Nuclear density measurement instrumentation is the only practical option for some applications from a standpoint of non-invasive and low-maintenance operation. But the question of calibration and performance is difficult since there is not a practical, physical material to use as a standard. You cannot easily “feel” if the density reading is correct.
The quality of a density measurement is composed of two parts: repeat-ability and accuracy. However, the accuracy of any nuclear density system is only obtained after it is installed and calibrated on live process material since it is an inferred, rather than direct, measurement technology.
Nuclear Density Calibration Procedure
After the density system is installed on the pipe and has been powered on for 30 minutes, the system can be calibrated or spanned. This requires running process material through the pipe and sampling data for a period of 60 to 120 seconds. The ideal situation is to have process material of two different specific gravities (SG), that differ by more than 20%, sampled for the two point calibration. Otherwise, one of the data points can be obtained with water (SG = 1.00) running through the line and the second point taken with process material at the nominal operating SG flowing through the line.
A small quantity of the process material running through the line at each data point needs to be collected and analyzed in the laboratory to determine its SG. The easiest way to determine the SG is by measuring the tare weight of one liter of the material, and dividing that weight (in grams) by 1000. The result is the SG of the slurry or solution. The use of a marcy scale is another easy method of determining the specific gravity of a slurry. After each of the data collection periods, the user inputs the laboratory measured value for the specific gravity via the interface software.
The Benefits of Using Absorbers
After the density meter is calibrated on a live process, an equivalent absorber can be created. The equivalent absorber is a shim set constructed of lead or steel plates that can be used to check the measurement quality of the density gauge, without sampling the process material.
After a successful calibration on process material, the SG value for the equivalent absorber can be established. With either air or water in the pipe, the absorber is positioned into the bracket. The shims can be removed or added to the absorber as necessary to produce the desired reading on the density meter. This SG value is then written or stamped onto the shim.
In the future, when a performance check is desired on the density meter, the shim is placed into the bracket with either water or air in the pipe, and the density meter should read the SG value stamped or printed on the absorber label plate.
A reading different from that indicated on the absorber plate indicates possible buildup or wear inside of the pipe. Using the configuration software, the gauge can be standardized so that it once again reads the correct value.