My Load Cell Calibration Does Not Match What the Certificate Says
Being in the business of calibrating force and torque instruments means you will run across scenarios where the equipment sent in for calibration performs differently than the last calibration.
You might also run into a situation where the end-user does some checks on their equipment. When the data does not agree with what the calibration certificate says, who is right?
This article provides several possible scenarios at a high level as to why your load cell calibration does not match what the certificate says.
There are likely thousands of variables that could impact the results, and we wrote a 200-plus-page guidance document available for free. Info on our free e-book is available here.
When Morehouse calibrates load cells, the conditions are ideal being an ambient temperature 23 degrees C ± 0.5 and machines, and adapters designed specifically to minimize errors. Our force machines are very plumb, level, square, rigid, and have very low torsion.
Any variation in loading conditions will likely cause errors. These errors can be very small from 0.001 - 0.005 % if the force measuring equipment purchased is compensated accordingly for side load and various other conditions.
The poorer equipment (not necessarily cheaper), though many cheap load cells, do not perform well, can have errors well exceeding 0.3 % - 1 %.
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Let’s start with why your load cell calibration does not match the calibration certificate.
The calibration provider messed up. It happens from time to time. Different laboratories employ people, and if there are not enough cross-checks, historical information, and detailed notes on the system, there is a much higher probability of a lab messing up the results.
When we send our equipment for calibration at Morehouse, we perform several cross-checks as we have been burnt badly with the dreaded OOT (Out Of Tolerance).
We had one of our four temperature monitoring probes out and were told it was off by about 50 %. This would not have been possible as the other three probes were used in the same lab, and the temperature control (thermostat) had to be off by a factor of ± 10 degrees.
It was not the first time this had happened. We have several stories of vendors not calibrating equipment properly.
So, did the provider mess up?
- Maybe
Before we blame our calibration provider, let us discuss some possible reasons why the load cell calibration does not match the certificate. There can be many reasons why the load cell calibration does not match previous data or what the certificate says it should be.
The first of these reasons is that the purchase order provided was incomplete.
Did the Purchase Order have explicit instructions on how the device is used and how it should be calibrated?
- Please calibrate to ISO/IEC 17025 and provide “As Found/As Left” data is a common statement that doesn’t tell the calibration lab much.
- A purchase order that states things that impact the results, like, please calibrate loaded through the threads in compression, using the supplied adapter threaded into the load cell approximately 1 inch, and use the mating compression pad, gives explicit instructions to the lab technician. Instructions that can help eliminate several variables about why your load cell calibration does not match the certificate.
If explicit calibration instructions are not provided, the provider will likely use different setups or adapters. Note: There is a lot more to cover on purchase order requirements. This article focuses on where proper instructions are given to allow the lab to perform the calibration and how to replicate use.
Force measurement is about keeping the line of force pure (free from eccentric forces) and replicating use.
Many of these errors are in our e-book. However, we can list many of the possible errors. They are:
Cable Stiffness and Mounting – Was the load cell setup correctly with the live end facing the force and with the cable mounting so as not to introduce a side load? If not, there could be a difference in the load cell output impacting the results. In this scenario, the first few test points should be impacted the most.
Using different adapters than what was used for calibration – This topic is pretty straightforward. If what is sent for calibration does not have mating adapters, then the calibration lab will likely use their adapters. Our paper on recommended compression and tension adapters covers this in detail. Using different adapters could easily be why the load cell calibration does not match the certificate.
Using Mass Weights instead of Force Weights – This is an error we often see, so much so that we wrote a 20-plus-page guidance document. Mass is not force, and errors of 0.5 % are not uncommon. Our guidance document on mass to force can be downloaded here.
Misalignment – Is the load cell aligned properly? Different load cells can produce large errors from the slightest bit of misalignment. An excellent way to test this is to vary the alignment of the load cell and see how much the output changes. Misalignment could be why the load cell calibration does not match the certificate
Thread Depth – Does the load cell have a hole that something needs to thread into? If so, many load cells are designed where the engagement of what is threaded into the load cell will change the output. We have a youtube video explaining more about threaded adapters.
Thread Engagement – Here’s a blog on some additional thread engagement errors. If the load cell has threads, this could be the reason the load cell calibration does not match the certificate.
Loading through the bottom threads in compression – Covered in our e-book. Some load cells are more symmetric when loaded through top and bottom threads. Morehouse shear web-type load cells have a 0.012 % difference when loaded against the base versus through bottom threads.
Calibration of Button Load Cells – The basics are that any misalignment will produce significant errors, and adapters are needed for calibration to align the load cells. This topic is covered in our e-book; here’s a video link.
Cable Length - Is a different cable being used than what was sent for calibration? If so, the differences caused by varying the resistance could be significant. More information on four versus six-wire cables can be found here.
Not Following Published Standards – We see this a lot in that many labs might not fully comprehend standards such as ISO 376 or ASTM E74. Our article on the top three mistakes can be found here.
Different Excitation Voltages or wavelengths – Is a different indicator or setting on the indicator different from what was done when the system was calibrated being used? This topic is covered in our e-book and several standards. Our free e-book is available here
Span Issues – Span errors can be large depending on the indicator and load cell. Our paper Converting a mV/V Load Cell Signal into Engineering Units explains more about these potential errors.
Errors From Used Batteries – Some indicators do not regulate the voltage well enough. More information on this topic can be found here.
Molecule Excitement Decline – Has the load cell been exercised to full capacity before use? If not, there can be differences in output. One of our published articles on this can be found here.
Proper Pin Sizes with Tension Links – Different size pins will significantly impact the results. In this video, we show an error 17 times the manufacturer’s specification and how to correct these errors.
Ascending versus Descending Curves – Are you making descending measurements, and your load cell is only calibrated with increasing forces? A load cell must be calibrated with a descending range if it is used to make descending measurements. Click here to learn more about hysteresis on load cells.
Timing Errors – Differences in timing can impact the results. Morehouse applies forces at each set point for approximately 30 seconds. More information is available here.
Appropriate Exercise Cycles (Especially when switching modes) – Any force or torque device should be exercised to capacity at least three times before measurements are made. Not doing so can lead to high zero errors, amongst other things. Click here to read more.
Not Switching Standards to Verify the Entire Loading Range – covered in our e-book.
Different Hardness of top adapters - The Hardness of the top block used for calibration can result in errors as high as 0.5 % of applied force. Click here for more information on why a different top block could cause your load cell calibration not to match.
The flatness of Load Cell and Adapters - A Load Cell with a Flat Base can be crucial to obtaining great calibration results. More information on the flatness of the load cell can be found here.
Temperature Differences – Most load cells are temperature compensated and operate at different temperatures than which the calibration is performed, which will have additional errors. Our post on the top five specifications to consider for load cell accuracy covers this topic.
Errors in the Calculations – Specifically, interpolated zero or the reduction in zero between data readings. Several laboratories use different methods for the reduction of zero. It might shock everyone that there are differences in how the ending zeros are reduced at the NMI level. We have a spreadsheet comparing these different methods.
Using the Appropriate Adapters – So much of the calibration comes down to proper adapters. Our e-book and paper on adapters cover this topic in more detail. It is one of the main reasons that load cell calibration does not match.
The equipment used to make the measurement is not sufficient – If your load cell calibration does not match the calibration lab, chances are your equipment to measure force, could be suspect. Good force equipment should be plumb, level, square, rigid, and have low torsion. More information on equipment used to measure force can be found here.
Additional information on load cell stability, namely errors in manufacturing and how these changes impact your uncertainty budgets, can be found here.
Other questions on why my load cell calibration does not match what the certificate says are below.
Did you switch something in the system, or did someone change the indicator settings?
- If an indicator setting was switched, a scaling factor was entered incorrectly, coefficients were not entered correctly (very common), or several other factors could impact the results. Our recommendation is to foolproof the system as much as possible and not use indicators that require input and read values to be entered anytime someone wants to switch modes or load cells. This is a recipe for disaster, as entry errors are easy to make without the ability to verify the forces.
Note: None of our meters use input or read values because these errors are too common. We do have meters that use coefficients from the calibration certificate. Careful care must be taken to validate that the coefficients are entered correctly. Double-checking these values is always recommended. More information about Morehouse indicators can be found here.
Was the cell damaged?
-Many customers become shocked to learn that their load cell has shifted because it was improperly used. That could be because a technician overloaded the load cell; it could have been dropped. Small load cells can be overloaded by shutting a case on them. an overload will shift the output and can be the reason the load cell calibration does not match the certificate.
Luckily, there is a device to test your load cells before they are sent for calibration. This device is a load cell tester.
Did damage occur during transport?
- Did Oops drop something? Did the item get damaged and repacked? The best advice is to buy cases to house your equipment and prevent damage. Our load cell cases have protected load cells for years, and out of thousands of shipments, we have only seen one major damage incident, compared with at least one damaged item per week from improper packaging or handling.
My Load Cell Calibration Does Not Match What the Certificate Says Conclusion
There can be numerous reasons why my load cell calibration does not match what the certificate says. The best advice we can give is to standardize your process, communicate with your calibration provider how you are using the load cell, and send in any fixturing you use with your load cell. Read our e-book, and subscribe to our newsletter to keep learning more about your equipment and best practices.
Lastly, purchase the right equipment to help eliminate entry errors, purchase a shipping case or make one with custom foam, and purchase a load cell tester to monitor your equipment. Do all this to reduce the chances of having to ask why your load cell calibration does not match that of the calibration provider.
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Everything we do, we believe in changing how people think about force and torque calibration. We challenge the "just calibrate it" mentality by educating our customers on what matters, like information about why the load cell calibration does not match the certificate., and what causes significant errors, and focus on reducing them.
Morehouse makes simple-to-use calibration products. We build awesome force equipment that is plumb, level, square, and rigid and provide unparalleled calibration service with less than two-week lead times.
Contact us at 717-843-0081 to speak to a live person or email info@mhforce.com for more information.
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