We have a high efficiency Rheem EverClean gas hot water heater (early
model, around 8 - 10 years old) that has developed a perplexing problem:
from time to time, the pilot light will go out, and I have trouble
relighting it.

Two potential causes come to mind:
1: the gas orifice is dirty;
2: the sensor bulb next to the pilot flame isn't getting hot or
actuating for some reason.

Is either of these likely to be the problem? What can I do to fix it or
diagnose the cause? If I need to clean the orifice, can anyone tell me
how to find out what size wire I need to use? If the sensor has gone
bad, should I think about replacing it myself (if I can find a new one)
or do I need to get a qualified HVAC geek do the work?

Related question: does the sensor bulb next to the pilot flame use a
thermocouple to electrically sense the flame, or is it purely
mechanical? There are no apparent electrical wires to the pilot and
sensor, and there appear to be two small copper gas lines feeding the
pilot and the sensor bulb. The sensor bulb looks as though it might have
a hole in its outer sheath. Must this bulb remain sealed?

BTW, this model seems to have an unrelated odd design defect. If I run
it out of hot water on a humid day, then when the tank refills with cold
water it causes water to condense on the tank. This water drips down and
extinguishes the pilot light. They should have put some sort of splatter
shield around the pilot flame to prevent this from happening.

Thanks.

Bill Christens-Barry