Load Cells and Tensile Testing

For many years I have been saying that BEFORE you plan your rigging, you need to know the direction and magnitude of the force that the rigging must hold. I have also been doing destructing testing of hardware and fabrics, so that I understand how they fail and under what loads. This helps me understand how to inspect these items and when they should be retired from service. Lately, other have begun to understand my thinking and there has been a great deal of interest in measuring forces in both the aerial and theatre communities. The purpose of this edition of The Flywire is to discuss some of the popular types of tensile measuring devices available and their appropriateness to your needs.
 
Types of tests
            Lets begin by looking at two measuring needs: Tensile loads and Shock loads. To begin, both of these tests involve measuring tensile (pulling forces). However, a tensile test usually measures a static (or relatively static) load. Destructive testing usually involves very slowly increasing the tensile force on an object until the object fails (breaks). Load cells are also used to measure the load hanging on chain hoists or suspended by a crane. Here also, the load is relatively constant. In contrast, shock loads involve very sudden changes in forces and are also very short in duration. The hardware and software needs to these types of tests can be very different.

Sampling rates
            Load cells, and their software, measure the data at different sampling rates -samples per second (sps).  Lets look at two extremes.
 
            The Ron StageMaster 6000 by Eilon Engineering is a system that allows the tracking of the loads on up to 96 wireless load cells on a computer. 
 
 
 
            These load cells sample the load at a rate of  only 1 sample per second. Pretty slow, but it meets the need of someone needing to know the loads on chain hoists on a concert rig. A similar device that is getting a good deal of attention is the BroadWeigh Wireless Crosby Load Cell Shackle. It has very similar specs to the StageMaster 6000, but is less than half the height.

            A dynamometer is another device that can be used to measure relatively static loads. This device uses a needle to display the load on its round dial face - they are not wireless.
 
            On the opposite end of the testing spectrum is shock load testing. S-type load cells, one of the most popular types, used with data acquisition hardware and software, can measure loads at up to 1,000 samples per second. The data acquisition software that runs on the computer allows users to set specific sampling rates, and measure the high (peak) load, lowest load, or average load over a specific period of time. Multiple meters can be displays to monitor several conditions at one time. This type of system is very versatile, but somewhat expensive - $1,800 range.
 
            The load cell is connected to a terminal box, that contains the instrument module(s), via a long cable.  Some terminal boxes can contain multiply instruments, allowing the user to view the data from several load cells at one time.
 
Below are the components for my testing system:

• Futek Load Cell (I have 3K, 10K and 25K capacity load cells)
• Futek Cable Assembly – 50 ft. (This connects to the load cell to the terminal box)
• DataQ Instruments' 718-B Screw Terminal Access (This box will take up to six instrument modules)
• DataQ Instruments' Strain Module (8B38-05)
• DataQ Instruments' WinDAQ Data Acquisition Software
• Dell Laptop running Windows 98 (I know that this is an "old" operating system, but this computer is dedicated to this one task and it works fine).

 
            Most load cells can be used with standalone data displays at much slower sampling speeds, but also much less expensive. They also do not give you the flexibility of the system described above.



Capacity
            You may have noticed above that I have load cells with different capacities and wondered why not just use the big one for everything? There is a good reason. Load cells systems are not 100% accurate - they are close, but their inaccuracy is amplified by their capacity. Lets say that load cells are 100% to 99.75% accurate. The table below shows the range of inaccuracy of the data reading based on this rate of accuracy.
 
Capacity (pounds)                Error range (pounds)
            50                                           0 - 0.125
           100                                          0 - 0.25
        1,000                                          0 - 2.5
      10,000                                          0 - 25
      25,000                                          0 - 75
 
            As you can see, the larger the capacity of the load cell, the greater the potential error.  Some standalone load indicators "round" the results to a nearest preset level - such as in 0.5, 1, 5, 10, or 25 increments. So, using a load cell whose capacity is as close as possible to maximum force that you expect to need to measure will give you the greatest accuracy.
 
            Users should also realize that the sampling rate can also effect the accuracy of the reading - the faster the sampling rate, the greater the potential inaccuracy, even though the software has move data to work with.  Software that allows the user to set the sampling rate, like the WinData software that I use, allows the user to chose the best sampling rate for for the type of testing and the accuracy required.

Note: Laboratory grade hardware and software allows for very accurate readings at very fast speeds (up to 10,000 sps), but at a much greater cost.
 
Rock Exocita's Enforcer Load Cell
            Rock Exocita's Enforcer load cell (also sold as CMC Rescue Enforcer) has sparked great interest because it is a wireless load cell that is moderately priced ($700) and it claims that it will work for both static weighing and dynamic shock load testing (it has a "Slow" mode that samples at 2 sps, and a "Fast" mode that samples at 500 sps ). While not as versatile as the S-type load cell systems described above, it has some impressive features, including the ability to create graphs of  load reading. I recently had an opportunity to "test drive" an Enforcer, thanks to my good friend Robert Dean at ZFX, and here are my thoughts it.



            The specs on Rock Exocita's Enforcer are a bit confusing.  Rock Exocita's website states that the Enforcer has a 4,500 lb capacity, but the Certificate of Calibration which came with my Enforcer states that it has a 4,000 lb capacity and a Max SWL of 4,500 pounds (see what I mean). Either way, this capacity makes it pretty good for testing of small hardware and most shock loads that involve performers. It has Minimum Breaking Strength is 8,100 lb or 9,000 lb, depending on which material you reference. These numbers are in line with what you would expect, considering the aluminum swivels on each end. On source says the accuracy the Enforcer is -/+2%. This means that the error range is 0-90 lb at 4,500 lb, which is not great. However, the Enforcer app states that the accuracy is -/+ 0.2 kN (45 lb) over the range. Users can set the output from the Enforcer to be displayed in either kN, lbf or kg, which is a nice feature. 
 
            The "Quick Reference Card" that comes with the Enforcer, or can be downloaded from http://rockexotica.com/enforcer/instructions.pdf, describes the Enforcer, including its display and what its four buttons do. The information on this card is minimal and does not describe how to use it for dynamic load testing. If you want to see loads remotely, get graphs of your data reading, or control some features of the Enforcer load cell, you need the "optional" Enforcer app (an additional $29.95). This app runs on Apple iPhone (4S and newer, with iOS 7.0 or newer) and iPads (Generation 3 or newer).  
 
            A "Recording Dynamic Event Tutorial" is available on Rock Exotica's website. These instructions, one page, explain how to use the app to set the trigger load (the force needed to start the Enforcer recording data). This load can be set as low as 450 pounds (2kN). When the Enforcer is triggered, it records four seconds of data (one second before the trigger and three seconds afterwards). The Enforcer can store four data recordings. Recordings can also be triggered manually by the user via the app, if you want to start recording readings a a force less than 450 pounds. Graphs can then be emailed to anyone from your iDevice.
 
            The Enforcer displays the current temperature, either in degrees Celsius or Fahrenheit, which at first seems like a strange feature. The temperature is important to the Enforcer (in Slow mode) because it apparently compensates the force based partially on the temperature, although how it does this is not explained on either the website or in the material that comes with the Enforcer. Temperature is NOT used to compensate the force in the Fast mode.
 
             While the Enforcer has a "reset" button (holding the "Max" button), "zeroing" the display, like with many load cells, is just not possible (unless you are using the "Rounding" feature available on the app).  Expect to see a reading of 4 to 8 pounds, even when there is no load on the load cell. When the Rounding feature is "On," readings on the Enforcer's display will be rounded to the nearest 10-lb increment, while readings on the app are still displayed in one-lb increments.
 
            Unlike most load cells that use Wi-Fi to communicate to their display unit (and can do so from up to 600 feet away), the Enforcer uses Bluetooth to communicate with the iPhone or iPad, and the distance is MUCH more limited.  One user reported to me that he experienced "dropping out" of data starting around nine feet. I had the same issue when I tried to take readings remotely.  In fact, my iPhone completely lost communication with the Enforcer at a distance of only 24 feet.
 
            When used as a stand alone unit (without the app) the load is displayed directly on the Enforcer's LCD display. The Enforcer can be set to display either the current load on the unit or the Max (Peak) load since the display was last reset.  While these features work in both Slow or Fast modes, it should be noted when in Fast mode, the Max load displayed may not be less than 26 pounds (and often more that twice that amount), even when no load is on the load cell.  Since the user is probably going to be measuring a Max load that is greater than 60 pounds, this is probably not a big issue, although it should be noted since it could be confusing. 
 
            When you are using the Enforcer in Slow mode, the current load indicated on the Enforcer's display and the current load indicated on the app are almost exactly the same (often off by no more than one pound).  This is also true of the Max load.  This is exactly what you would expect. However, in Fast mode, the the Max load shown on the Enforcer's display was always approximately 3% greater (around 45 to 60 pounds in my tests) than Max load indicated in the app. This seems to indicate some type of communication issue between the load cell and app. I would have liked to have had time to run more tests to see if I could find a pattern to the discrepancies in the readings, but I did not. 
 
            I also ran a comparison between the load indicated by my S-type load cell and the load readings from the Enforcer.  Using a "known load" my reading from my S-type load cell was "dead-on," while the Enforcer indicated a load approximately 3 to 6% greater, depending on if you were comparing it to the Max load on the app or the Enforcer's display.
 
            You can change between Fast and Slow modes on the Enforcer or via the app. And the app always displays the Max load read during the testing session, as well as the current load. However, there is no "Reset" button on the app, so if you want to "clear" the Max setting remotely from the app, you have to turn it "Off" and then restart it.
 
            Data is graphed on the Enforcer app as the graph scrolls from left to to right.  This means that you must read the graph from right to left - with the oldest readings on the right and the most resent readings on the left. This is backwards from how most people would expect the data to be read (it is also backward to how data is displayed with the WinData software, where you read the graphs from left to right). After a while you get used to the way the graph is displayed, but it was very confusing to me at first. On the positive side, the height of the graphs adjusts automatically to the magnitude of the load, which is a very nice feature.
 
            My final impression of the Enforcer is very mixed.
 
Pros:

• Setup is fast
• No electricity needed, so great in the field
• Two sampling rates give flexibility
• 4,500 lb (or 4,000 lb) capacity makes it useful for many purposes
   

• Accuracy is not as good as I had hoped, especially for the Max load in Fast mode
• Data cannot be read on the app at a distance of more than 24 feet from the Enforcer
• Display can be difficult to see in dim light - this is when the app is handy
• Graphing is backward from what I expect; and in Fast mode, it is limited to four seconds - but neither of these are major issues

            So, will I rush out and purchase one?  Probably not. While the Enforcer is a "good" load cell, it is not a great one. And while the price is good, $700, plus $30 for the app, it is still not cheap. It is certainly easy to set up and use, and there are times when it could be indispensible - when working away from electricity. The biggest limitation is the lack of distance that you can be from the unit. If I could be 200 feet away from the Enforcer and still read the data, I would purchase one immediately, and live with its other short comings. But, the Enforcer is really only useful if you can stay very close to it, or could get to it easily.
 
            If you need a good wireless load cell at a good price and can live with the fact that you must be near it at all times, then the Enforcer may be for you. 
 
 
 One Final Note
            I have only spent two days working with the Enforcer and its app, so I am sure that there are things about it that I do not know yet - some positive and some negative. I have tried to accurately describe its most important features and share with you my impressions of this device. If you have more experience with the Enforcer than I, and you have a different impression of the Enforcer, email me and I will be happy to share YOUR views with the readers of The Flywire in an upcoming issue.