Refrigerator monitor ideas?

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

We've got a 30 year old White-Westinghouse refrigerator/freezer in the
basement that's cosmetically shot, but still humming away, delivering
subzero temperatures in the icebox without major incident for years.

Several times in the past 10 years or so, the drain tube from the pan under
the freezer coils (it's an upright with a top freezer) has clogged, leading
the coils to become caked in ice.  When this happens, the first clue is that
the freezer temperatures begin to rise.  Other than placing a sight glass in
the freezer floor pan, what are some simple options to detect a coil
freeze-up condition.  I had previously had this on a yearly inspection
schedule but the addition of a number of shedding dogs to the house in
recent months has made that too infrequent a cycle.

What I would really like to do is design something simple that can catch the
formation of serious ice long before the coil became caked.  Simple
temperature sensing is inadequate because the self-defrosting feature causes
a sharp upward swing every night.

The conditions that occur during a "freeze-up" (in addition to an overall
increase of average freezer temperature each day it's building up) are that
very little water reaches the evaporation pan at the bottom of the fridge.
But checking that pan is about as inconvenient as unscrewing the drain tube
connector that runs from the center top of the refrigerator to the backwall
where it meets a drain tube that empties into the floor pan for evaporation.

One thought that had occurred was to modify the drain tube to include an
aspirin sensor between two spring loaded contacts at the top portion of the
tube.  But I am not sure that would work because the typical mode of failure
is an ice dam that occurs at the drain hole.  I would suspect that before
that happens, the entire drain tube in filled with water, but I can't say
for sure.

Probably the most convenient solution would be to stick a $12 CMOS board cam
in a baggie with some white LEDs to use as a video inspection port for the
evaporator tray on the floor.  I'd only have to power it up during
inspection times, so it wouldn't require a device that consumed power 24/7.
Any monitoring of the temperature changes will likely involve a lot more
power consumption than a video inspection "port" would.  On the other hand,
the video evaporator inspection method probably won't tell me the coils have
frozen until it's too late.

What I really want to know is when 5 on the freezer control no longer means
an average of X temperature in the freezer compartment.

--
Bobby G.