Types Of Strain Gauges: Construction & Specifications

Strain Gauges or Strain Gages are the geotechnical instruments used to measure strain or pressure applied on an object. This object can be as small as a machine or even as large as a TBM. Invented in 1938 by Edward E. Simmons and Arthur C. Ruge, strain gauges have been extensively used in geotechnical engineering since then.

While there are numerous types of strain gauges that are used to determine strain, here are the Encardio’s Models of the strain gauges along with their working principle, features, and applications.

Different Types Of Strain Gauges

Encardio mainly deals with four different types of strain gauges. We have the following Models:

  1. Model EDS-11V Hermetically Sealed Vibrating Wire Strain Gage
  2. Model EDS-12V Vibrating Wire Sister Bar Strain Meter
  3. Model EDS-20V-V Series Vibrating Wire Strain Gage
  4. Model EDS-30V Shortcrete Strain Gage

| Read More: Strain Gauges: Principle, Types, Features and Applications |

Let’s understand each one of them in detail with their working principle, specifications, and features.

Model EDS-11V Hermetically Sealed Vibrating Wire Strain Gauge

Model EDS-11V Vibrating Wire Strain Gauge

The Model EDS-11V hermetically sealed vibrating wire strain gauge finds its application mainly in soil, concrete structures, or on steel structures. It gives precise readings for the distribution of both the compressive, as well as, tensile strain varying with time.

Operating Principle Of Model EDS-11V Hermetically Sealed Vibrating Wire Strain Gauge

The Model EDS-11V has a magnetic wire with high tensile strength. One of the ends of the wire is anchored while the other one displaces proportionally with respect to the strain.

Even a minor deflection in the strain directly affects the tensile strength of the wire thereby, changing its corresponding vibrating frequency. The strain is proportional to the resonant frequency of the wire and is directly displayed through a readout unit.

Strain Meter & Its Components

EDS-11 Strain meter

In an EDS-11 strain gage meter, the coil magnet assembly is enclosed in a stainless steel ‘magnet enclosure’ which is an electron beam welded to two stainless steel tubes with integral stainless steel bellows which are rubberized for protection.

The purpose of the bellows is to reduce the modulus of elasticity of the strain meter such that it truly reflects the expansion and contraction of the concrete in which it’s embedded or the steel structure to which it is welded.

The bellows are a special feature of Encardio-rite strain meters. The ends of the stainless tubes are electron beams welded to two cylindrical end blocks, one of which has a ‘V’ groove around its circumference. The vibrating wire assembly is permanently anchored to the end blocks.

The leads from the coil magnet are terminated on the glass to metal seal which is integrally electron beam welded to the stainless steel body of the strain meter. The two pins marked red and black on the glass to metal seal are connected to the coil magnet.

The other two pins are used with a thermistor for measurement of temperature. A cable joint housing and cable gland are provided for the cable connection. Standard cable gland is NG 16 MS which is suitable for a cable size of 6-8 mm.

EDS-12/13 Spider For Strain Rosettes

EDS-12/13 Spider For Strain Rosettes

Encardio-rite manufactures spiders for five and thirteen position strain rosettes. A strain rosette is used to measure strains along different directions of the component that is being monitored.
The spider is precisely machined to the specified angles. The strain gage meters are screwed on to the 200 mm rods attached to the spider at the correct angular positions.

The Encardio-rite Model EDS-12 five position spider permits precise and accurate installation in a concrete dam or structure of four strain meters at angles of 0°, 45°, 90°, and 135° in one plane and one strain meter at right angles to this plane.

The Encardio-rite Model EDS-13 thirteen position spider has eight rods in the vertical plane at 0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°; four rods in a plane at 45o to the above plane at angular positions of 22.5°, 112.5°, 202.5°, 292.5° and one rod placed horizontally.

EDS-14 No Stress-Strain Container

EDS-14 No Stress-Strain Container

The strain gage meter is designed to respond to change in dimensions of the concrete in which it is embedded, whether the deformation is due to stress, creep, temperature change, moisture change or chemical growth of the concrete.

The main purpose of the strain gauge, however, is to indirectly determine stress. Change in stress is revealed simply by multiplying the measured strain by the modulus of electricity of the concrete in which it is embedded. However, for stress which develops over a long period of time, account must be taken of the changes in the modulus of elasticity and of deformation due to creep and to all causes other than stress.

It is often desirable to measure separately the deformation due to all causes other than stress. This is done by installing a “no stress-strain meter” which is exposed to the same conditions as the surrounding concrete except as to stress.

The EDS-14 no stress-strain container is a two-walled hollow cylinder with a dimension of 400 mm x 600 mm height. The welded leak-proof container has an outer wall of 2 mm thick mild steel and an inner wall of 1 mm thick copper. The gap between the walls is 50 mm.

The purpose of the gap is to prevent true stress from acting on the strain meter which is installed inside the container. This strain gage meter therefore only reads deflection which takes place in the concrete due to autogenously growth of the concrete.

EDS-15/16 Mild Steel Brackets & Stainless Steel Flanges

EDS-15/16 Mild Steel Brackets & Stainless Steel Flanges

The model EDS-15 mild steel brackets are used when the strain meter is used for welding applications on steel structures or attaching to concrete structures with saddle mounts. For ease in mounting, one of the brackets is split in the middle. The grub screw in the other bracket firmly locates it into the groove of the end block of the strain meter.

The model EDS-16 stainless steel flanges (48 mm dia) are used when the strain meter is used for embedment in concrete. One of the flanges fits into the ‘V’ groove of the left-hand end block of the strain gage meter. The other flange is fixed at a centre distance of 140 mm from the first flange.

EDS-17 Dummy strain meter

EDS-17 Dummy strain meter

The strain meter brackets are welded in position by placing a dummy strain meter in between them to maintain the correct distance and alignment. The dummy strain meter is finally removed and replaced by the actual strain meter.

EDS-18 Extender

The Encardio-rite model EDS-18 extender allows to increase the length as well as the sensitivity of the model EDS-11V strain meter. The extender has a male M6 x 10 thread which fits into the female tapping of the strain meter end block. The effective length of the strain meter is increased to 280 mm by the extender, thus increasing the sensitivity by 100 %.

Strain Gauge Specifications – Model EDS-11V Hermetically Sealed Vibrating Wire Strain Gauge
Type Vibrating Wire
Range ± 1500 μ strain
Sensitivity 1 μ strain
Active gage length 140 mm
Total length 190 mm
Accuracy ± 1 % fsd ( combined effect of non-linearity + hysteresis + repeatability)
Temperature limit Operational -10 to 80°C
Compensated 0 to 55°C
Over range limit 125 % of the range
Over range effect ± 0.1% fsd up to 120%
Insulation resistance 500 M Ohm
Coil resistance 120-140 Ohm
Enclosure Stainless steel
Protection IP-68 (NEMA 6)
Sealing Hermetically sealed by electron beam welding with a vacuum of 0.001 Torr. inside it
Vibration limit 2g, 20-500 Hz
Humidity limit 100% RH
Readout Portable strain indicator
Thermistor YSI 44005 or equivalent (3,000 Ohm at 25°C)
Cable connection Glass to metal seal solder pin connector

 

Model EDS-12V Vibrating Wire Sister Bar Strain Meter

Model EDS-12V Sister Bar Strain Meter

Encardio-rite Models EDS-12V/EDS-12V-EX vibrating wire sister bar strain meters are precise, low-cost, waterproof as well as reliable sensors for long-term installations. These strain gage meters are specifically designed for measurement of strain in concrete structures such as piles, diaphragm/slurry walls, bridge abutments, tunnel lining, dams, foundations etc.

The model EDS-12V vibrating wire sister bar strain gauge includes a hollow bar with vibrating wire strain gauge mounted coaxially inside it. The hollow bar is extended on both sides with 12 or 16 mm reinforced bars. The Model EDS-12V-EX has the sensor welded onto it, with proper protection for special applications.

Strain Gauge Specifications Of Model EDS-12V
Range 2500 μ strain
Sensitivity 1 μ strain
Dimensions 26 x1400 (for 12.0 φ)

30 x1400 (for 16.0 φ)

Temperature Limit -10 to +55°C
Protection VW strain gage encapsulated

inside hollow bar

Thermistor YSI 44005 or equivalent

Model EDS-20V Series Vibrating Wire Strain Gauges

Model EDS-20V Strain Gauges

The Encardio-rite Model EDS-20V Series Vibrating Wire Strain Gauges consist of two components – a basic gage and a sensor assembly. The basic gage incorporates a high tensile strength wire made of a magnetic material stretched between two stainless steel cylindrical end blocks.

The wire is sealed in a stainless steel tube by a set of double “O” rings fixed on each end block, ensuring resistance to corrosive, humid, wet and other hostile environmental conditions. The sensor houses a permanent magnet and a plucking coil assembly.

The wire, when plucked by the sensor, vibrates at its natural frequency that is proportional to the tension in the wire. Any change in strain, directly affects the tension in the wire, resulting in a corresponding change in its frequency of vibration.

The strain is proportional to the square of the frequency that can be measured and displayed directly in micro strain units by Encardio-rite’s EDI-51V vibrating wire indicator.

The main purpose of the strain gauge is to indirectly determine stress and its variation with time, quantitatively. Change in stress is determined by multiplying the measured strain by the modulus of elasticity.

Encardio-Rite deals with the following types of strain gauges to suit different applications:

  • EDS-20V-SW spot weldable strain gauge
  • EDS-20V-AW arc weldable strain gauge
  • EDS-20V-E embedment strain gauge
EDS-20V-SW Spot Weldable Strain Gauge

EDS-20V-SW Spot Wieldable Strain Gauge

The Model EDS-20V-SW consists of a stainless steel foil tab fixed on each end that enables the gage to be spot-welded or fixed in place by epoxy.  The strain gauge is pre-tensioned by a small compression spring.

The initial tension can be set during strain gauge installation so that there’s a wide range available for tension or compression as required. However, the stress gauge can only be used over flat surfaces.

The sensor assembly is encapsulated in a moulded protective cover. It is provided with a single part sensor coil rectangular housing suitable for mounting directly over the strain gauge and completely enclosing it forming a watertight enclosure. Pair of clamps positioned between a pair of embossed locating points over the sensor coil housing is spot welded to secure the housing to the substrate.

Strain Gauge Specifications Of EDS-20V-SW
Range 3000 μ strain
Sensitivity 1 μ strain
Active gage length 50.8 mm
Initial frequency 2,500-3,300 Hz
Effective gage factor around 3.896 x 10-4 μ strain/ Hz-square
Thermistor YSI 44005 or equivalent (3,000 Ohm at 25°C)
Temperature Limit -20 to 80°C
Cable 4-core shielded

1 m long

Gage l x b x h (mm) 62 x 12.5 x 7.5
Model EDS-20V-AW Arc Weldable Strain Gauge

Model EDS-20V-AW Arc Wieldable Strain Gauge

The Model EDS-20V-AW consists of two end pieces joined by a tube. A high tensile strength stretched magnetic wire is enclosed inside the tube by a set of double “O” rings fixed on each end piece. The double ‘O’ ring seals make sure that the gauge is safe from water ingression.

The tube is flattened in the middle so that a sensor assembly can be placed inside it. This sensor forms an integral part of the strain gauge assembly. Such strain gauge finds its application at locations prone to dripping or running water.

Two annular mounting blocks are carefully positioned and aligned with the help of a mounting jig and dummy gauge to mount the gauge. The assembly is welded to the structure while the dummy gauge is finally replaced by the actual strain gauge and fixed in its desirable position by a pair of set screws on each block.

Groutable reinforced bar annular mounting blocks are available for surface mounting the strain gauge to a concrete structure.

Accessories for EDS-20V-AW are as follows:

  • Mounting blocks – weldable or groutable
  • Installation kit consisting of mounting jig, dummy gage, Allen keys and grub screws (to be ordered separately)
Model EDS-20V-E Embedment Strain Gauge

Model EDS-20V-E Embedment Strain Gauge

Model EDS-20V-E vibrating wire strain gauge measures strain in underground cavities, tunnels, buildings, concrete, and masonry dams etc. Ideal for embedment in soil or concrete, the EDS-20V-E Embedded Strain Gauge is similar to the arc weldable strain gauge except for the fact that mounting blocks are replaced with stainless steel flanges.

Strain Gauge Specifications Of EDS-20V-AW & EDS-20V-E
Range 3000 μ strain
Sensitivity 1 μ strain
Active gage length 150 mm
Effective gage factor around 4.051 x 10-3 μ strain/ Hz-square
Thermistor YSI 44005 or equivalent (3,000 Ohm at 25°C)
Temperature Limit -20 to 80°C
Cable 4-core shielded 1 m long;
Size l x b x h (mm)

Model EDS-20V-AW

Model EDS-20V-E

174 x 28.5 x 30

170 X 28.5 X 30

Model EDS-30V Shortcrete Strain Gauge

Model EDS-30V Shortcrete Strain Gauge

EDS-30V has a much larger strain measurement range as compared to other conventional concrete embedded strain gauges like Encardio-rite Model EDS-20V-E or equivalent. It is especially useful in shotcrete applications where the freshly applied shotcrete can sometimes develop a compressive strain of much more than 3000 microstrain units which is beyond the measurement range of other conventional strain gauges.

The sensor is of stainless steel construction and is suitable for all types of hostile environmental conditions.

Model EDS-30V Shortcrete Strain Gauge Construction

Model EDS-30V Shortcrete Strain Gauge

The EDS-30V shotcrete strain gauge can be considered as a miniature high resolution vibrating wire displacement sensor in a stainless steel housing. It is pre-tensioned at around the middle of its measurement range using a very thin aluminium tubing so that it can measure both tensile and compressive strains of up to 30,000 microstrains unit.

Two short lengths of rebar are fixed, parallel to each other, at each end of the strain gage for gripping in shotcrete. The distance between the rebars is the gauge length of the strain gauge. The aluminium tubing is covered with a polyolefin sleeve to decouple it from the surrounding concrete so that the measured strain is the average strain in concrete lying between the two lengths of rebar.

The sensor houses a permanent magnet and a plucking coil assembly. The wire, when plucked by the sensor, vibrates at its natural frequency that is proportional to the tension in the wire. Any change in strain, directly affects the tension in the wire, resulting in a corresponding change in its frequency of vibration.

The strain is proportional to the square of the frequency. The sensor also includes a thermistor for measuring the temperature of shotcrete. Measurement of temperature enables the application of correction factors for more accurate results.

A high resolution vibrating wire indicator, like Encardio- Rite Model EDI-51V (version 5 and above), is needed to realize the high resolution of EDS-30V strain gauge. The EDI-51V indicator additionally provides direct readout in microstrain units. Individual gauge factors are supplied with each EDS-30V strain gauge for better accuracy. EDI-51V also displays the thermistor readings directly in °C.

Strain Gauge Specifications – Model EDS-30V
Type Vibrating wire
Measurement Range ± 15,000 micro strain
Overall length 200 mm nominal
Effective Gage length 166 mm
Coil resistance 160 Ohms typical @ 25°C
Operating temperature range -20 to 80°C
Thermistor 3 K ohms @ 25°C;
Protection class IP 68

This was all about the different Models of Strain Gauges dealt with by Encardio-Rite. If you have any queries or suggestions, feel free to drop a comment below.

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