Pressure Sensor References and Applications

Measurement compatibility

It is very important to ensure the compatibility between the pressure sensor and its actual application. Before making a pressure sensor selection, the following aspects should be considered: (1) material (2) chemical substance (3) concentration (4) temperature (5) exposure time (6) exposure form (7) failure criteria and (8) general information, such as application environment, device protection and other equipment components in the area.

In all cases, the user is ultimately responsible for ensuring the compatibility of the sensor with the measured medium.

Errors that affect pressure sensor performance

When calculating the total error of a pressure sensor, the following defined errors should be used. To determine the extent of the specific error of the pressure sensor you have selected, refer to the specifications for that sensor in this catalog.

Note: Some nominal specifications can be reduced or eliminated in specific user applications. For example, if a pressure sensor is used within half of its specified temperature range, the temperature error can be reduced by half. If automatic zeroing techniques are used, zero offset and zero drift errors can be eliminated.

Zero offset is the sensor output when the same pressure is applied to both sides of the diaphragm.

Range It is the algebraic difference between the output endpoints. Usually the two endpoints are zero and full scale.

Zero point temperature drift It is the zero point change caused by temperature change. Zero point drift is not a predictable error because each device can drift up or down, and temperature change will cause the entire output curve to shift up or down along the voltage axis. (Figure 1).

Figure 1 Zero offset error

Sensitivity temperature drift is the change in sensitivity caused by temperature changes, which will cause the slope of the sensor output curve to change (Figure 2).

Figure 2 Sensitivity offset error

Linearity error is the deviation of the sensor output curve from a calibrated straight line over the desired pressure range. One method of calculating linearity error is the least squares method, which mathematically provides the best fit straight line (B. FSL) for the data points (Figure 3).

Figure 3 Best fit straight line linearity

Another method is terminal base point linearity (TBL) or endpoint linearity. TBL is determined by drawing a straight line (L1) between the two end data points on the output curve. Then a perpendicular line is drawn from line L1 to the output curve, and the intersecting data points are selected to achieve the maximum length of the perpendicular line. The length of the perpendicular line represents the terminal base point linearity error (Figure 4).

Figure 4 End point linearity

Repeatability error is the deviation in the output reading for any given input pressure when all other conditions remain constant (Figure 5).

Figure 5

Hysteresis error Usually expressed as a combination of mechanical hysteresis and temperature hysteresis. (Figure 6).

Figure 6

Mechanical hysteresis : refers to the error of the output at a given input pressure (in different processes of rising and falling).

Temperature hysteresis is the deviation in output at a given input pressure before and after a temperature cycle.

Ratio Variation Ratio variation refers to the ratio of the sensor output to the supply voltage when other conditions remain constant. Ratio variation error is the change in this ratio, usually expressed as a percentage of the range.

Calculation accuracy or total error

When selecting a pressure sensor, the total error effect is important. The following method takes into account the individual errors as well as the interchangeability errors between components.

Two methods for calculating the total error are:

Root mean square sum of squares (RSS) and maximum and worst case errors. In practice, the RSS method gives the most accurate and realistic values, while in the case of the worst case error method, the probability that all errors of a pressure sensor are in the maximum case is very small.

example:

An application requires 0-15psig, 5°C to 50°C temperature range and 7VDC power supply, 142PCI5G

Step 1: Determine different error rates

parameter

Zero offset

Range error

Linearity

Zero/full scale drift synthesis

Error (calculating maximum and minimum application temperatures, use the larger of the two numbers) = 0.5%

Repeatability & hysteresis: 0.3%

1-year stability: 1.0%

Ratio error: 1.0%

Step 2 Calculate the total error

RSS method: Take the square root of the sum of the squares of the above determinations and errors

RSS maximum error maximum range.

Worst case error = 2.3%.

The worst case error is the sum of all maximum errors = 1.0+1.0+0.75+0.7+0.3+1.0+1.0.

Worst case error = 5.57% absolute maximum range.