| with any precise measurement device, calibration of a | | | | hard-pressed to find a more expert source available. |
| load cell, or the scale that uses it, is an exact science. | | | | Their suggestions for the calibration of a load cell are |
| Before delving into the subject of calibration, | | | | as follows: |
| letâs take a closer look at the intricacies of | | | | âAt full-scale load, the output it typically 2 |
| the device, to get a better idea of exactly what we | | | | mV/V or 3 mV/V, i.e., one part in 500 of the |
| will be calibrating. A load cell is a type of transducer | | | | excitation or one part in 333 of the excitation. There |
| (meaning that it converts one type of energy into a | | | | fore, for high-precision calibrations, a ratio |
| different type of energy) which reads an initial | | | | measurement is made. We use an 8-1/2 digit digital |
| amount of force or weight, and then converts this | | | | voltmeter in ratio mode, which has an error limit |
| force into an electrical signal. | | | | specification for this measurement of 35 ppm for a |
| This does not happen by way of a direct conversion, | | | | year from calibration. This DVM also communicates |
| but rather occurs in two separate stages. First, the | | | | with the PC by the IEEE-488 bus.â |
| force detected effects one or more strain gauges, | | | | It is also noted in the NIST requirements that these |
| effectively deforming the gauges, which causes its | | | | force calibrations are issued with a given 20 ppm |
| electrical resistance to change. This relative change is | | | | uncertainty factor for the force values, with an |
| then used to measure the strain. While load cells | | | | additional 50 ppm or 33 ppm uncertainty when |
| containing one or two strain gauges do exist, the | | | | considering the readout instrumentation. It is also |
| most common of these devices contain four strain | | | | mentioned that the accuracy of these measurement |
| gauges. The output of this electrical signal is then | | | | ratios can be improved significantly by more frequent |
| connected to circuitry which calculates and provides a | | | | calibration. To reduce the measurement uncertainty |
| digital value of the converted electrical signal of the | | | | by a factor of ten, a single-point ratio calibration at |
| force or weight originally applied to the load cell. | | | | full scale of the range used (100mV), or a 100:1 ratio, |
| After taking a more in-depth look at how load cells | | | | can be used. During NIST tests, a self-calibrating 100:1 |
| provide measurements, it is easy to see how the | | | | ratio divider was used, resulting in an uncertainty of |
| calibration of these devices can be a delicate science. | | | | only 0.5 ppm for this calibration. |
| In addition, any load cell in official use today must be | | | | As said before, the calibration of such delicate |
| calibrated to NIST (National Institute of Standards | | | | measuring systems can be quite a challenge, but with |
| and Technology) standards. NIST, being around since | | | | persistence and the required work, frequent |
| 1901, has worked hard to provide the most accurate | | | | calibration is guaranteed to improve the accuracy and |
| and effective guidelines for calibration of countless | | | | effectiveness of load cells and any other |
| types of measuring systems. This being said, when it | | | | measurement systems in use. |
| comes to the intricacies of calibration, one would be | | | | |