained by calculating the square root of the sum of the squares of all applicable errors specified for the sensor.
Nominal value:
It is the average of the specifications of a product manufactured during the first production run.
Normal Mode Error:
It is the error of the function (usually assumed to be proportional) of the main input variable (input pressure). For WF Sensors sensors, all range errors are normal mode errors.
Offset Calibration:
The error band defined by the maximum error in calibrating the offset voltage.
Dissonance Error:
The common mode error band defined by the maximum deviation of the out-of-phase voltage from its specified value. It may include calibration, temperature, repeatability and stability errors.
Offset Long-Term Drift:
The change in offset voltage that can occur over a specified period of time. The possible causes of this characteristic of piezoresistive pressure sensors have been investigated for decades. To date, no single or primary cause has been determined.
Since most WF Sensors low pressure sensors utilize dual-chip electrical cross-coupling compensation, there is inherent compensation for long term drift in the offset. The product WF Sensors warm-up shift test will typically identify any offset long term drift issues and will be rejected.
Offset position sensitivity:
The change in offset voltage due to a change in sensor position. Sensors used to measure pressures greater than 15 psi have little or no positional sensitivity. Because the sensor’s diaphragm has mass, and the ratio of mass to diaphragm thickness increases as the pressure range decreases, the sensitivity to position increases as the pressure range decreases. Since most WF Sensor low voltage sensors use dual-chip electrical cross-coupling compensation, even the most basic sensors have inherent offset position sensitivity compensation.
Offset Repeatability:
An error band indicates the ability of a sensor to reproduce an offset voltage measured at 25°C after exposure to any other temperature and pressure within a specified range.
Offset Stability:
The ability of an error band meter sensor to maintain an offset voltage at a constant pressure and temperature.
Temperature Coefficient of Adjustment:
The error band is defined as the maximum deviation of the offset voltage when the temperature changes from 25°C to any other temperature within the specified range.
Offset Temperature Offset:
The change in output offset voltage over the specified temperature range. For non-amplified sensors, the specification limits testing to three temperature points; 25°C, maximum temperature, minimum temperature, and then back to 25°C. For amplified pressure sensors, more data points are measured and compensation is mathematically appropriate for the data points.
There is usually no consistent equation to characterize the offset temperature shift. Since most WF Sensor low voltage sensors use dual-chip electrical cross-coupling compensation, even the most basic sensors have inherent offset temperature offset compensation.
Offset Voltage:
The output voltage of a sensor when the pressure differential across the diaphragm is zero. For absolute pressure sensors, the pressure differential across the diaphragm is zero when the absolute pressure of the sensor is zero. For gauge or differential pressure sensors, the differential pressure is zero when both sides of the diaphragm are under the same pressure.
Offset Preheat Shift:
The change in output offset voltage that may occur when a sensor is energized during the first hour of operation. All sensors test all low voltage sensors for this parameter. Since most companies use dual-chip electrical cross-coupling compensation for their low-voltage sensors, even the most basic sensors have inherent offset preheat offset compensation.
Operating Range:
The range of pressure over which a sensor has been tested. For sensors with millivolt outputs, this range can usually be extended to at least twice the specified range with minimal degradation of the specification. For amplified output sensors, the range can only be extended by 10% before the output is “railed” to the output voltage limit.
Output Span:
Is the output voltage that specifies the operating pressure range. For sensors without an internal voltage reference, the span is proportional to the sensor supply voltage. A change in the sensor supply voltage will result in a change in the output range of the applied pressure. Span is the difference between the output voltage and the offset voltage at full scale pressure.
OVERPRESSURE - MAXIMUM:
The maximum normal mode (measured) pressure that can be applied without altering the performance of the sensor or exceeding the accuracy of the specified limits. This will apply to either port of the differential sensor. This is also referred to as “proof pressure”.
Overall accuracy-calibration:
As opposed to the combined error bands of BSL, the mandatory reference is unique to a particular sensor. It does not include offset and sensitivity calibration errors. It includes all other offset and span errors: temperature, repeatability, stability, linearity, and lag.
Overall Accuracy - Interchangeable:
Combined error band relative to the response characteristics of an ideal sensor. It excludes stability errors, which are included in the specified calibration error. It includes all other offset and range errors: calibration, temperature, repeatability, linearity and hysteresis.
Withstand Voltage:
Is the maximum pressure that can be applied to the sensor without causing any change in the performance index.
Reference Pressure:
The pressure used as a reference when measuring sensor error.
Reference Temperature:
The reference temperature used to measure sensor error.
Repeatability:
A band of error that indicates the ability of a sensor to reproduce an output signal parameter (e.g., offset or span) at a specified pressure and temperature and at any other pressure and temperature within a specified range.
Sensitivity:
The ratio of the change in output signal voltage to the corresponding change in input pressure. Sensitivity is determined by calculating the ratio of the range to the specified input pressure range.
Sensitivity Calibration:
The error band defined by the maximum error of the calibrated sensitivity.
Range:
The arithmetic difference between the sensor output signal measured at a specified minimum and maximum operating pressure.
Range Error:
A band of normal mode error defined by the maximum deviation of the range from its specified value. It may include sensitivity calibration temperature, linearity, hysteresis, repeatability, and stability deviations.
Range Repeatability:
An error band indicates the ability of a sensor to reproduce its range after measurement at 25°C and exposure to any other pressure and temperature within a specified range.
Span Temperature Coefficient:
The error band defined by the maximum deviation of the span as the temperature changes from 25°C to any other temperature within the specified range.
Range Stability:
The error band indicates the ability of the sensor to maintain the range voltage at any pressure within the specified range while the temperature remains constant.
Stability:
The ability of an error band meter sensor to maintain an output parameter value (e.g., offset or range) at a constant temperature and pressure input.
Supply Voltage:
The supply voltage applied to the input terminals of the pressure sensor.
Temperature Coefficient (TC):
The band of error caused by the maximum deviation of the sensor’s output parameter (e.g., offset or range) when the temperature changes from 25°C to any other temperature within the specified range. It is usually measured in (ppm/°C or µV/V/°C).
Temperature compensated output:
The output of the pressure sensor is compensated for temperature-dependent errors, specifically for temperature dependence of span and offset.
The pressure sensor output parameters are sensitive to temperature changes, and in particular the pressure response (Span) and zero differential pressure output (Offset) exhibit a temperature dependence. To counteract this temperature dependence, WF Sensors provides internal circuitry for sensor products that compensates for temperature variations in Span and Offset over a specified temperature range. Temperature compensated outputs are available in analog (unamplified mV, amplified 4V) and digital (SPI and I2C) outputs. Vacuum: A perfect vacuum is the absence of gaseous fluids.
Vacuum Range:
The absolute pressure range between a perfect vacuum (0 psia) and a standard atmospheric pressure (14.697 psia).
Vacuum Sensor:
A sensor calibrated for pressure measurement in the vacuum range. This is usually an absolute sensor, but sometimes it is a gauge sensor.
Worst case error:
The error band obtained by simply adding all applicable errors specified for the sensor.
