Irrometer SR Tensiometer - Standard Range, 0-100 cb

Overview

The Irrometer SR (Standard Range) is the manufacturer’s most widely used tensiometer model. Its gauge reads soil water tension from 0 to 100 cb (kPa), and Irrometer recommends it for medium to heavy soils. The instrument is made by Irrometer and supplied by Agrinovo as part of its soil monitoring range.

Agrinovo supplies the SR in all six lengths, from 6 to 48 inches (15 to 120 cm). The length is chosen per order to match the depth to be measured, so the same model serves shallow and deep root zones.

Part of the Agrinovo tensiometer range: SR (Standard Range, 0-100 cb, medium to heavy soils), LT (Low Tension, 0-40 cb, coarse and sandy soils and potting media) and MLT (Miniature Low Tension, 0-40 cb, pots and small containers). The model follows the soil or growing medium; the depth follows the instrument length.

Key Features

• Standard Range gauge: 0-100 cb (kPa), for medium to heavy soils
• Six lengths: 15, 30, 45, 60, 90 and 120 cm (6 to 48 in), all supplied by Agrinovo and chosen per order by the depth to be measured
• Direct measurement: reads actual soil water tension, not an inferred value
• Replaceable ceramic tip: white threaded tip, suitable for a wide range of soil types
• Field gauge: 2.5 in (63 mm) dial in an AISI 304 stainless steel case with a polycarbonate window, rated IP67 and membrane vented for temperature and elevation compensation
• Large reservoir: for quick maintenance
• Self-contained: needs no electronics, batteries or calibration to operate
• Electronic options from the manufacturer: a voltage or 4-20 mA transducer, or an automatic switch, in place of the standard gauge

Choosing the Model and the Length

Two separate decisions, in this order.

1. The model follows the soil or growing medium. The SR, with its 0-100 cb gauge, is the model for medium to heavy soils. For coarse, sandy or potting-soil applications, where tensions above 30 cb are not expected, the LT (Low Tension) carries a 0-40 cb gauge. For small containers and pots, the MLT (Miniature Low Tension) is a compact 0-40 cb instrument.

2. The length follows the depth. Agrinovo supplies the SR in every length:

Length	Metric
6 in	15 cm
12 in	30 cm
18 in	45 cm
24 in	60 cm
36 in	90 cm
48 in	120 cm

A station that reads two depths uses two instruments of different lengths. Tell us the depth or depths you need to measure and we supply the matching lengths.

Applications

• Irrigation scheduling in medium to heavy soils
• Root-zone monitoring at a single depth, or at two depths with two instrument lengths
• Checking that irrigation water reaches the intended depth
• Sites with no power or connectivity, where the gauge is read on field rounds

How It Works

A tensiometer is a sealed, water-filled tube with a porous ceramic tip at the bottom and a vacuum gauge at the top. Installed with the tip in contact with the soil, it exchanges water with the soil through the ceramic. As the soil dries it draws water out of the tube, which creates a vacuum that the gauge displays. When the soil is wetted again, water flows back in and the reading falls.

The reading is soil water tension (matric potential): how hard a plant has to work to extract water. It relates directly to plant-available water, which is why a tensiometer needs no soil-specific calibration.

Understanding SR Readings

Readings are in centibars (cb), numerically equal to kilopascals (kPa). A tensiometer measures soil water tension: the force roots must overcome to take water from the soil. A reading of zero means fully wet soil with no tension, and the reading rises as the soil dries.

Irrometer publishes these general guidelines for its tensiometers:

Gauge reading	What it means
0-10 cb	Saturated soil
10-30 cb	Most soils are adequately wet. Coarse sands and potting media are already drying and in their irrigation range
30-60 cb	Typical irrigation range for most soils
60-100 cb	Usual irrigation range in heavy clay

Above 100 cb the soil is becoming very dry, which is beyond the SR gauge. For crops managed in that range, Irrometer points to the Watermark sensor, which covers the 100 to 200 cb range.

These bands are guidelines. The right trigger point depends on the crop, the soil and the growth stage, so set it from the trend over several irrigation cycles and not from a single number.

Why a Wet Reading Is Not Exactly Zero

With the cap closed, the weight of the water column inside the tube adds a little tension of its own: about 1 cb for every 10 cm (4 in) of instrument length. A 30 cm instrument reads about 3 cb in saturated soil and a 60 cm instrument about 6 cb. This is normal and is not a fault in the gauge.

The Best Time to Read

Readings can swing during the day with large temperature changes, especially when the instrument stands in direct sunlight. The effect is stronger when the instrument is due for servicing and has a large air bubble in the water tube. Under those conditions, Irrometer notes that morning readings are generally the most accurate.

Follow the Trend

Write the readings down and plot them over time. The rate of change is a key indicator of when to irrigate, and the history of each irrigation cycle helps show whether the crop is being under-irrigated or over-irrigated.

Where to Install the SR

A reading is only as useful as the place it comes from. Irrometer’s handbook gives these placement rules.

• Pick a representative site. Choose a spot that stands for a larger block with the same soil type, slope and sun exposure
• Stay in the active root zone and in the wetted area of the irrigation system, preferably on the side that gets more direct sunlight, where evaporation losses are greatest
• Use two depths for deeper roots. Crops with roots deeper than 45 cm (18 in) benefit from readings at more than one depth. A good starting point is a shallow instrument at 25% of the root zone depth and a deeper one at 75%

Situation	Recommended position
Trees	At the drip line of the canopy
Row crops	In the row itself
Drip irrigation	30 to 45 cm (12 to 18 in) from the emitter line
Micro-sprinklers	60 to 90 cm (24 to 36 in) from the sprinkler, inside the wetted pattern
Flood or furrow irrigation	Two-thirds of the way down the run, where water penetration is most critical

In flood irrigation, an instrument 15 cm (6 in) longer than the depth requires keeps the gauge above the water level.

Each depth needs its own instrument length. Agrinovo supplies the SR in every length from 15 to 120 cm, so tell us the depths and we match the lengths.

Preparing the Instrument

A tensiometer only works when it is full of water and free of air. Prepare it before it goes into the ground.

1. Remove the plastic wrapper from the ceramic tip and take off the cap
2. Fill to about three quarters of the reservoir. Irrometer supplies a green fluid concentrate, diluted at one capful per gallon (about 3.8 liters) of water. The fluid is optional: it makes the water level easier to see during servicing
3. Tap the top of the instrument with the palm of your hand until no bubble remains between the reservoir and the water tube
4. Soak the tip in clean water for at least one hour, with the cap off

If the instrument has to travel after soaking, put the plastic wrapper back on the tip and fit the cap, tightening it only a quarter turn past first contact with the stopper.

Installation Procedure

Install after an irrigation, when the soil is wet and easy to work.

1. Make the hole. In very loose soil a short instrument can simply be pushed in to depth. Heavier soils need a hole, driven with the Irrometer installing tool or a section of 1/2 in steel pipe (22 mm outside diameter)
2. Keep the hole tight. The instrument must sit firmly so the tip makes contact with the soil. If a larger hole cannot be avoided, fill it with a mud slurry first to grout the instrument in place
3. Match the depth to the length. Do not make the hole deeper than the instrument: the tip should rest against the bottom
4. Leave at least 25 mm (1 in) between the bottom of the gauge and the ground. This protects the gauge and lets its membrane vent work
5. Push the instrument in by the cap or the reservoir. Never push or pull on the gauge, and avoid twisting counterclockwise, which can loosen the replaceable tip
6. Mound a few centimeters of soil around the instrument so water drains away and does not pool
7. Service it right away with the hand pump, as described below

Servicing

Servicing means topping up the water and removing air from the gauge and the water column. It is done immediately after installation and then occasionally through the growing season.

How often depends on field conditions. While readings stay below 30 cb, servicing is rarely needed. The higher the readings run, the more often it is required.

1. Service when the soil is wet, so no extra air is drawn into the instrument
2. Add water before pulling a vacuum
3. Remove the cap and seat the hand pump cup against the top of the reservoir
4. Pull the pump a few times until the gauge reads in the upper half of its range, typically 70 to 80 cb on the SR
5. Hold the vacuum for a few seconds, then release it gently with the thumb release valve
6. Replace the cap and tighten it only a quarter turn past first contact

With Irrometer’s Basic service kit, a syringe takes the place of the pump: seat its tip in the small opening at the bottom of the reservoir, pull to the same gauge reading and release slowly.

On an instrument fitted with an RSU electronic transducer, never release the vacuum suddenly. A sudden release can damage the transducer.

Frost Protection and Storage

Freezing damages the gauge. When water inside the gauge freezes it expands and distorts the mechanism, which throws the gauge out of calibration. The operating temperature range is 0.5 to 65 °C (33 to 150 °F).

• Light, infrequent frost: an insulated box over the instrument can trap enough ground heat to protect it overnight
• Colder climates: remove the gauge, or the whole instrument, for the winter
• Draining is not enough on its own: the gauge can hold water even when the tube is drained. Hold the drained instrument with the gauge upright and pull a vacuum with the hand pump to remove the remaining water

Before storage, clean the instrument with clean water and a brush or cloth, then rinse it. To extend tip life, fill the instrument with water (distilled is best), take off the cap and let it drain through the tip by gravity. This flushes out some of the material that built up in the ceramic during the season.

Maintenance

Testing and Replacing the Ceramic Tip

Ceramic tips can mineralize over time, which slows the response. To test a tip, hang the instrument vertically, fill it to the top of the reservoir, remove the cap and let it drain for 24 hours. If less than 25 mm (1 in) of water has drained, replace the tip.

The SR tip is threaded. Unscrew the old tip, dip the threads and the O-ring of the new one in water, and screw it in until there is no gap between the tip and the tube. Do not tighten beyond that point, or the tip assembly can break.

Gauge

The gauge is removed and fitted with a 9/16 in or 15 mm open-end wrench. Do not tighten it by turning the gauge body by hand. The threads must be sealed with Teflon tape or gasket sealant, and at least one thread should stay exposed.

Cap and Stopper

The stopper inside the cap can harden over years of service, or be damaged by repeated over-tightening. It is replaced on its own or together with the whole cap assembly. This is why the cap is only ever tightened a quarter turn past first contact.

Troubleshooting

Low or zero reading and the instrument empties quickly

This points to a leak. Pull a vacuum with the hand pump and watch the water: a steady stream of large bubbles shows the leak. Small “champagne” bubbles are normal during servicing. A leak at the gauge fitting is fixed by removing the gauge and resealing the threads. A leak at the tip is fixed by cleaning and refitting the tip or replacing its O-ring.

Checking the gauge

Gauges can be damaged by freezing or by a knock. With the cap off and the instrument vertical, a healthy gauge reads zero. A higher reading means the gauge is damaged and should be replaced. A gauge that zeroes with the cap off and pulls up to 70 cb or more with the hand pump is likely good.

Slow response to irrigation

The ceramic tip may be mineralized. Run the 24 hour drain test above.

The reading is not what you expected

If the instrument is serviced, has no leaks and the gauge checks out, the reading is probably correct. Either the real conditions differ from what was assumed, or the site is not representative because of an irrigation problem, hardpan, an installation issue or another variation in the soil. Consider reinstalling in a similar spot nearby.

The gauge never reaches zero in wet soil

This is the water column offset described above, about 1 cb per 10 cm of length.

Service Kits and Replaceable Parts

The cap, the stopper, the gauge and the ceramic tip are all replaceable, so a SR is maintained, not thrown away. Irrometer lists these items for the SR:

Item	Irrometer part
Service kit with hand pump	1001 (Standard)
Service kit with syringe	1000 (Basic)
Service kit with test gauge, for checking gauge calibration	1002
Green fluid concentrate	1004
Threaded replaceable tip	SRT
Tip O-ring	SR-O
0-100 cb gauge	1008
Cap assembly with stopper	CAP

Ask us about service kits and spare parts when you request a quote.

Electronic Options

The SR is a mechanical instrument read on its gauge, with no RS485, Modbus or SDI-12 output. For electronic reading, Irrometer offers three options that replace the standard gauge:

• RSU-V: voltage output transducer, 0.5 V to 4.5 V
• RSU-C: 4-20 mA output transducer
• AVS: automatic switch that triggers at a set tension

If you need continuous logging or automation, ask us about availability when you request a quote.

Construction

• Body: C.A.B. plastic
• Other materials: ceramic, neoprene, EPDM, bronze, brass and stainless steel
• Gauge: 2.5 in (63 mm) dial, AISI 304 stainless steel case, polycarbonate window, membrane vented, IP67
• Gauge accuracy: +3-2-3% of gauge span, ASME B40.1 Grade B
• Reservoir section: 130 mm high, 55 mm diameter
• Water tube: 22 mm (0.875 in) diameter
• Weight: 0.44 kg for the 30 cm instrument, plus 0.114 kg per additional foot of length
• Manufacturer warranty: one year

Related Guide

For how tensiometers work, how to read them and when to use them, see Tensiometers for Irrigation: the complete guide.