SDP500 datasheet(6/10 Pages) ETC2

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5. Conversion to Physical Values

5.1

Signal scaling and physical unit

The calibrated signal read from the sensor is a signed

INTEGER number (two's complement number). The

INTEGER value can be converted to the physical value by

dividing it by the scale factor (pressure = sensor output



scale factor). The scale factor is specified in Section 2.

5.2

Temperature compensation

The SDP600 sensor series features digital temperature

compensation. The temperature is measured on the

CMOSens® chip by an on-chip temperature sensor. This

data is fed to a compensation circuit that is also integrated

on the CMOSens® sensor chip. No external temperature

compensation is necessary.

5.3

Mass flow temperature compensation

A sensor output proportional to mass flow is necessary for

measuring mass flow in a bypass configuration. Even

though the output of the SDP sensors with mass flow

temperature compensation is still differential pressure, the

temperature compensation is adapted especially for mass

flow measurements in a bypass configuration. At

calibration temperature both calibrations are equivalent.

Please find the application note “Bypass Configuration

Differential Pressure Sensor SDPxxx” on our website.

5.4

Altitude correction

The SDP600 sensor series achieves its unsurpassed

performance by using a dynamic measurement principle.

The applied differential pressure forces a small flow of gas

through the sensor, which is measured by the flow sensor

element. As a result, any variation in gas density affects

the sensor reading. While temperature effects are

compensated internally, variations in atmospheric

pressure (elevation above sea level) can be compensated

by a correction factor according to the following formula:

DPeff = DPsensor  (Pcal / Pamb)

DPeff:

Effective differential pressure

DPsensor: Differential pressure indicated by the sensor

Pcal:

Absolute pressure at calibration (966 mbar)

Pamb:

Actual ambient absolute pressure.

Altitude correction factors:

Altitude

[meters]

Ambient pressure (Pamb)

[mbar]

Correction factor

(Pcal / Pamb)

1013

0.95

250

984

0.98

425

966

1.00

500

958

1.01

750

925

1.04

1500

842

1.15

2250

766

1.26

3000

697

1.38

Example: At 750 m above sea level and a sensor reading

of 40 Pa, the effective differential pressure is 41.8 Pa.

Note: In many HVAC applications such as air flow

measurement in a bypass configuration, the described

dependence on absolute pressure is actually welcome

because the quantity that must effectively be controlled is

the mass flow and not the volume flow. Mass flow is

dependent on differential pressure and absolute pressure.

For details please refer to our application note “Measuring

Flow in a Bypass Configuration”.

6. OEM Options

A variety of custom options can potentially be

implemented for high-volume OEM applications. Contact

Sensirion for more information.

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