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Encoders such as the ACK A-30 use uncompensated silicon pressure sensors (e.g. MPX100 series according to this video) which are kept at a constant temperature. I presume they are heated so as to stay on the same voltage to pressure conversion line (Figure 3 MPX100 documentation). Is this presumption correct? More importantly, however, I was curious how the conversion from pressure to pressure altitude is carried out. I have come across this formula, but I was not able to find any documentation that specifies what conversion calculation/look-up table is actually used. Perhaps this information is contained in AS8003, but that document is behind a paywall.

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Altitude encoders are relatively simple devices. They are designed to be as inexpensive and reliable as possible. They are so cheap that if it fails, buying a new one is cheaper than a repair (and you get a new warranty.)

But on to your question of design. It's basically a sensor and linear data flow from the transducer to the output. And most of the basic ones used Gillham output coding on nine to 11 parallel discrete lines.

The static line input is connected to the pressure transducer. We bonded the back of it to a voltage regulator and covered it with a foam insulating block. We referred to this as the oven. Heat from the voltage regulator heated it to about 130 deg F. There was a thermistor controlling the voltage regulator to maintain a constant temperature. Doing this avoids having to compensate the transducer output to a variable temperature. You heat it above ambient as it's easier to maintain a constant temperature as the external temperature varies.

The analog voltage output of the transducer is then fed into an A/D converter. The digital output then goes to a custom ASIC that is essentially a lookup table. During calibration the table is populated with the Gillham code for the voltage range associated with the altitude range. Avionics shops have a test set that can set the pressure output line to a specific altitude calibrated to the ICAO standard atmosphere (basically, the reverse of the encoder.) And to answer your other part of the question, the standard atmosphere is defined using a table.

For the parallel output the Gillham code is put through a serial-to-parallel converter to set the output discretes. Some more expensive units would provide the serial output in ARINC 429 or RS232 formats.

To be clear, you don't really care what the voltage output of the transducer is at a specific pressure altitude as long as it is relatively linear across the range. You provide a calibrated altitude source to the unit and read the voltage output and then plot it against altitude. You then build the lookup table setting the Gillham code for the range of voltages. An example would be the voltage range output from a pressure altitude input of 3950 feet to 4050 feet will be set to the Gillham code for 4000 feet [ 0 0 1 1 1 1 0 1 0 for the A1 A2 A4 B1 B2 B4 C1 C2 C4 lines.]

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Have a look at the ICAO standard atmosphere (Doc 7488). It contains formulas and tables for conversion between altitude and pressure and vice versa, but only for the standard atmosphere (1013.25 hPa /29.92 inHg). For QNH correction there is a separate formula, which is documented in AS8003 and other sources (I can't find a good public source).

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