Contactless liquid level measurement using reflectometer chips

Figure 4. Balun and transmission line used in the liquid level sensing example.

Figure 5. Discrete balun and terminated transmission line, before connection to water tank.

Now, the transmission line might be connected to the side of the tank, as shown in Figure 6. It is normal to see a slight reduction in return loss when connected to an empty tank, due to the detuning effect of the tank wall material acting as an additional dielectric layer to the transmission line.

Figure 6. The example design shows the transmission line connected to the side of the water tank.

Example test results

The complete test setup is shown in Figure 7. The transmission line is connected to the side of the tank, and the tank is configured to control the amount of water that is filled and drained.

The Analog Devices evaluation kit DC2847A is used to easily read the measurement results of the ADL5920 reflectometer. This evaluation kit contains a mixed signal processor MCU to read the analog voltages of the forward and reflected detectors. The PC software automatically loads and displays the results (in both graph and time format). The calculation of return loss is very simple: the difference between the forward and reverse power measurements. Figure 7 shows the entire test setup for the design example.

Figure 7. Entire test setup for the design example.

In this design example, the level of the liquid is determined by activating a pump in one of two tanks. When the pump is running, the mass flow is relatively constant, so ideally the water level in the tank rises linearly with respect to time. In reality, the cross-section of the tank is not exactly the same from top to bottom.

Figure 8 shows the test results when the liquid level changes from full to empty. As the liquid is pumped out of the tank, the forward power remains constant and the reflected power decreases linearly.

At t = 33 seconds, there is a noticeable change in slope. This is likely due to the design of the tank. The cross-sectional area at the bottom of the tank is reduced, as shown in Figure 7, to make room for the pump motor. This results in a nonlinear measurement that can be easily corrected in the system firmware if necessary.

Figure 8. Example test results vs. liquid level. The liquid level measurement is linear and unchanged, except for the unexpected behavior caused by the tank design described in this article.

calibration

To achieve the highest accuracy, the reflectometer must be calibrated. Calibration corrects for manufacturing variations in the RF detector inside the reflectometer—that is, the slope and intercept. The DC2847A evaluation kit supports individual calibration, as shown in Figure 8.

At higher levels, calibration of the liquid level and return loss may also be required. This may be due to the following sources of uncertainty:

● Differences in manufacturing distances between the transmission line and the tank wall.

● Difference in thickness of the water tank wall.

● The dielectric properties of the liquid and/or tank walls change with temperature.

There may be system nonlinearity issues, such as the slope variation shown in Figure 8. If linear interpolation is used, then in this case, a three-point or higher calibration is required.

All calibration coefficients are typically stored in the system's nonvolatile memory, which may be unused code space in the embedded processor application, or a dedicated nonvolatile memory device.

Level measurement limitations

A key specification for any reflectometer is directivity. When the transmission line is accurately terminated with its own ZO, the reflected power is reduced to zero, and the reflectometer measures its own directivity, ignoring balun losses. The higher the directivity, the more accurately the reflectometer can distinguish between the magnitude of the incident and reflected waves.

For the ADL5920, directivity is typically 20 dB at 1 GHz, increasing to approximately 43 dB at 100 MHz or less. This makes the ADL5920 ideal for measuring liquid levels in tanks with heights of approximately 30 mm or more (see Figure 3).

Application Extensions

In some applications, the basic non-contact level measurement principle can be extended in several ways. For example:

● Measurements may be performed at a low duty cycle to save power.

● If the fluid level remains constant, the return loss measurement can be correlated to another relevant fluid property; for example, velocity or pH.

● Every application is unique. For example, some technologies may provide better accuracy at the top of the range than at the bottom, or vice versa, depending on the application.

● If the water tank is made of metal, the transmission line needs to pass through the inside of the water tank. Depending on the specific application, the transmission line may need to be immersed in water.

● Measurements at multiple RF power levels can be used to determine if external RF interference is causing errors. Many single-chip PLL devices support this feature, making it a good test for testing system reliability, or self-testing reliability.

● Transmission line sensors on two or four sides of the water tank can compensate for the tilt of the tank along one or two axes respectively.

● If it is used to measure liquid level thresholds, using one or more shorter transmission lines running at a higher frequency would be a good solution

in conclusion

The development of single-chip reflectometer devices such as the ADL5920 has enabled new types of applications, such as fluid level instrumentation. Reliability is greatly improved by eliminating moving parts, such as mechanical floats that have been used for many years. Oil level monitoring has also become possible, driving many new industrial and automotive applications.

footnote

1The presence of liquid affects the impedance, loss and propagation speed of the transmission line.

2 Industrial, Scientific and Medical frequencies. Visit en.wikipedia.org/wiki/ISM_band.

3ATLC: Arbitrary Transmission Line Calculator (for transmission lines and directional couplers). Visit atlc.sourceforge.net.

4. If the impedance is set too large, it will increase the difficulty of transmission line design and cause the transmission line loss to exceed the limit.