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

[fw/altos] / doc / micropeak.txt

diff --git a/doc/micropeak.txt b/doc/micropeak.txt
index 3621a2dcc17cffe23a7bb8a9999fafb1ea3c2d73..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
 
@@ -327,10 +364,16 @@ Keith Packard <keithp@keithp.com>; Bdale Garbee <bdale@gag.com>
        sensor out of direct sunlight.
 
        The barometric sensor sampling ports must be able to
-       "breathe", both by not being covered by foam or tape or other
-       materials that might directly block the hole on the top of the
-       sensor, and also by having a suitable static vent to outside
-       air.
+       "breathe", both by not being covered by solid foam or tape or 
+       other materials that might directly block the hole on the top 
+       of the sensor, and also by having a suitable static vent to 
+       outside air.
+
+       One good solution is to use a small rectangle of Poron 
+       50-30031-12X12P or equivalent to cover the sensor.  This is an
+       open cell foam in 1/32" thickness with an adhesive backing.  It
+       seems to do a good job of blocking sun while still allowing
+       airflow to and from the sensor internals.
 
        As with all other rocketry electronics, Altus Metrum
        altimeters must be protected from exposure to corrosive motor