Robust, simple temperature loggers
Note: One problem with this configuration was that the PCV pipe heaved and jacked the thermistors out of the ground. In the future it might be better to make a small soil pit and drill in the bottom to install the thermistors directly into the ground.
In the Permafrost Lab we often have the need to measure just a few ground temperatures and nothing else. The most affordable and reliable loggers we have been able to find for this purpose are the HOBO loggers from Onset, specifically the U12. While they do make an outdoor version of the U12 (U12-008) it is far from animal resistant and then their is the complication of what to do will all the extra cable. Our solution has been to use the indoor version of the U12 (U12-006) in an enclosure adapted for how it will be installed. This is an example (parts list) of what we have used when we want to measure a few shallow temperatures, down to 1.5m or so.
We have been using a polycarbonate enclosure that is waterproof, impact resistant, and easily modified. It comes in many different sizes but 6"x6" seems to be big enough to accommodation any extra wire and a battery pack. To this we add fitting to allow us to attach a short length of liquid-tight metallic conduit and a built up right angle adapter to hang the thermistors in a surface borehole cased with 1/2" PVC. Here are the basic steps with photos:
- Drill a hole in the enclosure for the liquid-tight fitting. Install the fitting and use some silicon around the outside to make sure you have a water tight seal.
- To assemble the right angle transition piece you will need a 1/2" right angle conduit body. Into this you will glue a short piece of 1/2" PVC and then to that the 1/2" female thread adapter. Into the 1/2" thread adapter you will screw the 1/2" liquid-tight fitting.
- On the inside of the right angle body a small hole is drilled for a screw and fender washer. These are used to hold the position of the thermistors in the borehole, like a clamp.
- The datalogger and battery pack can be mounted to the inside of the enclosure using some double sided mounting tape. The photo shows the logger mounted to the bottom of the enclosure but we have since found out that mounting it to the lid is much more convenient for accessing the logger to download data.
The photos pretty much tell the story of how to put this together, it is simple but robust and easy to install. In the parts list there is some information about the drill setup we have been using to drill these surface, 1.5m, boreholes. Basically it consists of a large carbide tipped ship auger type drill bit, extension, and a 18V hammer drill. This seftup is lightweight and can drill to 1.5m in a few minutes in most soils.
Here are also a series of photos for a larger box we have put together for measuring some deeper, 10m, boreholes. It is an 8"x8" box and has two of the U12 loggers. It also has a hole and adapter on the bottom so it can be mounted on a 1" PVC pipe, which our boreholes happen to be cased with.