4.1Calculation of Sensor Elevation
Readings can be used to calculate the elevation of the sensor and to plot them on a graph versus time. The graph should also show the elevation of the fill above the sensor at the time of each reading. A plot of temperature can also be included. For the standard 4660 settlement system, using type 4500SV or 4500ALV transducers, the readings will get smaller as the sensors settle relative to the reservoir.
For these sensors, the elevation (E) of the sensor is given by:
E = E0 – (R1 – R0) G + ΔERES
equation 1: Elevation
Where:
E0 is the sensor elevation at installation.
ΔERES is any change of the fluid level inside the reservoir sight glass.
(If the fluid level falls, ΔERES is negative. If the fluid level rises, ΔERES is positive.)
R0 is the initial sensor reading.
R1 is the subsequent sensor reading.
G is the calibration factor supplied with the sensor. (A typical calibration report supplied by the factory is shown in Figure 10.)
Please note: The calibration report shown in Figure 10 was developed using a simple manometer and is good only over a range of three meters (ten feet) height differential between reservoir and sensor. If this range is exceeded by the initial setup, or by large amounts of settlement, then there are two options:
1.Continue to use the calibration report shown in Figure 10.
2.Use the second calibration report, supplied with the equipment, which was developed by calibrating the pressure sensor over a wider range.
For Example:
If:
E0 = 541.62 meters
R0 = 9030 digits
R1 = 8800 digits
G = -0.001765 meters/digit
ΔERES = –10 mm (i.e. the level of water in the reservoir sight tube is 10 mm lower than the level measure at the time of the initial reading).
Then the new sensor elevation equals:
E = 541.62 – [(8800 – 9030) x –0.00175] + (–0.010)
E = 541.204 meters
In other words, there has been a settlement of 0.416 meters.
4.2Correction for Settlement or Heave of the Reservoir Terminal
Periodic level surveys should be made of the elevation of the concrete pad on which the reservoir terminal is located. Any measured settlement of the reservoir should be subtracted from the calculated sensor elevations.
4.3Corrections for Temperature
Temperature effects on liquid volume (liquid density) and on the expansion and contraction of the liquid confines can be quite complex and, in some ways, self-canceling. Liquid lines in fills are generally well insulated so that temperature effects tend to be insignificant. Systems exposed to the atmosphere and to sunlight may suffer from rapidly changing temperatures at different parts of the system causing significant fluctuation of the readings. In such cases, precautions may be necessary to obtain readings at times of maximum temperature stability, and/or to apply "system temperature corrections", as described below.
Temperature effects on the sensor can be corrected for but are usually quite insignificant, especially if the sensor is buried, and can be ignored in most cases.
The elevation (ET) corrected for changes in sensor temperature is given by:
ET = E0 – [(R1 – R0) G + (T1 – T0) K] + ΔERES
equation 2: Elevation, corrected for temperature
Where:
T0 is the initial temperature.
T1 is the current temperature.
K is the temperature correction factor included on the calibration report.
A thermal factor for the sensor alone is given on the calibration report. A thermal factor for the entire system could be determined empirically by measuring the temperature as well as the sensor outputs at times when no settlement is taking place and then calculating the K factor from the slope of the line of a plot of temperature v readout digits x gauge factor.
In practice, system temperature corrections are more effective in minimizing the effects due to changing temperatures. A system temperature correction factor is detemined empirically, by measuring the temperature of the reservoir (using the thermistor so provided) at 2 or 3 times (or more) different ambient temperatures, along with the corresponding sensor readings, at times where no settlements taking place. The system temperature correction factor is then determined from the slope of the line from a plot of temperature vs sensor reading x the calibration factor (provided on the calibration sheet).
Where systems are connected to dataloggers, apply system correction factors rather than sensor temperature correction factors. Do this by connecting the thermistor in the reservoir to the datalogger via a separate cable,by using a 3-pair cable from the reservoir to the datalogger, or by simply ignoring the sensor thermistor (by disconnecting its green and white leads), and connecting the reservoir in its place (see Section 2.4).
Figure 10: Typical Calibration Report