We have been asked a bit more frequently lately about the benefits of calibrating everything as a system versus calibrating the load cells and indicators separately and intermixing them using a calibrated load cell simulator.
Being able to intermix any load cell and meter combination makes a lot of sense. If someone needs two load cells and a meter to do a specific job, they can check out the two load cells and a meter and adapters and perform the calibration or test. The other load cells that are not used would then be available for someone else to use. If done correctly, there is quite a bit of benefit. However, there are quite a bit of disadvantages and obstacles to overcome.
Let’s start by dealing with the requirements of both ISO 376 and ASTM E74 standards. These are standards required for calibrating force-proving instruments, most commonly known as load cells, to calibrate other force-measuring instruments, force machines, hardness machines, testing machines, using ASTM E74, ASTM E4, ASTM E-10, ASTM E-18, ISO 376, ISO 7500 and so on.
ISO 376 and ASTM E74 requirements for meter calibration
ISO 376 in section C.2.11 Effect of a replacement indicator states,
“The deviation between the two indicators should be determined (there are several methods, e.g., calibration of both indicators, use of a common bridge simulator), and the uncertainty of this deviation should be estimated (including factors such as the calibration uncertainty of the indicators and the stability of the common bridge simulator).
If corrections are made, the uncertainty of the deviation should be taken into account. If no corrections are made, the deviation and its uncertainty should be considered.”
Additionally, ISO 376 makes mention of programming indicators using span points.
If one does not use the calibration equation and programs points into an indicator that allows points from the calibration curve to be input so that the display is in units of force or torque but carries out linear interpolation between these points, the effect of this approximation to the curve should be investigated, and an uncertainty contribution should be included.
ISO 376 section 3.1 defines a force-proving instrument as a “whole assembly from the force transducer through to, and including, the indicator.”
One might be thinking, I do not calibrate following ISO 376. Maybe one only uses the ASTM E74 standard or a commercial calibration.
ASTM E74 is a bit more prescriptive in the requirements for substitution. Section 12 is explicitly titled Substitution of Electronic Indicating Instruments Used with Force-Measuring Systems. The standard acknowledges that it might be desirable to treat the indicator and force-measuring instrument separately.
A huge benefit in doing this is if you purchase the same indicators, one could be used as a backup if the primary unit fails. The expensive calibration of the entire system could potentially be avoided.
Then the standard goes on to list conditions that shall be satisfied to substitute a metrologically significant element of the electronic indicating instrument.
ASTM E74 Section 12.1.1 specifically states, “The electronic-indicating instrument used in the initial calibration and the instrument to be substituted shall each have been calibrated and their measurement uncertainties determined. The electronic indicating instrument to be substituted shall be calibrated with traceability to the SI over the full range of its intended use including both positive and negative values if the system is used in tension and compression.
The calibrated range shall include a point less than or equal to the output of the force transducer at the lower force limit and a point equal to or greater than the output of the force transducer at the maximum applied force. A minimum of five points shall be taken within this range. The measurement uncertainty of each electronic indicating instrument shall be less than or equal to one-third of the uncertainty for the force-measuring system over the range from the lower force limit to the maximum force.”
To summarize, you will need to have a simulator calibrated to comply with the standard. The simulator needs to be capable of both positive and negative values if the load cells are used in both positive and negative directions. The simulator must have at least one point less than or equal to the lowest force point value in the range and one for the highest point.
Below is a picture of a Morehouse simulator. This simulator likely cannot be used to satisfy these requirements.
Figure 1 Morehouse Budget mV/V Load Cell Simulator
The first point is 0.5 mV/V and the last one 4 mV/V. If someone had a 4 mV/V (10,000 Force Units) load cell and the range of verified force was 500 through 10,000 Force Units, the simulator at 0.5 mV/V is 1,250 Force Units. If the verified range of forces started at 200, a 0.08 mV/V first step would be required.
Note: The best high-end simulators typically have the first step of 0.04 mV/V or lower as 0.04 mV/V on a 2 mV/V load cell equates to a 2 % llf. A simulator that starts at 0.1 mV/V would equate to a 5 % llf on a 2 mV/V load cell.
2 mV/V is 5,000 Force Units. Using this simulator with a 0.5 mV/V first point, the end-user would need to raise their Class A verified range of forces to 1,250 FU. A situation that does not work for many as they want to capture force values from the first non-zero calibrated point, typically below 5 % of the load cells capacity.
The ASTM E 74 standard gives further guidance by stating the measurement uncertainty of the indicator shall be determined by one of the methods in Appendix X2. It recommends the simulator has a series of mV/V steps of the measurement range with similar impedance characteristics and then states this requirement in section 12.1.2.
“The measurement uncertainty of the transducer simulator shall be less than or equal to one-tenth of the uncertainty for the force-measuring instrument.” ASTM E74 further states, “Excitation voltage amplitude, frequency, and waveform shall be maintained in the substitution within limits to ensure that the affect on the calibration is negligible.
It is a user responsibility to determine limits on these parameters through measurement uncertainty analysis and appropriate tests to ensure that this requirement is met. Substitution of an interconnect cable can have a significant affect on calibration. If an interconnect cable is to be substituted, see Note 15.”
This is interesting as the interconnect cable for the simulator does not always share the same connection as the load cell. If the system is not 6-wire, meticulous care will need to be made to ensure the same gauge wire and length is used for the simulator to meter connection as that of the load cell. Note 15 goes into more detail, and Morehouse has an article explaining why 4-wire systems are not ideal. That article can be found here.
Appendix X2 details all the steps necessary to determine the uncertainty. Morehouse fully supports ASTM E74 and feels the membership is incredible. For under $ 100.00, one can join and get access to a catalog of standards. This author’s opinion is that this is one of the best deals in the industry. Signing up is simply at astm.org. The committee for standards such as ASTM E74, ASTM E2428, ASTM E10, ASTM E18, and ASTM E4 is the E-28 committee on Mechanical Testing.
Summary of Top Requirements for Load Cell Simulator Calibration Needed for Meter Substitution
The summation of what is needed is as follows:
1. At least five readings for each polarity over the range need to be taken.
2. The points need to be less than or equal to the first point in the Class A or AA verified range of forces, and the capacity needs to have a point equal to or greater than the maximum output observed during calibration. So, if loading a 10,000 Force Unit load cell to 11,000 Force Units, which might read 4.4 mV/V, a 4.0 mV/V simulator is not good enough.
3. The simulator shall provide at least one point for every 20 % interval throughout the range. (Interesting tidbit here as the standard says five points, though the simulator likely needs to have the low force point and an additional 5 points to cover up to capacity or higher for a total of six points throughout the range)
4. Section 8. Calculation and Analysis of Data of the ASTM E74 standard provides guidance to determine the standard deviation Type A uncertainty component for calibration of the simulator.
Okay, so the benefits might still outweigh the additional headache of using a simulator and being able to separate one’s load cells from the indicator or decouple the system.
However, there are a lot more error sources one needs to be aware of.
These include Calibration Uncertainty (Gain Error), Zero Offset, Temperature Effect on Sensitivity, Quantization Error, Normal Mode Voltage, Power Line Voltage Variation, Non-Linearity, Temperature Effect on Zero, Gain and Zero Stability, Common-Mode Voltage, Noise, Electrical Loading, Error Signals due to thermal EMF, Difference in cabling if not a true 6-wire system. All these error sources should be evaluated.
Typical Error Sources for Meter Substitution
When calculating Measurement Uncertainty for a meter to be used for substitution, the following are typical error sources:
Simulator Uncertainty includes the resolution of the meter, calibration of the simulator and the associated reference standard uncertainties, stability of the simulator, and the ratio uncertainty. At Morehouse, we achieve about ± 0.00005 mV/V uncertainty on our high-end simulator using different cables for positive and negative output as the polarity switch introduces additional uncertainty.
On the meter side, Non-linearity, Stability, Environmental, Ref Uncertainty from the Simulator, Additional Cable Uncertainties, Noise or Resolution, Repeatability, and Reproducibility.
In our experience, most who choose to use meter substitution add about 0.02 % – 0.04 % uncertainty to their systems. This is too much uncertainty for ASTM Class AA calibrations that are expected to be better than 0.05 %, too much for ISO 376 Class 00, 0.5, and likely too much for Class 1 & 2. For an ASTM Class A the requirement is to be better than 0.25 %. The contribution to uncertainty is often significant, though somewhat manageable.
Morehouse does offer calibration of load cell simulators to comply with either standard. Below is a page from our calibration report for one of our reference standard simulators.
These simulators have high-quality aged resistors and steps from 0.04 – 4.4 mV/V. The standard deviation is less than the resolution, hence the importance of having the resolution as part of the overall measurement uncertainty.
Figure 2 Morehouse Page from Simulator Calibration Report in mV/V
Morehouse Budget Load Cell Simulator
Back to the simulator, that likely is not good enough to calibrate the meters for substitution. Why would anyone want this? The best answer is cost. The simulator is under $ 600.00 compared with a higher-end model that costs over $ 4,500.00 plus calibration. So, what? It does not allow me to calibrate my meter. That is technically correct, though it is a very powerful tool.
Our simulator allows the end-user to do the following:
1. Perform cross-checks on equipment
2. Help control stability/drift
3. Verify coefficients are correctly entered in our 4215 plus, C705P meter; both use the actual coefficients from the calibration report. Verify coefficients for other programs such as Morehouse calibration software.
4. Check for linearity issues in any meter.
5. Use as a diagnostic tool to rule out the load cell meter, leaving the load cell, cables, or adapters as the issue.
6. It can be used to calibrate A/D offset and gain setting.
7. It can be used to set up a new indicator prior to system calibration.
Meter Substitution Conclusion
Morehouse is not the calibration police, and we are here to serve our customer’s requirements best. Personally, I feel it is much better to calibrate everything as a system. I always strive to do what yields the lowest overall measurement uncertainty to limit the overall risk.
There is a risk/reward scenario for separately calibrating the indicator and load cells. There is a lot of additional work required to comply with either ISO 376 or the ASTM E74 standard. If that extra work saves time and money, it might be worth it. Plus, the overall uncertainty increases by an additional 0.02 – 0.04 %, which will be absorbed by everyone else down the metrological traceability pyramid.
Though not suitable for meter calibration following ISO 376, or ASTM E74, our budget simulator can save a lot of time when troubleshooting equipment and verifying everything was keyed incorrectly via coefficients from a calibration report without breaking the bank. We can provide higher-end simulators for indicator substitution, though the cost is likely over $5000.00 depending on the exact steps and requirements.