Discussion of Error Band as it Relates to Pressure Sensors

A Whitepaper from PMC

Pressure sensor manufacturers commonly group the errors associated with pressure measurement into two groups, specifically Static Error Band and Total Error Band. While certain industries and application areas may occasionally list other errors, the two above are by far the most common and can be found in marketing literature from almost any sensor Manufacturer. Sensor errors are invariably referred to as %FS (percent of Full Scale).

In the simplest terms, Total Error Band (TEB) is the maximum deviation from “perfect” performance under any combination of environments to which the sensor may be subjected. Therefore, as long as the sensor is operating within the parameters defined by the manufacturer’s specification, the user should always be able to rely on the sensor output being accurate within the TEB limits.

For most modern sensor technologies, the largest contributor to TEB is thermal error. Thermal error may be as much as 90-95% of the total error, particularly when the sensor is expected to operate over a wide temperature range.

By far the most prominent error term referenced in marketing literature is Static Error Band. Most sensor companies define Static Error Band as the combined effects of nonlinearity, hysteresis, and repeatability errors. However, this definition can be (and is) misleading at times, specifically because it does not include two important but often ignored error sources, specifically the zero setting and span setting errors.

Users often assume that readings from pressure sensors at room temperature will always be accurate within the static error band limits. When zero set and span set are not included however, large errors often occur. For example, some manufacturers list 0.25% static error band, but separately list 0.5% zero set error and 0.5% span set error. In this example, the user may rely on a reading being within 0.25% of the true value, but in reality the reading could be in error by as much as 1.25%.

Standard practice at STS-Sensors is to include these setting errors in the Static Error Band definition. Under the STS definition, where Static Error includes the combined effects of non-linearity, hysteresis, repeatability, zero-set and span-set errors, the user can be assured that readings taken at room temperature will indeed be within the stated 0.25%.

Because the setting errors are generally large relative to the non-linearity, hysteresis, and repeatability errors, an STS sensor specified as a 0.25% device is usually equivalent to a 0.1% device from other manufacturers, and an STS 0.1% device is about equivalent to 0.04% from others.

The following definitions further explain these errors, all of which are expressed in Percent of Full Scale (%FS):

Full scale – The pressure at which the sensor outputs its maximum rated electrical signal.

Hysteresis – The difference in sensor output at any given test pressure, depending on whether the test pressure was achieved by increasing or decreasing pressure. A perfect sensor with zero hysteresis would retrace the same line as the pressure is increased or decreased. In reality, hysteresis error causes the output signal to form a loop as pressure is increased and decreased.

Linearity (non-linearity) – The deviation of the electrical output from a perfect straight line as the pressure is changed from zero to full scale, or from full scale to zero. Always expressed as %FS, but several methods are used, including BSL (Best Fit Straight Line), TSL (Terminal Straight Line), or forced zero BSL. BSL is by far the most common, and FZ-BSL is rare. In the BSL method, a Best fit Straight Line derived by a linear regression analysis is used as the reference line for error measurement. In the TSL method, the reference line is draw between the two endpoints of the output curve.

Repeatability – The difference between consecutive readings taken at exactly the same pressure, when the test pressure is attained from the same direction.

Span – The change in the output of the sensor when the pressure is increased from zero to full scale

Zero-setting error – Sometimes called zero balance, this is defined as the deviation of the output from the correct value, when zero pressure is applied.