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Interpreting specifications of pressure sensor accuracy
Unfortunately there is no universally adopted convention for specifying the accuracy of pressure sensors. Although there are standards such as IEC60770 and DIN16086 most manufacturers do not specify them in their data sheets. It is therefore up to the user to analyse each data sheet to understand what parameters are included in the accuracy statement and what technique was used so that a true comparison can be made.
The accuracy of a pressure sensor can be broken down into a few contributing components which are linearity, hysteresis, short term repeatability, temperature errors, thermal hysteresis, long term stability and zero & span offsets.
The linearity of a pressure sensor is rarely indicated as a separate component. The linearity refers to the straightness of the output signal at various equally spaced pressure points applied in an increasing direction. It should not be confused with accuracy which refers to how close a measured output or reading is to the actual pressure rather than how straight they all are.
The proportion of pressure hysteresis can vary depending on the sensor technology and typically it is incorporated with linearity to define the precision of the pressure sensor. The pressure hysteresis is measured by taking the difference between 2 output signals taken at the exactly the same pressure but during a sequence of increasing and decreasing pressure.
The precision of a pressure sensor sometimes includes short term repeatability errors. This is an indication of how stable a pressure sensor is after a series of pressure cycles. The same pressure point from each pressure cycle is compared with the first cycle to determine the amount of change. This is rarely shown as a separate error on a data sheet and is usually incorporated into a combined non-linearity, hysteresis and repeatability (NLHR) error statement.
The accuracy performance of a pressure sensor over a range of temperatures is normally quoted in a separate section of the technical data sheet. The temperature errors are normally specified over a maximum and miinimum temperature which is called the compensated temperature range and does not necessarily refer to the operating temperature range which is often wider than the compensated temperature range for a pressure sensor.
The temperature error is either expressed as a percentage of full scale for the total compensated temperature range or as a percentage of full scale per degree Celsius, Fahrenheit or Kelvin. Very often the thermal error component at zero pressure and the thermal error of pressure sensitivity (span) are split out and indicated separately.
Thermal hysteresis refers to the change of a specific pressure point at a particular temperature measured during a sequence of increasing temperature and decreasing temperature. Thermal hysteresis is unlikely to be mentioned in a pressure sensor specification so it is difficult to determine whether it has been included in the overall temperature errors or not. If thermal hysteresis is indicated it will be expressed as a percentage of full scale over the compensated temperature range.
Long term stability is a measure of how much the output signal will drift over time under normal operating conditions. The long term drift is expressed as a percentage of full scale over a period of time normally a period of 12 months. Sometimes the zero and span long term stability is quoted separately especially if one is much larger than the other. Long term drift is really only a figure for comparing one technology with another and cannot be relied on for a particular application. This is because the amount of pressure cycling, temperature cycling, vibration and shock the pressure sensor will be subjected to over its service life is not easily predictable. All of these factors will affect the pressure sensor’s performance to varying degrees depending on amplitude and frequency.
Zero and span offsets are the actual signal outputs at zero and full span. They are either expressed as percentages of full span or as electrical units such as millivolts or milliamps. Typically they are indicated as separate items on a pressure sensor data sheet. If the pressure sensor is going to be calibrated when installed the zero and span offset can be easily eliminated but if the pressure sensors have to installed or replaced without calibration they must included in the overall accuracy of the pressure sensor.
Related Products
High accuracy pressure sensors and instrumentation
Associated Topics
Calibrating with a highly accurate digital pressure sensor
Installing pressure transmitters for improved performance
Selecting pressure range for optimum long term performance
How accurate are pressure sensors over time
What is best straight line and terminal straight line accuracy
Site Last Updated: 09/02/12
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