Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 116

Service cap

"O" ring cap seal

Depth level

Port, molded into
side wall of
body tube

Heavy clear plastic
cover plate

Vent screw

Bourdon vacuum
gauge

Backup washer

"O" ring stem seal

Plastic body tube

High air-entry
ceramic cup

Figure 4.7 Conventional tensiometer manufactured by Soilmoisture Equipment Corporation.

be exposed to the air and evaporation can be allowed for the
buildup of tension in the water in the tensiometer.
The response time of a tensiometer can be checked by
allowing the development of a negative water pressure of
approximately 80 kPa. The negative pressure is developed
by allowing evaporation from the ceramic cup. The ceramic
cup is then immersed in water. The negative water pres-
sure in the tensiometer should increase toward atmospheric
pressure within 5min after immersion of the ceramic tip in
water. A sluggish response may indicate a plugged porous
cup, the presence of entrapped air in the system, or a faulty
gauge that requires rezeroing.
The ceramic cup must be kept immersed in water prior to
its installation in order to avoid desaturation due to evapo-
ration from the cup. The prepared tensiometer can then be
installed in a predrilled hole in the field or in a soil spec-
imen (e.g., a compacted soil specimen) in the laboratory.
It is important to ensure good contact between the ceramic
cup and the soil in order to establish continuity between the
pore-water in the soil and the water in the tensiometer tube.

4.2.5.2 Jet-Fill Tensiometers
A jet-fill-type tensiometer is shown in Fig. 4.8 and is an
improvement on the regular tensiometer. A water reservoir
is provided at the top of the tensiometer tube for the purpose
of removing air bubbles. The jet-fill mechanism is similar to
the action of a vacuum pump. The accumulated air bubbles
are removed by pressing the button at the top to activate the
jet-fill action. The jet-fill action causes water to be injected
from the water reservoir to the tube of the tensiometer, and
air bubbles move upward to the reservoir. The jet-fill action
also leaves the tensiometer reading at zero, and therefore,
time must be allowed for the tensiometer to return to a value
indicative of the suction in the soil.

4.2.5.3 Small-Tip Tensiometer
A small-tip tensiometer with flexible coaxial tubing is shown
in Fig. 4.9. The tensiometer is prepared for installation using
a similar procedure to that described for the regular ten-
siometer tube. A vacuum pump can be initially used to