In case you missed May’s webinar with Sartorius Intec on load cells, we’ve put together some of the key points:
Let’s start with the most common weighing element: the strain gauge load cell.
A strain gauge load cell can be found in bench platforms, floor scales, and in some cases, tank and hopper systems. It is the most common load cell used in the weighing industry.
The most common electrical strain gauges are thin, rectangular strips of foil with maze-like wiring patterns on them, leading to a couple of electrical cables. You stick the foil onto the material you are measuring and wire the cables up to your computer or monitoring circuit.
When the material you are monitoring is strained, the foil strip is very slightly bent out of shape, and the maze-like wires are either pulled apart and stretched slightly thinner, or pushed together and become slightly thicker.
Changing the width of a metal wire changes its electrical resistance because its harder for electrons to carry electric current down narrower wires. So, all you have to do is measure the resistance typically using a wheatstone bridge, and with a bit of conversion, you can calculate strain.
If the forces involved are small, the deformation is elastic and the strain gauge eventually returns to its original shape and you can keep taking measurements over a period of time.
In weighing applications, it is important to understand that stress does NOT equal strain.
Stress refers to force. It is a measurement of how much internal pressure a material is under when a force acts upon it. The bigger the force, or the smaller the surface area, the more stress there will be, and the more likely the material will deform, or change shape.
Just like pressure, we measure stress:
Strain is what happens as a result of stress. If a material is stressed by force, it often changes shape and gets a little bit longer, if you pulled it apart, or shorter, if you pushed it together.
Strain can be defined by the change in length the force produces divided by the materials original length.
In a bar strain gauge load cell, the cell is set up in a Z formation. The torque is applied to the bar and the 4 strain gauges on the cell will measure the bending distortion: 2 measuring compression, and 2 measuring tension.
When these 4 gauges are set up in a wheatstone bridge, it’s easy to accurately measure the small changes in resistance from the strain gauges.
Each strain gauge as a different sensitivity to strain, expressed quantitatively as a gauge factor. The gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length. The typical gauge factor for metallic strain gauges is about 2.
Advantages of Sheer Beams and Bending Beams:
- Low cost
- Can be used as a structural member of the scale
- Works well in floor scales and bench scales
Disadvantages of Sheer Beams and Bending Beams:
- All torque in any direction is converted into signal
Sheer beam design is “good” – but not good enough for a tank…
- All cells have a slightly different output. If they tank has side load, the output will change because the calls are not matched. If a cell is replaces, the system must be recalibrated.
- Junction boxes require trim pots to balance the load, adding a weakness to moisture causing frequent maintenance.
- Every calibration is unique and must be done by spanning a mass. If a cell is replaced, the system must be recalibrated and a full load must be placed on the scale.
- Not optimized for resistance to creep due to inconsistent metallurgy. Tanks have constant load, unlike a bench scale that is loaded and unloaded.
- Not optimized for drift because of inconsistent gages. Lower mV/V output cells create less heat and better bonding is required for higher precision
- Temperature compensation is limited
Tank cells and mounts need to meet a higher standard:
- Tanks are often difficult to calibrate with a mass
- Tanks face motion, side load, temperature swings and require exceptional linearity and resistance to creep
- Tanks are often called on to offer higher precision
- Tanks used in processing can cause tremendous financial loss when there is a failure
Sheer beams are good for use in tank and silo weighing when:
- The product has low value so low precision is not needed
- The time lost in production has no value and the several day wait for a new load cell installation with calibration with a mass is not an issue
The canister style load cell is the earliest load cell design. Its either hermetically sealed or welded to protect the gauges. There are two types of canister constructions: single column and multiple columns.
- Single column canisters cannot typically withstand a side load of over 15%.
- Canister cells range in size from 100 up to 500,000 lbs.
- Off center loads should travel through the beam exactly through the center without causing a change in load cell output
For more load cell basics, watch the complete webinar recording below:
Download the presentation slides [2.4 MB .PDF]
Learn more about Sartorius load cells for weighing applications
Lesman is the premiere stocking representative for process valving, controls, and measurement instrumentation, serving customers in Illinois, Indiana, Wisconsin, Eastern Missouri, Eastern Iowa, and Michigan’s Upper Peninsula.
About the presenter:
James Heaphy joined Satorius Intec as a tank and hopper specialist in early 2015, and currently serves as national sales manager. Prior to that, Jim spent 10 years at Avery Weigh-Tronix as a technical sales account manager. Over his career, he also held sales and service positions at companies like SPX GSE and Controlled Power, and served as a US Navy electronics technician for six years.