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Maintaining precise cable tension is crucial across various industries, including engineering, construction, aerospace, and manufacturing. Cable tensiometers serve as indispensable instruments for measuring and monitoring this cable tension. However, the calibration of cable tensiometers is often riddled with issues.

Issues such as clear communication between calibration labs and end users are essential to ensuring that calibration accurately reflects how equipment is used in real-world situations. This is especially crucial because different calibration methods can sometimes lead to discrepancies of over 20%, potentially casting serious doubt on the equipment’s functionality.

This article will examine the essential aspects of successful cable tensiometer calibration: selecting the right equipment, implementing the optimal process, and understanding the end-user’s requirements. We will delve into three crucial factors that can significantly impact calibration accuracy:

1. Cable Length: The impact of different cable lengths used during calibration.
2. Calibration Method: Analysis of various calibration techniques and their influence on results.
3. Cable Type: How variations in cable types (e.g., GAC vs. SSAC) affect calibration outcomes.

We aim to initiate a conversation and encourage best practices for better alignment between calibration practices and real-world requirements, enhancing the reliability of cable-dependent systems across diverse industries.

What is a Cable Tensiometer?

Cable tensiometers are devices used to measure the tension or force in cables, wires, and ropes. They typically feature a load sensor or gage that provides real-time cable tension readings.

Calibrating a cable tensiometer involves knowing the proper requirements, having the right equipment, and the right processes.

How to Calibrate Cable Tensiometers: The Right Requirements

What is needed to accomplish this task? Does cable tensiometer calibration affect the type and length of cable used?

Several variables can influence the readings of the cable tensiometer. The standard variables are:

1. The Length of the Cable.
2. The Method Used for Calibration.
3. The Type of Cable GAC versus SSAC.

To ensure meaningful data, one must determine the appropriate test method that best replicates how your customers use the equipment.

Are they clamping a tensioned cable or adjusting the tension based on the calibration chart?

How to Calibrate Cable Tensiometers: The Right Equipment

The right equipment may also consist of cables of at least three feet or greater free from defects. That means the cable should lie flat and not have any bends or kinks. Some systems use cables that are 18 – 24 inches, and these shorter cables can lead to different results.

Some of these systems even use torsion cells meant to calibrate torque wrenches. Torque equipment used to calculate the force on a cable typically utilizes shorter cables and is likely not the right equipment. This approach is complicated by the calculation of force from a torsion cell and introduces measurement uncertainty and error components that are extremely difficult to quantify and often quite large.

For calibrating any cable tensiometer, it is essential to read the manufacturer’s recommendations, which often include cable lengths and types.

These are key features that can ensure accurate and reliable tensiometer calibration:

• Minimized Strain: A load cell positioned at the base eliminates the influence of any extra strain caused by hanging and clamping the tensiometer.
• Precise Frame: Make sure the frame is meticulously designed to be plumb, level, square, and rigid, minimizing torsion and offering exceptional control during calibration.
• Versatile Calibration: We recommend a machine that allows for two calibration methods: clamping and adjusting the tension, pre-loading the cable to a specific value, and clamping. We recommend cables up to 5 feet long and that generate tension forces as high as 2,000 pounds-force (lbf).

This combination of features includes minimizing strain, utilizing a precise frame, and offering versatile calibration methods to calibrate tensiometers. When evaluating the right equipment, one may want to investigate frames with versatile calibration options.

How to Calibrate Cable Tensiometers: The Right Processes

For cable tensiometers, this might be as follows:

Apply the reference force: Attach the reference standard to the tested cable or wire and apply the known tension or force. Then, clamp the device to the cable, adjust the force to the desired reading, unclamp it, and clamp it again. This method would be equivalent to one fixed point, such as the hanging weight method. There is another method of clamping after a known force is applied.

Note: These methods produce a significant difference depending on the cable size.

Compare readings: Read the tension measurement on the cable tensiometer while the reference load is applied. Compare this reading to the known reference standard value.

Repeat as needed: Take as many measurements as deemed necessary; in our testing, we usually take six measurements at each test point to have confidence in the repeatability numbers.

Record calibration data: Document the calibration process, including the reference standard used, the measured values, and any adjustments made.

Note: It is essential to follow the manufacturer’s guidelines and recommendations for calibration specific to your cable tensiometer model, as the process can vary between devices. 

Cable Tensiometer Known Error Sources: The Length of the Cable

The length of the cable can be a significant error source. This error source makes sense because a shorter cable will be more rigid. Therefore, lower readings will show the most differences between the cable lengths.

In some of our testing, we used the same tensiometer and calibration method to compare cables made from the same material. This calibration method employs a “clamp and adjust” technique. It involves applying a force close to the desired reading, clamping the cable, and then fine-tuning the force back to the intended value before recording the measurement.

The overall results for the “clamp and adjust” technique using different digital tensiometers show a similar trend, which makes sense. The higher the tested value, the lower the percentage difference between cables.

We can draw an analogy between cables and springs. When subjected to the same force, a longer cable, like a spring with double the length, will experience greater stretching. Consequently, compared to a shorter cable stretched by the same amount, a longer cable may exhibit a smaller relative increase in tension.

Think about a rubber band and how a thinner or longer one will stretch more than its thicker or shorter counterpart. A cable acts the same. Steel is much less stretchy, so the length changes are much less.

Also, we are only varying the cable length from a minimum to a longer length. Depending on the thickness of the cable, those who use cables from 18 inches to 24 inches in length will observe higher differences compared with 60-inch cables.

If you were testing a solid piece of wire, the formula for elongation of a uniform cross-section in tension summarizes that force (F), length (L), width (area, A), and material (modulus of elasticity, E) are all related to elongation (e):

e=(F∙L)/(A∙E)

Rearranging gives:

F/e=(A∙E)/L

F/e is the spring rate, which is the amount of force (e.g., lb.) per displacement (e.g., in). So 50lb/in means if you apply 100 lb., it will stretch two inches. Thinking about the cable, we can see that increasing area increases the spring rate, increasing material stiffness (modulus of elasticity) increases the spring rate, and increasing length decreases the spring rate. A lower spring rate means less change in force (tension) when applying the tensiometer.

The cables used for tensiometers are stranded wire and are much more complicated. The same basic principles apply, though there is also torsion and bending, which are not entirely negligible for the measurement.

The Machine and Method Used for Calibration

There are two common calibration machines used to calibrate cable tensiometers:

• Deadweight calibration with one fixed point: The hanging deadweight avoids introducing additional tension, as clamping the cable does not affect the measured force.
• Machine calibration with two fixed points: Clamping the cable in this method shortens the distance between two fixed points, introducing additional tension. 

Calibration Using Deadweight

The Tensiometer Deadweight Calibrator revolutionized calibration processes. A fixed point (shown in the red circle) eliminates the need to secure cables to beams and manually hang weights. This design prioritizes safety and simplifies the calibration technician’s work. The operator secures the cable and selects desired weights up to 2000 lbf.

The design allows the weights to hang in the air, so we call this a machine with one fixed point, as clamping the cable will not create any additional force.

A Cable Tensiometer Machine with two fixed points (shown in the red circles) also eliminates the need to secure cables to beams and manually hang weights. One of these points is attached to a more solid fixture in a load cell connected to a jack (bottom), and the other is connected at the top where there is a spring. The spring can either be set up to be very rigid or have some flex.

However, this machine uses two fixed points to apply tension to the cable, meaning that when we use a tensiometer to clamp the cable, we create additional tension. Cable tensiometers work this way. They have a system that clamps the cable and thus tries to shorten the length between the two fixed points.

Using a cable tensiometer machine like the one pictured above allows us to evaluate two methods. One of the methods simulates that close to that of deadweight.

Method 1 Clamp the tensiometer and adjust the force. This method is when the force is applied close to the desired reading, the cable is clamped, and then the force on the cable is adjusted back to the nominal value. In this case, we might load to 460 lbf, clamp the cable, get a reading of 495 lbf, and adjust the force value to 500 lbf. Since we clamp and then adjust to the force value, the results are like using deadweight as we have manually compensated for the increased tension of the tensiometer by decreasing the distance between the two fixed points.

Method 2 Clamp tensiometer. This method is when the force is applied at the desired reading, the cable is clamped, and the value is read. In this scenario, the applied force might be 500 lbf, and when clamped, the value might jump to 528 lbf.

Replicating Real-World Use:

While the calibration process typically involves securing the tensiometer and applying a known tension first, the cable loading method (Method 2) better represents real-world usage.

In this method, the force is applied to the desired level, the cable is clamped, and the reading is taken. This captures not only the actual cable tension but also the slight internal tension introduced by the tensiometer during measurement, which is present in actual field usage.

For consistency, both calibration methods were tested using the same setup: a single cable, load cell, and a reference machine.

Comparing these two methods using five-foot long 3/16-inch cables yielded a significant difference. The most considerable difference was at the lower test points.

In principle, this makes sense as we shorten the distance between the two cables by grabbing it with the tensiometer. That is a fixed distance that is repeatable; thus, the difference is close to a constant one.

Clamping the tensiometer shortens the effective length of the cable. This modification in effective length influences the increase in tension and forces read, which directly relates to the overall “spring rate” of the system.

Note: Our tests used a digital tensiometer, which does not introduce as much internal tension as some analog tensiometers.

The Type of the Cable GAC versus SSAC

GAC is short for Galvanized Aircraft Cable, while SSAC is Stainless Steel Aircraft Cable.

We used “Method 1” for the comparison tests with two 3/32-inch cables and two tensiometers with the following specifications below.

The Data

Overall, the digital tensiometers performed very well. The standard deviation from test multiple points was typically one lb. or better. The variation between six repeated measurements varied from 1 to 3 lb. per point. The repeatability data varied from 0.41 to 1.03 lb.

Comparing the different cable types SSAC versus GAC

We compared the GAC and SSAC on two different cable tensiometers. The highlighted cell in the above figure from the comparison using S/N 564 shows the measurement in which the unit does not meet the specifications of ± 8 lb. from 100-300 lb., which might result in a failed calibration and an adjustment when the tensiometer is within the specification.

Note: When considering measurement uncertainty, the expanded measurement uncertainty at a confidence interval of 95 % is around ± 0.5 lb., which would decrease the acceptance limits to ± 7.5 lb.

The tension readings between 3/32” SSAC and 3/32” GAC cables vary. The end-user must assess if their specific tensiometer meets their testing requirements for both cable types, considering the observed variations and their associated risks.

The manual for the digital tensiometers confirms this by mentioning that SSAC needs a different calibration than GAC.

If the measurement is at the tolerance limit, there would be a 50 % risk, and the unit is either good or bad. If the specification were from 30 – 100 lb. to be within ± 4 lb. & from 101 -400 ± 8, then S/N 544 would fail using a GAC cable, while S/N 564 would pass.

How to Calibrate Cable Tensiometers Conclusion

Calibrating cable tensiometers involves using the right equipment, such as a calibrated load cell and appropriately sized cables and following specific processes to ensure accurate tension measurements for various cable types.

However, a crucial question remains: which calibration method best reflects the actual end-use conditions for the equipment? Are calibration laboratories actively engaging with their clients to determine the method that best replicates how the equipment is used?

Failing to do so could result in profound implications. If one method is used on an airplane, a pilot may complain about the rudders being too tight, which can impact the ability of a pilot to turn an airplane.

Would you want to be a passenger on that airplane?