The cells are kept in an aluminium canister with silica gel in the walls to keep the humidity low, and stored in a refrigerator when not in use. The cell must be accurately weighed before placing in an environment for monitoring. A precision of 10-5 g is required, with an accuracy of 10-4 g. The aluminimum canisters must be discarded when they increase in mass by 2g.
The cell is then immersed in fresh water in a small bottle lined with PVC, and sealed and placed in the environment to be monitored and left for a period of time. If the integrated mean annual temperature is required, the cell is left for 365 days. In the case of the experimental kumara gardens, we wished to know the integrated mean temperature over the growing season, so they were placed in the ground when the tubers were planted, and removed during the harvest.
After removal from the ground, the cell is removed from the bottle of water and dried with a cloth and placed in the special aluminium caniser for a week at room temperature so the cell wall can equilibrate to dryness.
Next the cells are re-weighed to the same precision as with the first weighing. We noted that there is a significant rise in weight over a period of about 30 minutes in the special weighing room. This is due to the cell membrane taking up moisture from the air in the laboratory when outside the aluminium canister. It is obviously preferable to have the thermal cell out of the canister for the minimum time during weighings.
The integrated mean temperature in degrees C can be calculated by:
where B = 11.364 and M = -5047.9
These constants relate to the specific batch of cells used for the
experimental gardens and marine envionments being monitored.
Q = the net weight gain in g over the exposure period
t = the exposure period in days
For example:
The initial weight of a cell used at one of the experimental gardens
weighed 11.64976 g before burial, and 12.15820 g after removal.
This represents a weight gain of 0.50844 g over the 173 days of burial.
The integrated mean temperature is therefore 20.44 degrees C.