Strain Gauges: Real-World Considerations and Typical Applications

Previously, strain gauges were introduced as those sensors responsible for measuring the displacement of an object as compression and extension forces are applied, resulting in the property known as ‘mechanical strain.’

 

Real-World Considerations

Foil strain gauges can be shorter than 1 mm long or as long as 30 mm. Most typically, the active area covers 2 to 10 square millimeters, but they can be shorter or longer. The resistance often is either 120 or 350 ohms, but there are others. One source refers to 60 ohms.

As described in the previous article, there needs to be a backing material. The foil and its backing are almost always bonded together; it would be very difficult to adhere them separately. The most common backing materials are either cyanoacrylate for shorter-term use, or epoxy glue if it needs to last for months or years. Other possibilities include anything from paper to ceramic cement.

The following table lists some of the most common sensor and backing material pairs. Wire is not used to make strain gauges as much anymore, so most devices today are created with foil.

Sensor Material	Backing Material
Constantan Foil, Constantan Wire	For limited use, cyanoacrylate For long life, epoxy or phenolic glass Polyimide Paper, Bakelite
Nichrome Foil, Nichrome Wire	Phenolic glass Removable Teflon glass tape
Iso-Elastic Wire	Bakelite, paper
Dynaloy Foil	Polyimide, polyimide glass
Stabiloy Foil, Stabiloy Wire	Polyimide, phenolic glass Removable Teflon glass tape
Platinum-Tungsten Wire	Phenolic glass Removable Teflon glass tape

For Constantan foil with epoxy backing, the recommended range is about -75 to +205 ºC, while the maximum temperature is -200 to +315 ºC. If you need really high (and low) temperatures, platinum-tungsten wire with removable Teflon glass tape may be the solution. The recommended temperature for this combination is -130 to +650 ºC. The maximum range possible is -270 to as much as +800 ⁰C.

Alloys are available to produce outputs that compensate for temperature variations. Of course, even the best compensation is only approximate. The best compensation alloy might change depending on the temperature range needed. Another temperature compensation method is to change one of the bridge resistors to an appropriate temperature-sensitive resistor.

 

How Do You Calibrate Strain Gauges?

One way to calibrate a gauge is to add a high parallel resistance on one of its fixed resistors. This will slightly reduce the voltage and, if properly designed, set the bridge voltage to zero at the desired point.

Most types of strain gauges have a gauge factor of approximately 2. For precision gauges, the exact value might be printed. Also, if the temperature varies, the bridge might include a temperature-compensating resistance to at least approximately correct it.

 

Rosette Strain Gauges

Sometimes you need to have more than one strain measurement at the same time, often three or four, to compute strain at more than one angle. The three strain gauge elements (or four, or two, or sometimes something else) are combined in something called a rosette.

 

Figure 1. A few examples of strain gauge rosettes. Image (modified) used courtesy of Veda

 

The most common rosettes have three elements arranged in either 0º, 45º, and 90º; or 0⁰, 60⁰, and 120⁰. Others include 0⁰, 120⁰, and 240⁰, and others. There also are two-element rosettes arranged at 0⁰ and 90⁰, and four-element rosettes arranged at 0, 45, and 90 degrees. There are others.

There are planar rosettes and stacked rosettes. The stacked rosettes are stacked one on top of each other. The planar rosettes are side by side, not on top of each other.

Each element needs its own Wheatstone bridge; in other words, usually three Wheatstone bridges per rosette. Then, each bridge usually has signal conditioning to turn it into a digital quantity. After that, some fancy calculations can compute the strain at multiple points simultaneously.

 

Typical Applications

Fully understanding strain gauges is important in design and maintenance, but where are these sensors actually used?

 

Industrial Applications

For industrial uses, most people would think about weight and force measurement. A typical device is a load cell. Here’s an assortment of industrial load cells.

 

Figure 2. An assortment of industrial load cells. Image used courtesy of Interfaceforce

 

Weight is usually a static measurement, varying from grams or ounces to kilograms or tons (or tonnes). Applications run from counting parts to weighing trucks and monitoring railroad tracks.

Here’s an interesting weight application: parts counting. Each part can be small, but the total weight may be large and change constantly. Even very small product in large volumes, such as the load in a dump truck, or the seed content of an automated ag seeder, can be measured with load cells.

 

Figure 3. Load cells are used to measure the amount of seed in a cultivator. Image used courtesy of Control Automation

 

Here are some other applications: performing torque and power measurements in rotating equipment. Crushing machines, which need to detect problems if the machine jams. Measuring stress in paper mills to maintain correct alignment.

 

Utility & Civil Engineering Applications

In utilities and civil engineering, strain gauges are used while testing new design ideas and then again after the design is finished. This includes cable bridges, wind tunnels, buildings, dams, tunnels, and more. More strain gauges often are permanently built in to allow repeat monitoring months and years later.

 

Figure 4. Strain gauges are used in many utility and civil engineering applications. Image courtesy of Adobe Stock

 

Medical Uses

Medical uses include blood pressure measurement, dialysis machines, and syringe pumps. And, of course, weight measurement.

 

Other Uses of Strain Gauges

Strain gauges are used in liquid level and gas pressure measurement. Household uses include scales and robot vacuum cleaners. Other uses include boat and marine testing

Here’s an interesting use: crash test dummies. Also, weight measurement to find out whether an air bag needs to be inflated, and maybe whether the occupant is a child (less pressure).

 

Summary of Strain Gauges

Strain gauges serve a wide variety of uses. In the first article, we studied the basics then, in this article, we learned about some real-world considered and typical applications.