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Which calibration method should I use?
There are four types of calibration, which include:
1. Hardware Calibration Using Certified Weights
2. Hardware Calibration Using Material Substitution
3. Hardware Calibration Using Test Weights with Material Build
4. C2® Electronic Calibration (Hardy Instruments only)
Most users ask when do I use which method? To answer, you first need to understand the different methods.
Hardware Calibration Using Certified Weights is typically used in an environment where multiple scales are in the same process and the outputs are compared. Where a standard is set, and that standard must comply to allow a product release. Also, when the product weighed is the same or general range.
A scale is most accurate at zero and the span weight applied. To improve weighing in a particular weighing range, use that weight as the span value in the calibration procedure. For example, to weigh 5 kg bags on a checking scale first calibrate at zero and then span using a 1kg test weight. This creates a very accurate scale for that target weight. Scales that measure a variety of weight use the 80 to 100% of the highest expected load as the span weight.
The recommended minimum weight you should use when performing a hard calibration is 80-100% of your target load. Loading 500 lb. bags, the span weight is 400 lb. to 500 lb. This gives you the most accurate and repeatable weight readings. This method uses known calibrated weights to apply a load on the load-receiving elements. Because the traceability of the standard is known and maintained, it is the most accurate calibration technique. Accuracies within + .05% of the applied load can be achieved with this method.
This procedure, however, may be restrictive for some industrial scales, as large capacity weights are not easily transferred to the site or are often unavailable in the field. In many other cases, the load receiving element may not be able to accept placement of the weights or be in a poorly or inaccessible location. An example of this would be a vessel hanging high above the factory floor or an open top vessel without any place to install test weights of a required amount.
To use the certified weights method of calibration, a set of Class "F" accuracy weights (within 0.01% of denomination) that meet the following regulations are required:
a. A total weight of 80% to 100% of the system capacity.
b. At least three weights between 10% and 100% of the system capacity to check the mid-range.
c. Several low-capacity weights equivalent to one or two instrument divisions.
d. This precaution is necessary due to the nature of a Test calibration. Test weight calibrations can hide or mask scale problems like non-linearity and binding. Without various levels of weight testing the scale health cannot be determine, just by a hardware calibration.
e. Most local Weights and Measures organizations have such weights available. Typical weight values are 10, 25, 50, 100, 500 and 1000 pounds.
If the scale is a device other than a platform, some method of loading the weights on the structure must be devised. Also ensure the test weights are placed as close as possible to the center of the scale and the load is balanced. Too many weights placed on one side or the other can cause a weighing moment that is not present in the normal operation of the scale. In that case material substitution would be a better calibration solution.
When certified test weights are not available or usable, accurately weighed material may be used to calibrate a system. In this method, material weighed on a secondary, calibrated scale is delivered to the site of the scale to be calibrated. The secondary calibrated scale should be of the same accuracy or greater and have a capacity approximately equal or less. A smaller scale has a finer resolution and produces a better accuracy. Material introduced, either a liquid (water) or a dry product (sand) is dependent on what the vessel is designed to weigh.
The methods used for delivering the material for calibration must not be overlooked since they may lose or gain weight during transit (i.e. if a truck is part of the delivered weight, it loses fuel during the transfer). Snow, rain, or other environmental characteristics might alter the precision of the weights. A liquid such as water can be measured by volume and its weight calculated. Accuracies of up to +0.15% of applied load can be achieved.
A commonly used method of material substitution called "Warm Body Cal." is easy but yields a much lower accuracy. With this calibration method, multiple workers are weighted on a secondary scale, and then climb onto the scale (vessel) being calibrated. Extreme care must be taken to ensure no injuries to personnel or damage to equipment occurs. This method is used when test weights will not physically fit on the scale or are not available in large enough quantities. If it is not possible to transport enough weighed material a combination of method one and two may be used.
This method is used when calibrating large capacity scales for which is not feasible to obtain certified weights to equal 80% of capacity. This method employs a combination of certified test weights and material substitution. It requires class "F" certified weights equal to a minimum 5% of full capacity and a means of attaching the weights to the scale. This method also requires a means of filling, transporting, and discharging the material (dry or liquid) to the scale being calibrated.
In this method, the scale is calibrated with the certified test weights. The test weights are removed, and material is added to the calibrated span weight. The weight is then recorded, and the certified test weights are placed back on the scale to verify the material weight plus the test weight totals agree.
This process is repeated until 80% to 100% of the scale capacity is reached. At that point the scale must be re-spanned to give a calibration point at the 80-100% of target load. This method can produce accuracies of up to +0.25% of the applied load.
With all these methods there is another method that removes any of the guesswork in calibration. It is a combination of electronic pre-calibration testing and test weight for verification. This is the Hardy Process Solutions C2® calibration method described next.
Each individual load point (load cell with C2® and mounting hardware) in a C2® system has its performance characteristics electronically stored in a memory device at the end of its cable or in the case of Advantage Load Points internal to the load cell body. These performance characteristics decide how that load point will perform. They are measured on a NIST traceable test standard and recorded when the load cell is manufactured. The C2® system uses these parameters, the instruments' characteristics, and a reference point to mathematically calibrate the scale. If the C2® memory device is destroyed, the load point will still act as a standard load cell that can be calibrated utilizing hardware techniques.
As with any of the described calibration methods, test weight must be used to verify any scale calibration. C2® calibration only needs 5-10% of the target weight to determine if the scale is accurate. Through field-testing of actual installations, Hardy has found C2® to be equal to and in many cases better than calibrations using test weights. Using certified test weights with weight equal from eighty to 100 percent of the scale capacity yields the most accurate calibration. However, conditions for calibrating are often less than ideal. Many vessels lack a place to put or attach the required capacity of test weights. In addition, there may not be a way to distribute the weights equally on (or in) the scale vessel. Some vessels are mounted high above the plant floor or in areas offering limited accessibility. Still others have weight capacities far in excess of the available test weights.
Multiple calibrations over time can also yield dissimilar results. This can be due to different personnel using diverse calibration methods, using different test weight quantities, or to the placement of these weights on the scale. The result of these real-world issues likely would produce an error prone calibration. With C2®, these considerations are no longer an issue. As soon as the scale system is installed, it can be C2® calibrated, and a proper scale installation verified. The result is a calibration that is easier, quicker, safer, and typically more accurate, without the need for highly skilled labor.
Requirements for C2® Calibration:
1. C2® certified Hardy Process Solutions load points
2. C2® certified Hardy Junction Box to handle extra wires (if using more than one load cell)
3. C2® certified eight-conductor cable to handle the extra extra signals.
4. Sense jumpers removed and the sense lines installed.
5. A mechanically correct load cell and scale installation.
a. Level
b. Correct piping installation
c. Correct parameters set in the instrument.
d. Installation instructions properly followed.
6. C2® certified weighing instrument: Hardy Process Solutions weight controller, weight processor or weighing indicator with C2 functionality built in.
7. Certified test weights equal to one or two graduations and 5-10% of the target load.
8. Some instruments can adjust for errors in the system. These correctable errors are limited to gravity and electronic instrumentation error. Errors caused by binding, unstable mounting or outside interferences should be repaired, not corrected electronically. Refer to the troubleshooting guide in the back of the Hardy instrument's User Guide to help clear large errors. Typical system errors are 0.15% to 0.2% of rated capacity. Error for Gravity is around the 0.07% range and varies the closer and further your system is from the 45th parallel.
C2® Calibration enables spot calibrations to be done quickly and easily by onsite personnel, but there’s more to preventative maintenance than just calibration. To ensure the best-continued performance of your processes, we recommend periodic inspection and calibration of all weighing equipment using a Hardy Service Technician. Hardy technicians are capable of servicing any manufacturer's equipment, and calibration can be scheduled around planned maintenance cycles to minimize manufacturing interruption.
For a fast and easy quotation, please contact one of our service specialists at 800-821-5831, or 858-278-2900 option 4, or email us at hardysupport@hardysolutions.com.
