doc: Update for 1.9.1

[fw/altos] / doc / micropeak.txt

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--- a/doc/micropeak.txt
+++ b/doc/micropeak.txt
@@ -1,8 +1,8 @@
 = MicroPeak Owner's Manual
 Keith Packard <keithp@keithp.com>; Bdale Garbee <bdale@gag.com>
-:revnumber: v1.9
-:revdate: 8 Oct 2018
-:copyright: Bdale Garbee and Keith Packard 2018
+:revnumber: v1.9.1
+:revdate: 25 Sep 2019
+:copyright: Bdale Garbee and Keith Packard 2019
 :stylesheet: am.css
 :linkcss:
 :toc:
@@ -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 thousands of
+       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