MAINTENANCE AND SERVICE OF A TENSIOMETER

In a moist and wet soil, the tension readings are low; air will not accumulate under the plug of the reservoir cup. If the soil is relatively dry and the readings reach 40–60 cbars, air can rapidly accumulate in few days. Air bubbles increase the response time to changes in tension, thus lowering the precision of the instruments.

After installation, the instrument must be revised frequently and daily. Excess air is removed and then water is added to the reservoir cup. With periodic service and maintenance, less air will enter into the system. Then the maintenance can be done once a month. If the tensiometer is not serviced and maintained for a long period of time, then the water level in the reservoir is lowered significantly. It must be filled up again with water. Air bubbles must also be removed with a hand vacuum pump. Under saline conditions, maintenance is essential to keep the ceramic tip free of salts [2, 3].

Vacuum Hand Pump

This is an instrument to remove air from the tensiometer [9]. It has a universal suction tip that adjusts itself to all models of tensiometer. The cap of the reservoir compart-ment is removed and the suction to the instrucompart-ment is applied with the pump. Four or five strokes should produce a reading no greater than 80 cbars. The pump should stay attached to the top of a tensiometer without holding it. If the manual vacuum pump is not available, one can use a small flexible polyethylene tube (spaghetti) to remove the air bubbles from the tensiometer (See Figure 3.7).

Figure 3.7. Vacuum hand pump.

Figure 3.8. Suction method to test the operation of a tensiometer.

Suction Method

This method consists of placing the tensiometer inside a “Matraz Erlenmeyer flask”

through a rubber plug (see Figure 3.8). A known amount of suction is applied around the ceramic tip by opening a stopcock. The vacuum gage will show a reading corre-sponding to the applied suction. Then the suction valve is closed. If the reading of the instrument remains unchanged, then it implies that there is no loss of suction from the seal of the ceramic tip and the gage. If the reading changes, then there is a loss of suc-tion somewhere in the instrument. To find this spot, we add water through the reservoir cup and apply the suction again. The instrument is allowed to flood and the top of the reservoir is covered. If the reading remains unchanged, then there is no loss. If it is changed, then there is an escape and the spot is easy to locate. This method reduces the testing time to approximately 10 minutes.

STORAGE

After crop season is over, the tensiometer must be removed, emptied and stored. For removing the tensiometer from the soil, rotate to loosen it from the soil. Then grasp the main tube and pull it upwards. The ceramic tip should never be allowed to dry up.

Immediately after removing it from the soil, cover the ceramic tip with a wet towel to keep it wet, while it is being packed. Even a few minutes of exposure to the air can permanently seal the pores. Wet packing is recommended for temporary packing. Fill up and cover the tensiometer. Clean the external ceramic tip and submerge it in water.

This method keeps the instrument in operating status and ready for immediate instal-lation. Dry packing is only recommended for long periods of storage. Carefully, clean and wash the ceramic tip with a brush. Clean the surface of the plastic with a soap solution. Place the instrument in an inverted position. Hang it in a clean and dry place free of dust. Cover the ceramic tip with a transparent plastic bag.

LIMITATIONS

The tensiometer is filled with water and is limited by the physical properties of water.

The gage indicates the difference in suction inside and outside the plastic tube. This suction depends on the atmospheric pressure. The operational limit of the tensiometer is 80 cbars at sea level. For each 300 meters of height above sea level, pressure lowers by 3.5 cbars. A reading of 80 cbars in a tensiometer at sea level equals 62.5 cbars at 1500 meters of elevation:

[(3.5/300 × 1500 = 17.5), and (–17.5 + 80 = 62.5)].

Tensiometer with a common vacuum gage does not respond quickly. A good preci-sion is possible for changes in tenpreci-sion with tensiometer equipped with electric digital sensors.

SUMMARY

A tensiometer is an instrument to measure the tension with which the water is adhered to the soil particles. The tensiometer shows the changes and fluctuations that occur on the water film surrounding the soil particles. Readings of the tensiometer can be

related to the water available to plants, but can not be used to determine the amount of soil moisture. The parts of the tensiometer are plastic tube, porous ceramic tip, reser-voir and vacuum gage. This chapter discusses the procedure to prepare and calibrate a tensiometer; to select and locate the site for installation of a tensiometer; to install a tensiometer; to provide adequate service and maintenance; and to store the tensiometer for short and long terms.