Strain Gauge Technology
Strain gauges are electrical devices used to measure the strain on the object to which they are bonded. On a fundamental level, strain gauges consist of a variable resistor whose resistance value changes when a force is applied to the material resulting in strain. As a result of this simple – yet powerful – working principle, strain gauges are useful in the wide variety of applications in which the ability to measure resistance change corresponds to external force applied on the object resulting in compressive or tensile deformation is a crucial component. This deformation causes linear changes in resistance on the strain gauge – a phenomenon that will be explained in detail – and understanding this change in resistance is key to measuring the applied force, as well as the force applied to the object in question.
Though strain gauges have been used in many creative ways, they primarily see wide-spread use as part of a pavement management systems (PMS). These are often installed by local municipalities to help monitor the condition of concrete or asphalt structures, such as roads and other forms of paving. This page will explain the working principle of strain gauges and the extreme durability and quality of measurement that Jewell Instruments’ strain gauges offer.
What Is Strain?
Strain is defined as the change in length of a material divided by its original length in a specified dimension. Strain is denoted by the Greek letter ε.
Strain is caused by “stress”, which is the force per unit area acting on the material. The diagram below shows a weight (or force) pulling and stretching. The length (L), is now stretched, and the change in length is represented by ΔL. The strain, ε can be calculated by dividing ΔL by the original length.
Transverse and Longitudinal (X & Y axes) strain can be measured and calculated. These quantities can be used to calculate Poisson Strain, which is used to determine how much a material can withstand being stretched or compressed before it fails. Poisson’s Strain is defined as the negative ratio of transverse to axial (longitudinal) strain as shown below:
How Is Strain Measured?
Strain gauges work by measuring changes in resistance proportional to physical strain. Changes in resistance are usually measured via a simple Wheatstone bridge circuit, which is a four-element circuit arranged in a diamond shape as displayed below. One can think of this circuit as a pair of potential dividers configured parallel.
If each resistor in the circuit has equal resistance, then the voltage read from points D to B via the voltmeter would be 0 Volts, since the bridge is in a balanced state. If RX is adjusted so that its resistance does not equal R1, R2 or R3, then the bridge will be unbalanced, and a potential difference will appear across D-B.
In this case, a strain gauge element could be used in place of any of these resistors – or potentially all of them. This is referred to as a “full bridge” strain gauge. Below is an example of such a circuit, as shown in our CSG-3000 datasheet. The total equivalent nominal resistance is 350Ω.
The strain gauge’s output is in units of mVolts per Volt, mV/V, meaning the voltage across the bridge terminals is in mV compared to the supply or excitation voltage. For example, a 1V excitation with a 1000µε strain, causing a +0.24Ω resistance change would result in a Vout of 0.0004995V or 0.4995mV/V.
Jewell Instruments Strain Gauges
Jewell Instruments currently offers two types of strain gauge: the ASG-3000 and the CSG-3000, which are both horizontal, full bridge units. One for designed for asphalt applications, The ASG-3000 and the other for concrete, the CSG-3000.
Both the ASG & CSG models are configured to measure axial strains along the length of the rod and ignore any bending strains the mechanism is subjected to. They both have a nominal gauge resistance of 350Ω and possess an output sensitivity of ~1.3mV/V at 1000µε, with a nominal supply voltage of 2 – 5 VDC. The gauges stiffness is of the same range of magnitude as the asphalt and concrete, so the true strain within the surrounding material can be measured. The ASG has been designed to be both temperature and moisture resistant, so it can perform excellently even under circumstances where the temperature swings, including the ambient environment in general, as well as the extremely high temperatures of the asphalt during installation. Specifically, the ASG has an operating range of -34°C to +200°C, whereas the CSG Is ranged from -34°C to +100°C. The ASG-3000 achieves its thermal and moisture resistance due to its use of Gilsonite emulsification, and the threaded rod flexure made of Ultem black. The emulsification process is an added step that helps to seal the installation and protect the plastic components for immersion in the asphalt. The strain gauge installations themselves are protected with two acrylic layers, an RTV layer, a wax layer and a shrink wrap layer. This emulsification procedure is the final step, which coats the entire strain gauge. The CSG uses a combination of M-coat A & M-Coat C and electrical tape as a protective coating.
