106 Part | II Mechanical Measurements TABLE 10.1 Some guidelines for selecting a suitable level sensor. Adapted from Lazenby (1980) Is remote control or indication desirable? With level indicators, is time spent in taking a reading important? Can the sensor contact the material being measured? Must weight be measured rather than height? Are there objections to mechanical moving parts? Is the application in a liquid? Is the application for a powdered or granu- lar material? Do level indicators have to be accurate to about 2 percent? A yes excludes mechanical float gauges, sight glasses, dipsticks, and other devices designed specifically for local control. A yes excludes dipsticks, sight glasses, mechanical float gauges, and certain balance systems. A no eliminates all but ultrasonics, radiation, radar, and optical and load cells. A yes means the choice is limited to load cells, but in uniform-sided tanks, other devices such as capacitance meters can be calibrated for weight, particularly if a liquid is being measured. A yes gives one a choice of sight glasses and capacitance, ultrasonic, radiation, conductivity, radar, load-cell, optical, thermistor, and bubbler devices. A yes eliminates vibrators and certain paddle types. A yes eliminates dipsticks, sight glasses, floats, themistors, conductivity devices, pres- sure (except pressure switches) instruments, bubblers, displacers. A yes eliminates thermistors, vibrators, paddles, optical devices, suspended tilting switches, and conductivity instruments, as those types only provide control. All other types can be considered, but the poorest accuracies probably come from float gauges and radiation instruments. A yes reduces the list to dipsticks, some of the displacers, and balance devices. Do level indicators need to have an accu- racy that is a lot better than 1 percent? 10.3 calIbratIon of level-measurIng systems Contents that are traded for money, such as petrochemicals, foods, milk, and alcohol, must be measured to standards set by the relevant weights-and-measures authority. official approval of the measuring system and its procedures of use and calibration is required. in such cases the intrinsic value of the materials will decide the accuracy of such measure- ments, which often means that the system and calibrations must comply to very strict codes and be of the highest accu- racy possible. The use of the indirect process of determining volumet- ric or mass contents, based on a level measurement, means that a conversion coefficient or chart of coefficients must be prepared so that the level measurements can be converted into the required measurement form. Calibration tables for a large fabricated tank are most easily prepared using the original engineering construc- tion drawings. This, however, is not an accurate or reliable method. This is especially true because of the difficulty in obtaining accurate "as-built" details on many vessels other than custom-built process reactor vessels. A more accurate and traceable method (known as strap- ping) is to actually survey the container's dimensions after it is built. This can be a time-consuming task. The values can be used to calculate the volume corresponding to different levels. From this is compiled a conversion chart for manual use or a "lookup table" for computer use. By far the most accurate method, however, is a direct volumetric calibration of the container by which a suit- able fluid (usually water) is pumped in and out to pro- vide two readings, the tank passing it through an accurate flow-metering station. While this is in process, level data are recorded, enabling the conversion factors to be provided for each level measurement value. These results will often require correction for temperature, as has already been dis- cussed. Another volumetric calibration method is to pump the liquid into a tanker vessel, usually a truck, which can then itself be weighed and the tare of the empty vessel sub- tracted to produce an accurate measure of volume. Highly accurate level measurement requires continuous monitoring of the various error sources described earlier so that ongoing corrections can be made. A continuous mainte- nance program is needed to clean floats and electrodes and to remove unwanted sediment. in many instances the use of hand dipping is seen as the ongoing calibration check of the level measurement. For this, rods or tapes are used to observe the point at which the contents wet the surface along the mechanical member. obviously this cannot be used for dry substances; for those, the rod or tape is lowered until the end rests on the surface. in each case it is essential to initially establish a perma- nent measurement datum, either as the bottom of the tank where the rod strikes or as a fiducial mark at the top. This mark needs to be related to the transducer system's readout and to the original calibration.