doc: Describe what sunlight does to MicroPeak and how to protect it

Show flight data from micropeak in sunlight, a picture of foam
installed on micropeak and the resulting data.

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/doc/micropeak-flight-foam.png b/doc/micropeak-flight-foam.png
new file mode 100644 (file)
index 0000000..c7d37e3
Binary files /dev/null and b/doc/micropeak-flight-foam.png differ

diff --git a/doc/micropeak-flight-nofoam.png b/doc/micropeak-flight-nofoam.png
new file mode 100644 (file)
index 0000000..ecf83f7
Binary files /dev/null and b/doc/micropeak-flight-nofoam.png differ

diff --git a/doc/micropeak-foam.jpg b/doc/micropeak-foam.jpg
new file mode 100644 (file)
index 0000000..73a4040
Binary files /dev/null and b/doc/micropeak-foam.jpg differ

diff --git a/doc/micropeak.txt b/doc/micropeak.txt
index b6dd0ddb49c84003a6a51cc5e7b26971ae9abdca..f1feeec41423a557948c19d4e9c14c9bf451bb82 100644 (file)
--- a/doc/micropeak.txt
+++ b/doc/micropeak.txt
@@ -299,6 +299,43 @@ Keith Packard <keithp@keithp.com>; Bdale Garbee <bdale@gag.com>
                        these applications, change in one application
                        will affect the other.
 
+== Protecting MicroPeak from Sunlight
+
+       The MS5607 barometric sensor is sensitive to direct light. When light
+       shines through the holes in the cover to the components inside, the
+       reported pressure can vary wildly from the actual pressure. This
+       causes the reported altitude to have errors of as much as 3000-4000
+       feet.
+
+       MicroPeak should be installed in an opaque compartment in the airframe
+       and not subjected to sunlight. Alternatively, a small piece of
+       adhesive-backed open-cell foam can be attached to the device so that
+       it covers the barometric sensor and protects it from direct light.
+
+       Here's what happens when MicroPeak is exposed to sunlight. At apogee,
+       I exposed MicroPeak to varying amounts of sunlight and you can see the
+       wild swings in altitude resulting from that:
+
+       .MicroPeak in Sunlight
+       image::micropeak-flight-nofoam.png[width=430]
+
+       You can carefully cutting a piece of adhesive-backed open-cell foam
+       and attach it to MicroPeak. It's important to press the adhesive to
+       the circuit board and not to the top of the barometric sensor or the
+       sensor may become blocked and not operate at all. Once you've attached
+       the foam, you should test MicroPeak on the ground to make sure it's
+       still working.
+
+       .MicroPeak with Foam
+       image::micropeak-foam.jpg[width=430]
+
+       That MicroPeak was in the same barometric chamber as the one which
+       generated the above results and the resulting flight data looks
+       correct:
+
+       .MicroPeak in Sunlight with Foam
+       image::micropeak-flight-foam.png[width=430]
+
 [appendix]
 == Handling Precautions