Ultrasonic sensing, part 1: an effective and improved approach to gas-flow metering


Ultrasonic sensing is an emerging technology in the industrial and residential gas-flow metering space. In fact, ultrasound is a better alternative to mechanical flow metering because it enables the measurement of much lower flow rates while consuming very little power. Because there aren’t any moving parts in an ultrasonic flow meter, these meters are also not subject to problems introduced by mechanical wear and cost less to manufacture.

Ultrasonic gas-meter solutions typically rely on a time-of-flight measurement determined with a timed threshold crossing of an upstream and downstream ultrasonic signal. TI’s solution captures these upstream and downstream signals with an integrated analog-to-digital converter (ADC) and determines time of flights via digital correlation. Our solution subsequently determines the volumetric flow rate and the user-definable geometric properties of the flow tube.

Because the correlation of the signals acts like a filter, this approach is less susceptible to noise. And because amplitude variations in the signals don’t significantly impact their correlations, our solution is also robust in the context of these variations. The excitation of transducers over a broad frequency range makes it possible to achieve higher measurement accuracies without requiring recalibration as transducers age or are exposed to varying temperatures.

Ultrasonic gas meters often specify registration errors over anticipated flow rates, along with minimum detectable flows. Typical solutions offer minimum detectable flows of 40-120 liters per hour (lph). The zero flow drift of the meter over temperature (typically -35°C to 65°C) determines the minimum detectable flow rates.

Typical solutions operate with 5 to 20ns of zero flow drift, while our solution can enable products with less than 1ns of drift. Because broadband frequency information is encoded in the excitation signal, TI’s advanced proprietary algorithms can accurately register low-amplitude signals, eliminating the need for high-voltage excitation and reducing associated system cost and power.

In conjunction with this reduced excitation voltage, TI’s ultra-low-power low energy accelerator enables much lower measurement currents by enabling multiple vector operations performed in parallel without central processing unit (CPU) intervention. As can be seen from Table 1, TI’s ultrasonic gas metering solution offers a significant improvement in zero flow drift, minimum detectable flow, and power consumption over other existing ultrasonic solutions.

 Ultrasonic gas-metering solution

 Zero flow drift

 Minimum   detectable flow

 Power consumption

 Typical solutions 

 5-25ns

 40-120lph

 >100µA measure/second

 TI ultrasonic gas-metering solution 

 <1ns

 <7lph

 <20µA measure/second

Table 1: TI’s ultrasonic gas meter solution vs. existing solutions

TI also provides an ultrasonic gas-meter software library and PC graphical user interface (GUI) as part of a  reference design that enables accelerated development and testing of ultrasonic gas-metering products. The GUI enables the characterization of new ultrasonic meters and provides a convenient method for evaluation and testing.

Figure 1 is a GUI snapshot of the ADC downstream and upstream captures. The upstream waveform is the red line and the downstream capture is the blue line. The GUI enables you to immediately determine the impact of changes in ultrasonic excitation or flow-tube design.

Figure 1: ADC capture of ultrasonic upstream and downstream waveforms

Many of the high resolution ultrasonic sensing techniques described here can also be applied to a variety of other applications. In the next ultrasonic sensing blog I’ll discuss how accurate absolute time of flight measurements can be calibrated to enable gas concentration measurements with less than 0.5% error.

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