What kind of air pump could cycle air from a tube located in the ground to surface for a long period of time?

Question

I wish to constantly pump air from an approximately 1/4" tube in the ground to sealed container on surface. I wish to do this so I can determine the relative humidity below ground surface. The relative humidity measuring device is to large to put below ground so I need to constantly pump the air to surface. I have looked at a peristaltic pump and an air mattress pump. Peristaltic pump would work but I wanted something that I wouldn't have to order online and the air mattress pump seems like it would burn out. Any suggestions as to an air pump I can get that is relatively cheap, readily available and will not burn out if it runs constantly.

Answer 1

Simple answer is a diaphram sampling pump which is positive displacement.   Unfortunately you will most likely have to mail order something.   Air pumps that you find in the store like for a paint sprayer or aquarium are desigen to supply room air under pressure. As a result these will not have a inlet connection.  You could attempt to enclose an air pump in a sealed box which has a penetration for the compressed air and power cord.  A third connection to the box would then supply a pipe fitting connection to the tubing. The air pump would such air from inside the box which would then be drawn throught the suction tube.

Relative humidity is also based on the temperature at the sampled location. If you are using a chilled mirror you should have the ability to use an external temp probe.  If you are using an RH meter with a capacitive sense element mant of these units have a wand with the RH and temp sensors placed together in the wand.

  Also the sample as its drawn through the line will be subject to cooling especially at night and in winter. This will cause condensation  in the line and dry the same as well as form a sluge of water.  You should place a moisture trap ahead of the test equipment to prevent condensation from being drawn into the unit and damaging it.

To help prevent condensation it is advisable to use a heated and ininsulated sample line.  By heating the line your will keep the sample above its dew point. You may also have to keep the instrument heated as well to prevent condensation from forming within the instrument.    

Answer 2

try a fish-tank (aquarium) pump.

They run all the time making bubbles in an aquarium, so should last awhile.

and they are pretty cheap.

Answer 3

Use an air pump designed to pump air for aquariums. Cheap and readily available at your local pet store.