Whether performed automatically or manually, precise sampling is crucial to accurate performance measurement. Today, realizing the importance of sampling accuracy, more and more processors are automating the sampling procedure. Automated sampling eliminates human errors associated with manual sampling such as inconsistent sampling durations, especially when short second to second samples are required, and streamlines the process of data handling. Automated sampling involves the use of a precision scale with output to a computer. Software controls the acquisition of weight data as the feeder discharges material onto the scale.
The recommended procedure for automated sampling is called differential dynamic sampling. This highly accurate method involves employing a simple scale that has a full scale capacity that is sufficient to weigh accumulated weight of 30 samples and has a scale resolution that is atleast 10 times greater that the smallest individual sample weight desired. The sample scale must also be able to communicate sampling weight as frequently as six times per second, automatically computing the difference between successive ‘micro-samples’. These values are then totalized over the desired sampling size or period to form a single ‘macro-sample’. This process is repeated until the desired thirty macro-samples (for repeatability measurements) or ten macro-samples (for linearity measurements) are obtained. Conventional statistical processing is then automatically performed to determine repeatability performance (@ 2 sigma) or average sample weight (for linearity).
Note that automated sampling is the only means available to reliably determine feeder accuracy over timescales shorter than one minute. When taking short duration samples, human error in timing the samples becomes too great a factor to produce a meaningful result.
While the trend is toward automated sampling, manual sampling is still frequently employed when calibrating a feeder in the operating environment. Tools include a watch, two containers, a sampling scale, a record keeping worksheet, and a calculator. Whether testing for linearity or repeatability, the procedure is basically the same. With the desired setpoint value dialed in and the feeder running, material flow is channeled from the process by a flap-type flow diverter (or similar means) into one of the containers. At the start of the timed catch sampling period the sampler quickly slides a clean, empty container into the material stream, positioned so that all material is discharged into the container. At the end of the timed sample interval the sampler cycles the other container into position and, while it is receiving material, records the weight of the contents of the first container. The sampler proceeds in this fashion, weighing one sample while the next is being obtained, until the desired number of consecutive samples is taken.
To minimize errors in manual sampling several safeguards must be observed:
- Since there will probably be a difference (however small) between the weight of the two empty catch sample containers, each container should be tared separately. If the scale being used to weigh the samples does not have provisions for storing two tare values, the heavier container should be tared out and weights affixed to the lighter one to bring its weight up to that of the heavier one.
- The sample weight must be large enough to make human error in sampling negligible. Most feeder manufacturers specify that samples should be a minimum of one minute in duration or one pound in weight, whichever is greater.
- To minimize variations in sampling technique, the same individual should catch all samples.
- Samples must be taken consecutively.
- The resolution of the sampling scale must be sufficient for the purpose, with 0.1% of sample weight a minimum resolution requirement.