20 Jan 98

ADDENDUM -- Kitplanes Altitude Encoder Article April '98

When I first submitted this article, Kitplanes editor Dave Martin told me that the information was great, but that there just wasn't room in the magazine for an article with four extra pages of tables and two extra pages of drawings. His suggestion, and I agree, is that detail work like this belongs on a website for download if you intend to actually manufacture the device described in the article.

We therefore give you the following information for your constructing pleasure:

1. TABLE 1

This table lists altitudes from -200 MSL to 2000 MSL. Note how the C1, C2, and C4 values repeat their pattern every 1000', and how they do this repetition starting at 300'. Another way of saying this is that the C-values are the same for 300', 1300', 2300', 3300', and so on up through 32,300. The same thing holds true for 400', 1400', and so on, as well as 500' & 1500', 600' & 1600', and so on.

Note also the strange "mirroring" effect at the 800' values -- the C values stay the same from 700' to 800' but that the A or B values change at that point to give a unique code. The C-values then count BACKWARDS through 1200'.

If you think of how an optical shaft encoder (like in an encoding altimeter) has to work with all those little holes and LEDs, then you can see that making the optics is a lot easier doing it this way, but that decoding the altitude with electronic logic is a royal pain in the tush.

 2. TABLE 2

Since the C-values repeat themselves every 1000' starting at the 300' point and going through the 1200' point, doing a table for every 100' from Sea Level to 32,000' would require a table 320 lines long. That is just a little much when the pattern is so easy to get from Table 1 and Table 2. Table 2 shows how the A and B values change at the 300' and 800' points. To get the intermediate C-values simply refer to the 100' increments shown in Table 1 above.

For example, use Nevada County Airpark at 3104 MSL and presume that the pressure altitude and field elevation are equal at 29.92" Hg. From the table below, we see that the A and B values are 001 100 (from the line "2800 to 3200"). We can also see from the table above that the C values are going to come from the "100" foot line, since 104 is between 50 and 150. The C values for this line are 110.

Therefore, the 9-line code we expect from our encoder would read 001 100 110 (reading from A values on the left to C values on the right).

Drawing 1 -- It shows in "schematic" form how to make the vacuum source described in the article.

Drawing 2 -- It shows a schematic diagram of a test box for altitude encoders.