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.
 
                        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
 
 [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
        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
 
        As with all other rocketry electronics, Altus Metrum
        altimeters must be protected from exposure to corrosive motor