Tensiometer

Theory

Use of tensiometer

construction of tensiometer

Theory of tensiometer

The tensiometer measures the soil water potential in the soil A description of soil water potential and its relationship to soil water content is present here

Construction of tensiometer

Reference to a company trade name is made for identification purposes only and does not imply approval or recommendation. Other equivalent parts can be used to build a tensiometer.

To construct a tensiometer use 1/2 inch schedule 80 pvc pipe and tensiometer cups from Soil Moisture Equipment Corporation in Santa Barbara. The size of the cups are 0655X 01 B1M1.

Clear acrylic tubing from Baker plastics in El Paso Texas is used on the top of the tensiometer and is part 44023 5/8 od and 1/2 id. The septum at the top of the tensiometer in the clear plastic is 3/8-1/2 inches in size and is purchased from pharmacy

Cut the pvc tubing 6 inches longer than the depth that you desire to measure the soil moisture tension. Drill the bottom of the pvc tubing with a 9/16inch drill at least 3/4 inches. Smooth the entrance of the pipe using a 1/2 grinding stone. Place a light coat of epoxy glue on the tensiometer cup end and let dry. The ceramic cup absorbs the glue. Put a second coat on the tensiometer cup and glue into the bottom of the pvc pipe.

On the top end drill a hole 1/2inch deep with a 5/inch drill and cut a 4 inch piece of acrylic tubing and epoxy the tubing into the top of the pcv.

Fill with water and install the rubber septum in the top. Test the tensiometer by setting up a air source set at 800 cm of water pressure and connected to a needle (no 25G 5/8 ). Put the tensiometer in water and apply the pressure. Look for air bubbles.

A pressure transducer can be attached to the tensiometer and the transducer connected to a data logger to record the data automatically. One type of transducer is a Microswitch (a Honeywell division)136pc 15g 0-15 sold by Lcomp 206 W. Julie Unit 2 Tempe Az 85293 tel 1-800- 325-1393

Use of tensiometer

Fill the tensiometer with water to the top and put the tensiometer in a container of water and let it sit for 24 hr.

Install the tensiometer in the soil by creating a hole using a 1/2 inch steel pipe to the depth you wish to put the tensiometer Push the tensiometer down the hole until it reaches the end and makes a tight fit. Fill the tensiometer with distilled water up to 1/2 inch below the rubber stopper.

Read the tensiometer using a pressure transducer called a digital manometer model MPL-3200-Dm that reads in cm of H20. Set the zero scale on the manometer before using.The pressure transducer is built by Micro Pneumatic Logic, Inc 2890 NW 62 St Fort Lauderdale, Fl 33309

Buy a no 25G 5/8 needle at a pharmacy to install on the end of the transducer. Push needle into septum on the top of the tensiometer and read the soil water potential.