Measure the flow of hydrogen by means of ultrasonic waves
As fuel cell vehicles, which run by reacting hydrogen with oxygen in the air, have been put into practical use, there are an increasing number of hydrogen stations in the streets. General households, also, are entering the stage where they practically use home-use pure hydrogen fuel cells that use hydrogen supplied through pipelines. The usage of this supplied hydrogen needs to be measured accurately like that of city gas or liquefied petroleum (LP) gas, in order to ensure fair charging and payment. We are among the first to research hydrogen gas meters to which ultrasonic gas meter technology developed for city gas and LP gas is applied, with an eye towards the forthcoming hydrogen society.
Hydrogen is one of the gases that are the least conductive to ultrasonic waves, as the signal intensity of hydrogen is about a quarter of that of city gas or LP gas (hereinafter referred to as fuel gas). Also, hydrogen characteristically has four times higher sound speed. Its low conductivity to ultrasonic waves decreases the S/N ratio, and also its higher sound speed increases variations in measurements; all these characteristics can cause decrease in measurement accuracy. Moreover, normal gas meters do not require external power supply, running only on an internal battery for ten years; therefore, we aimed to develop hydrogen gas meters that run on an internal battery for ten years. Our hydrogen gas meters were developed with the aim of solving these issues and achieving accuracy and usability equivalent to those of normal gas meters.
Removal of noise
The measurement performance of hydrogen gas meters is affected by noises that are never problematic for normal gas meters, because characteristically, hydrogen gas meters decrease the intensity of ultrasonic waves and therefore are less conductive to signals. To solve this issue, we increased the transmission voltage beyond that of normal gas meters, and also increased the amplification factor. However, simply increasing the amplification factor also increases noise components and does not lead to improvements in the S/N ratio. Thus we took the following measures to remove these noise components to improve the S/N ratio.
■ Removal of enclosure noise (patented)
In flow measurement that uses ultrasonic waves, there are not only ultrasonic signals propagating through the fluid under measurement, but also ultrasonic waves propagating through the enclosure, and ultrasonic sensors unintentionally receive these unwanted ultrasonic waves (these are called enclosure noise).
In fuel gas or the air, enclosure noise is sufficiently small as compared with the intensity of ultrasonic signals, and also the propagation velocity of enclosure noise is much higher than that of ultrasonic signals, making it easy to separate ultrasonic signals and enclosure noise on the time axis. These are why enclosure noise is not a significant problem for measurement in fuel gas or the air.
In hydrogen, however, enclosure noise is relatively high due to ultrasonic signals being small, and high sound speed makes it difficult to separate ultrasonic signals and enclosure noise on the time axis. These are why enclosure noise greatly affects measurement in hydrogen.
Thus, we made the shape of the enclosure complex as shown in the figure below to lengthen the propagation path to promote the attenuation of enclosure noise, making it possible to minimize the effects of enclosure noise during measurement.
■ Removal of transmission noise
In hydrogen gas meters, ultrasonic signals are weak and therefore are affected by electrical noise that is not problematic for measurement in fuel gas. We found that the physical distance between the transmission and reception circuits for ultrasonic signals, in particular, affects the intensity of electrical noise (transmission noise). Thus we reviewed the circuit patterns and distanced the transmission and reception circuits to reduce transmission noise.
Increasing the resolution of measurement
In order to measure flow by using ultrasonic waves, it is necessary to precisely measure the difference in the time between the transmission of ultrasonic waves from upstream to downstream or from downstream to upstream and the reception of the signals (arrival time). Normal gas meters measure this difference in arrival time several times and calculate a Removal of noisemore precise arrival time from a value obtained by processing the differences. However, since the sound speed in hydrogen is about four times higher than that in fuel gas or the air, if a hydrogen gas meter measures arrival time the same number of times as a normal gas meter, then the variation in measurements becomes larger and leads to decrease in measurement accuracy. Thus, in order to ensure sufficient accuracy, our gas meters collect a several tens of times larger amount of data than a normal gas meter to perform an averaging process.
Normal gas meters run only on an internal battery for ten years. We aimed to develop hydrogen gas meters that also run only on a battery for ten years. However, increasing transmission voltage to increase the intensity of ultrasonic signals, or increasing the number of measurements to ensure measurement accuracy (these are devices for measuring hydrogen) necessitates the use of more electrical power than normal. Therefore, we further improved technologies for reducing current consumption, and developed hydrogen gas meters that run only on an internal battery without needing external power supply.
■ Transmission of ultrasonic signals with reduced current consumption (patented)
To transmit ultrasonic waves, a voltage higher than the battery voltage, 3V, is generated by a voltage boosting circuit, and is applied to an ultrasonic sensor. However, as the voltage is increased, the power consumption becomes larger. Therefore, we improved a method for applying voltage to an ultrasonic sensor to increase voltage boosting efficiency so that sufficient transmission pulses can be generated by voltage boosting that is half of conventional levels. This is how we reduced current consumption.
■ Minimizing the operating time of circuits (patented)
In ultrasonic gas meters, electricity is saved by powering circuits on only when they need to operate and by powering them off when they do not need to operate, in order to reduce power consumption. In our hydrogen gas meters, current consumption is further reduced by controlling this electricity saving operation in a more timely manner.
Voice of engineer
■ What issues or findings were there in the development of hydrogen gas meters except for those mentioned?
When we started this development, we desperately lacked information on ultrasonic measurement of hydrogen, getting a series of unexpected test results. For example, measurements were able to be carried out in the air without problems, so we expected that one fourth of the signal intensity there could be obtained in hydrogen, but ultrasonic signals disappeared immediately after hydrogen was measured. We found that this was caused by a slight gap in a certain part of the flow path, but instructional texts and literature on audio say that the extent of this gap is not problematic at all. There were many difficulties similar to this, in which applying theoretical values obtained from instructional texts or literature did not produce expected results. On the other hand, some findings made the design easier; for instance, we found that although an increase in flow speed causes ultrasonic signals to weaken in a normal gas meter (dependency of ultrasonic signals on flow speed), signal intensity hardly depends on flow speed in hydrogen measurement.
■ What approach or solution did you find from that situation?
When we filled the gap for other tests, we happened to see the signal intensity recovering. This is when we found the fact that the gap hinders the transmission of ultrasonic waves. We became keenly aware that in such a situation where there is a lack of information, all kinds of data and their analysis can be hints on solutions to various issues, correlating with each other.
■ Talk about further and future improvements.
Currently, we are collecting data on practical operations by installing a hydrogen gas meter in demonstrative tests for pure hydrogen fuel cells that are operated by suppling hydrogen through actual pipelines. In addition, we have a plan for hydrogen gas meter installation. We would like to offer proposals for newer hydrogen gas meters for the forthcoming hydrogen society, by combining data obtained from the plan with gas meter technologies we have developed until now.