- Author: Michael D Cahn
- Author: David Chambers
Tensiometers are useful for monitoring soil moisture in vegetable and row crops so that plants are not over-watered nor become water stressed. As the name implies, tensiometers measure soil moisture (water) tension, otherwise referred to as matric water potential. Soil water tension is essentially a measure of the force that plants need to exert to suck water from the soil pores into the roots. A high tension means that the soil is dry and a plant needs more energy to extract water from the soil compared to when the soil is moist. Tensiometers function without batteries and wires, although they can be interfaced to dataloggers by adding a pressure transducer. Also, the readings are not affected by soil texture, temperature, and salinity unlike many electronic soil moisture sensors.
A previous article published in this blog described how to build a tensiometer from PVC parts and ceramic cups that can be purchased from a supplier in California. The design results in a dependable and inexpensive tensiometer that can be built by anyone comfortable using a few hand tools. The most challenging step was gluing the cup to the end of the PVC shaft using epoxy cement which can sometimes result in a vacuum leak if the bond is weak. Since publishing this article, we have built and used more than a hundred of these tensiometers and can suggest some modifications to the original design and a few other words of wisdom. For example, we observed that the PVC “T” can crack if the gauge is threaded too tightly or during cold weather conditions which can cause the plastic to contract, so we suggest using a different type of “T” and not to over thread the gauge. We have also collaborated with a supplier of the ceramic cups, SoilMoisture Equipment Corporation, whose engineers have developed a cup that can be bonded to the PVC shaft using standard PVC cement and primer rather than a specialized epoxy (Fig. 1). This improvement greatly reduces the time required to build the tensiometer which is why we call it the “ten-minute tensiometer.” Once you have some experience, you will probably need less than 10 minutes to assemble one of these tensiometers from the parts shown in Fig 2.
Materials needed:
Ceramic cups
Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525, www.soilmoisture.com)
Part Number Y2630C, Dimensions: 0.875 inch OD x 2.75 inch length
Cost: $25 ea. Note that this is a new part that must be special ordered.
Vacuum gauge
Vender: Zoro.com/Grainger.com
Part Number 4FMK3, Description: ¼ inch MNPT 2 inch diameter test vacuum gauge
Weblink: https://www.zoro.com/zoro-select-vacuum-gauge-test-2-in-4fmk3/i/G0040792/?q=4FMK3 Cost: $18.99 ea.
#1 size rubber stopper
Vender: Grainger.com
Part Number 8DWU6, model RST1-S, Description: 24 mm neck, bottom diam. = 14 mm. Top diam. = 20 mm.
Weblink: https://www.grainger.com/product/GRAINGER-APPROVED-Stopper-8DWU6?internalSearchTerm=Stopper%2C+24mm%2C+Black%2C+PK52&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true&suggestConfigId=8&searchBar=true
Cost: $18.55 / 52 pieces
Schedule 40 PVC pipe (½ inch diameter)
Vender: irrigation supply or hardware store
Sand paper (60 or 80 grid)
Vender: hardware store
½-inch PVC “T”
Vender: irrigation supply or hardware store
Description: ½ inch Female slip x ½ inch Female slip T. Note that this T replaces part number 402-072 from Spears Inc., which we found to sometimes split during cold periods. Also, the previous part was often not in stock.
PVC bushing
Vender: irrigation supply or hardware store. Description: ½-inch PVC Male slip x female ¼” thread bushing.
PVC cement (gray) and purple primer (Fig. 3)
Vender: irrigation supply or hardware store. Note that the gray cement provides a stronger bond than the clear product.
Gas pipe thread sealant (white or blue paste type)
Vender: irrigation supply or hardware store
Painters masking tape
Vender: hardware store
Petroleum Jelly (Vasoline)
Vender: pharmacy. A coating of petroleum jelly improves the seal between the rubber stopper and PVC tube.
Rubber gloves
Vender: hardware store, paint store, etc. Description: Nitrile disposable gloves to protect hands from PVC primer and glue.
Tools needed:
- PVC saw
- Miter box
Procedures
- Cut PVC pipe sections in the following lengths
1 foot depth tensiometer: top shaft = 4 inches*, bottom shaft = 17 inches
2 foot depth tensiometer: top shaft = 4inches, bottom shaft = 30 inches
We advise first cutting the bottom shaft about 1-inch longer than indicated above using the miter box or an electric miter saw to assure that the cut at the lower end is at a 90-degree angle (Fig. 4). The ceramic cup will fit crooked on the end of the shaft if the cut deviates from 90- degrees. After assuring that the cup fits well, the top end of the shaft can be cut to the exact length.
*Note that the top shaft can cut shorter than 4-inches so that when the tensiometer is installed in the field it has a lower profile to the ground, thereby reducing chances of being hit by tractor implements.
- Check that the ceramic cup fits into the bottom of the PVC shaft and is aligned straight. Sand the neck of the cup and/or the interior walls of the PVC shaft if the cup cannot be inserted at least half-way into the PVC. Be careful not to over-sand the cup or the fit between the neck and the PVC shaft will be loose and not bond well when glued. The fit between the neck of the cup and the PVC should be very tight such that a lot of force is needed to insert the cup into the bottom of the PVC shaft. If the cup is not in alignment with the PVC shaft, then recut the end with the miter box at a 90-degree angle and recheck the fit.
- Wrap the bottom of the PVC shaft and the top of the ceramic cup with painter's tape to prevent cement from coating the outside of the ceramic cup (Fig. 5).
- In a well-ventilated location, apply PVC primer to both the interior of the PVC shaft and the outside of the PVC top of the ceramic cup. Then apply gray PVC glue to both surfaces, and push the parts together, and hold in place for about 30 seconds to 1 minute (Figs. 6-9). Tip: slightly twist the parts by about 30-degrees immediately after gluing to assure a good bond before the cement begins to set.
- Next glue the top and bottom shafts and the bushing into the ½ inch, PVC slip “T” using the PVC primer and cement. After inserting each of these parts into the T, slightly twist them and hold in place for about 30 seconds while the cement sets (Figs 10-12). Also cover the non-glued areas with painter's tape to prevent the outside from becoming covered with primer and cement.
- Coat the ¼ inch male threads of the gauge with pipe thread sealant and hand screw on the vacuum gauge. Tip: do not over tighten or the PVC “T” may crack.
- Fill the tensiometer fully with degassed distilled water. The water can be degassed by boiling it and allowing it to cool.
- Coat the lower end of the rubber stopper with a thin film of petroleum jelly and insert into the top end of the tensiometer with a light twist to firmly seat the stopper (A loose stopper is the main cause for vacuum leaks).
Conclusion
So that is it—the tensiometer is ready for testing and installation (Fig. 13). Hopefully, it did not take too long to build and with practice one should be able to assemble these tensiometers in 10 minutes or less. Please visit our previous blog article on how to test and install tensiometers in the field. Please let us know if you have any questions or feed back.
- Author: Michael D Cahn
- Author: David Chambers
A tensiometer is a very useful tool for monitoring soil moisture status of vegetable and berry crops. Compared to other sensors that often require equipment such as dataloggers or a computer to collect readings, tensiometers can be easily read by irrigators in the field. Also, tensiometer readings are not affected by variations in soil texture, temperature, and salinity and they can operate without electricity (no batteries needed).
What is tension? Tensiometers measure soil moisture in units of negative pressure also known as tension. Tension is a measure of the force that plant roots need to exert to pull water from the soil pores. Large pores hold water with less force than small pores. As plants extract moisture from the soil, water is first taken up from the largest pores. As the soil dries roots need to exert more force to pull water from the smaller pores. Hence, high tension values mean that the soil is becoming dry.
How do tensiometers work? Tensiometers are filled with water (preferably distilled) that has been degassed by boiling. A key component of the tensiometer is a porous ceramic cup which allows water in the shaft of the tensiometer to freely pass into the soil without air bleeding though the small pores in the cup (Fig. 1). If the soil is not saturated, water will move from inside the cup into the unfilled soil pores. Because air cannot replace the space vacated by the exiting water, a vacuum develops in the shaft of the tensiometer that can be measured with an accurate gauge. Water will stop migrating from inside the tensiometer cup into the soil when the internal vacuum pressure of the tensiometer equals the soil tension, or the force needed to pull water from the soil pores. The vacuum gauge measures tension in units of kPa or cbars, which are equivalent (1 kPa = 1 cbar).
Interpretation of tension readings Because the tension value provides a sense of how much energy a plant would need to exert to suck water from the soil, tensiometer readings can be easily related to water stress in crops. At high tension values a plant experiences more water stress and growth slows. In addition, a tension reading has a similar meaning in terms of water stress whether the soil has a sandy, clay or loam texture.
Reliability of tensiometers The one Achilles' heal or weakness of the tensiometer is that if any air leaks into the instrument it will not retain a vacuum and the readings will be unreliable. There are several brands of commercial tensiometers available. Some are relatively inexpensive and simple to use, and others are more complex and can be interfaced with dataloggers to provide continuous readings throughout the day. Based on our experience, some of the most popular commercially available tensiometers often leak air and lose vacuum pressure, and in many cases the gauges do not provide accurate readings or are not durable. The loss of vacuum pressure means that the tensiometers need to be frequently refilled with degassed water. Also, irrigators may mistake a low reading to indicate that a crop has adequate moisture when in reality the soil may be dry.
A dependable tensiometer design We designed and tested a version of a tensiometer in 2018 that was simple to build and provided accurate readings for a material cost of less than $55 . The design improved the ability of the instrument to retain a vacuum at high tensions. Under moderately moist soil conditions the tensiometer usually required refilling with degassed water less than once per month. Even when the soil dried to tensions above the maximum range of the tensiometer (> 80 kPa), these tensiometers continued to hold a vacuum for about two weeks until all of the water in the shaft was depleted.
The following paragraphs describe the materials needed (Fig. 2) and procedures to build a tensiometer. The vendors of the materials are examples of ones that we use, but you may identify different or cheaper sources for these components. By carefully following these instructions, one should be able to build a dependable tensiometer that provides accurate tension readings. An update to this design can also be found in a more recent blog article.
Materials needed:
Ceramic cups
Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525) Part Number 0655X01-B01M3, Dimensions: 0.875 inch OD x 2.75 inch length. Cost: $30.80 ea.
Epoxy (ceramic/plastic)
Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525)
Part Number 0980V004, Description: 4 oz: epoxy and 4 oz hardener. Cost: $106 ea. Note that the epoxy/hardener is a sufficient volume to make several hundred tensiometers.
Vacuum gauge
Vender: Zoro.com/Grainger.com Part Number 4FMK3, Description: ¼ inch MNPT 2 inch diameter test vacuum gauge. Cost: $18.09 ea.
#1 size rubber stopper
Vender: Grainger.com Part Number 8DWU6, model RST1-S, Description: 24 mm neck, bottom diam. = 14 mm. Top diam. = 20 mm. Cost: $18.08 / 52 pieces
Schedule 40 PVC pipe (½ inch diameter) Vender: irrigation supply or hardware store
PVC “T”
Vender: irrigation supply or hardware store, Manufacturer: Spears Inc. Part number 402-072, Description: ½ inch slip x ¼ inch threaded reducing "T."
PVC glue (gray) and purple primer
Vender: irrigation supply or hardware store
Gas pipe thread sealant (white or blue paste type)
Vender: irrigation supply or hardware store
Painters masking tape
Vender: hardware store
Petroleum Jelly (Vasoline)
Vender: pharmacy
Tools needed:
- PVC saw or PVC cutting tool
- Aluminum Oxide grinding stone, Manufacturer: Forney Part Number: A11 60028 Description: 7/8 in [23 mm] diam. x 2 inch [50.8mm] length
- Power hand-held drill
- Miter box
- Pocket knife
Procedures
1. Cut PVC pipe sections in the following lengths
1 foot depth tensiometer: top shaft = 4 inches, bottom shaft = 17 inches
2 foot depth tensiometer: top shaft = 4inches, bottom shaft = 30 inches
It is advisable to cut the bottom shaft about 1-inch longer than indicated above and then carefully cut the lower end of the shaft using the miter box or electric miter saw to assure that it is cut at a 90-degree angle. The ceramic cup will fit crooked on the end of the shaft if the cut deviates from 90 degrees.
- First glue the top shaft and then the bottom shaft to the ½ PVC “T” using the PVC glue. Make sure that you do not glue the end of the bottom shaft that was trimmed to 90 degrees. In a well-ventilated location, apply PVC primer to both the end of the shaft and the inside of the “slip” end of the “T”. Then apply gray PVC glue to both sides, and push the parts together, and hold in place for about 30 seconds to 1 minute. Tip: slightly twist the parts by about 30 degrees immediately after gluing to assure that the parts are secure. Also cover the non-glued areas with painter's tape to prevent the outside from becoming covered with glue.
- Slightly bevel the inside of the lower end of the bottom shaft using the handheld drill and grinding stone (Fig. 3). Alternatively, one can use a knife to bevel the end. Whether using the drill or the knife to bevel the inside of the pipe, stop periodically and test fit the ceramic cup. This way you will not remove too much material, and will quickly get a feel for the appropriate amount to remove.
- Use epoxy to glue the ceramic cup to the lower end of the bottom shaft. Protect the ceramic cup during the gluing process by covering the outside with painter's tape (Fig. 4). Check that the ceramic cup fits snuggly into the PVC tube and is aligned straight. If using the epoxy from SoilMoisture equipment epoxy, mix up 1-part epoxy with 1-part hardener. Mix thoroughly. Only a small amount of epoxy is needed to coat the throat of the ceramic cup and the inside of the PVC tube, so it may be best to glue several tensiometers at the same time so that the epoxy is not wasted. One can usually glue no more than 20 to 40 cups at a time becaue the epoxy begins to cure after an hour. Approximately 20 ml of epoxy is needed for 20 tensiometers. The cure time is temperature dependent. Full cure is 8 hours at 77 °F. It is best to allow more time for curing. After gluing, painter's tape can be used to secure the cup to the shaft. Take care when securing the two with the tape to assure that the cup is aligned with the PVC shaft. Let the glue set for at least 24 hours with the tensiometer supported with the cup-end up in a vertical position. Tip: best if parts are glued at temperatures above 65 °F. More hardener may be needed at lower temperatures. Also, it is advisable to first test a small batch of epoxy to assure that the proportion of hardener to epoxy is enough for epoxy to set up hard.
- Coat the ¼ inch male threads of the gauge with pipe thread sealant and hand screw on the vacuum gauge. Tip: do not over tighten or the PVC “T” will crack!
- Fill the tensiometer fully with degassed distilled water. The water can be degassed by boiling it and allowing it to cool.
- Coat the lower end of the rubber stopper with a thin film of petroleum jelly and insert into the top end of the tensiometer with a light twist to firmly seat the stopper (A loose stopper is the main cause for vacuum leaks).
Preparing the tensiometer for testing and field installation
The tensiometer should be filled with degassed water (preferably distilled) before testing. Tap water will work too, but if it is hard water (contains a high concentrations of calcium and carbonate) it could cause precipitates to form inside the ceramic cup. Boiling will expel much of the dissolved air from the water. We do not recommend using a vacuum pump to remove dissolved air from the water. Boiling works best for degassing water. It also helps to soak the ceramic cup end of the tensiometer in water for a few hours so that the pores of the ceramic cup are saturated before testing and/or installation.
Testing the tensiometer for air (vacuum) leaks
After filling the tensiometer with water and sealing it with a rubber stopper, wrap a dry paper towel on the end of the ceramic cup and hold it tightly (Fig. 6). If the tensiometer is filled with degassed water, the tension should quickly increase to about 20 to 30 kPa as the towel absorbs water from the cup. If the gauge does not increase above 0, air is likely leaking into the tensiometer. Check the glue joints and assure that the stopper is tightly in place.
If the tension quickly increases to more than 20 kPa, then leave the tensiometer out in the sun to assure that the tension rises to above 70 to 80 kPa. This may take some time, minutes to hours, depending on the ambient temperature. If the tension does not increase to a high value, then check glue joints and the stopper. Also check that the gauge is securely threaded into the PVC “T.”
Installing tensiometers in the field
Proper installation of a tensiometer in the field will achieve close contact between the ceramic cup and surrounding soil. Using a soil probe with a ½ inch diameter shaft, make a pilot hole to a depth a few inches shallower that the depth of installation (Fig. 7). Make a soil water slurry by thoroughly mixing soil with the water to a pancake batter-like consistency. Add some slurry into the hole and push the tensiometer to the desired depth (Fig. 8). The soil slurry assures that water can freely move between the ceramic cup and the surrounding soil and fills the voids between the hole and tensiometer shaft. Formation of air gaps between the ceramic cup and the soil will lessen the accuracy of tensiometer readings. After two days of equilibration, the tensiometer reading should accurately reflect the tension of the soil.