Tag Archives: ms5611

Feb 16

Air Pressure Sensor Options

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The application this article concentrates on is how to decide slight variations in altitude. It becomes plain that the sensor should be very attuned to the smallest pressure changes, and the pressure reading should be absolute, or compared against a sustained value. If this value isn’t constant, the pressure changes would not be detected, as the reference pressure (atmospheric pressure, as an example) might change more than the pressure being measured.

Two popular absolute air pressure sensors being utilized by electronics enthusiasts for their projects, such as radio controlled copters, GPS devices, etc, are the MS5611 made by MEAS Switzerland, and the BMP085, manufactured by Bosch Sensortech. The BMP085 offers a resolution of roughly 0.03 hPa, which corresponds to a change of approximately twenty-five cm in altitude, at an effective range of three hundred hPa to 1100 hPa. The MS5611 optimizes for a resolution of roughly 10 cm change in altitude, with a range of 400 hPa to 1150 hPa. The ranges correspond to altitude changes from sea level to about 30,000 feet. These sensors provide one or two methods of operation where there’s a tradeoff between conversion rate (about 1-8ms), energy consumption, and precision.

Both of these ICs use the piezo-resistive sensor technology, which can supply high accuracy and linearity, together with low hysteresis, low noise, and high equilibrium of both the pressure and temperature signals. The piezo-resistive sensors are typically built in a strain-gage transducer configuration connected as a Wheatstone bridge. This bridge structure contains a pressure sensitive surface which is formed by single crystal silicon. This formation is founded upon the Advanced Penetrable Silicon Membrane (APSM) process. This process hermetically seals a reference vacuum, which is employed as the constant pressure value for absolute pressure readings. Any slight change in the curvature of the membrane can then be converted to an electric signal.

Each pressure sensor package is internally calibrated at the factory for 2 temperatures and two pressures. These 2 readings are normally the zero scale value and the full scale value. This calibration process also figures out a few coefficients needed to provide compensation for process variations and temperature differences, and stores them in read-only memory. These values are employed by a programme written for the main microcomputer chip to work out correct values for temperature and pressure as the binary values for this information is extracted from the sensor. The calculation for temperature is comparatively easy, as this value’s found simply by multiplying and then adding coefficients to the binary difference between the actual value from the sensor and reference temperature. But the calculation for pressure becomes awfully complicated, because the pressure sensitivity and the pressure offset parameters are both impacted by temperature. The user shouldn’t have to stress about this, though, as the sensor manufacturer provides working code for the calibration routines.

In several altitude measurement applications, there’s a lot of noise in the atmosphere, so it would be smart to place the sensor in a sealed, insulated chamber so it can generate a steady signal as the altitude is changed. The pressure sensors are also attuned to acceleration, so if they’re employed in applications like model rocketry or R/C aeroplanes, the sensor should be mounted ninety degrees to the g-axis, or two sensors can be mounted ninety degrees apart. An algorithm for taking averages would then be used to calculate temperature and pressure readings.

Embedded Adventures is a fantastic web site for new and interesting electronic modules such as the MS5611 air pressure sensor module and the handy real time clock module, together with alphanumeric displays such as sixteen and 14 segment displays.

Feb 05

Intro To Air Pressure Sensors

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The technology of the air pressure sensor has advanced to the stage now where it can be simply included in pocket-sized electronic widgets, such as mobile phones, GPS devices, bicycle speedometers, and now even wrist-watches. Some of the more recent ones come in miniscule IC packages (even less than 5mm * 5mm), and have extraordinarily low energy consumption (less than 3uA). They offer extreme precision and enough resolution to detect a miniscule change in altitude (10 to 25cms in altitude). They can be attached to an existing circuit via a straightforward serial communication interface, eg I2C or SPI.

The analogue outputs of the older sensors have been replaced by digital outputs, so the analogue to digital conversion occurs right within the chip. This minimises the length of the analogue signal lines and reduces interference for way higher accuracy. They use a sensor that is susceptible to both pressure changes and temperature changes, so temperature readings can be added as a bonus.

To understand how pressure measurements are made, it might help to understand the difference between relative pressure and absolute pressure. Relative pressure sensors are used for most applications, such as tire pressure and blood pressure readings. In these sensors, the pressure being measured is generally referenced relative to ambient air pressure. In some economic applications, this reference price could be a variable pressure of another fluid or gas. The reading from a relative pressure measurement is always the difference between the two pressures.

Some applications require an absolute pressure reading, for example barometric pressures, atmospheric pressures, altimeter pressures, and deep vacuum pressures. According to the definition of absolute pressure, the pressure being measured is referenced against an ideal vacuum. But in real life applications, this reference might be any sustained value of pressure, such as average atmospheric pressure at sea level.

Pressure sensors enable you to predict weather – changes in air pressure often accompany localised rainfall for instance, or decent weather. They can also help with altitude measurements (this is how altitude is measured on an airplane for instance). As these sensors become smaller, they can add weather prediction or altitude measurements to applications where this was not possible in the past.

Embedded Journeys is a great web site for finding fun electronic modules such as the MS5611 air pressure sensor module or useful things like a real time clock module.