Clever conduits: smart flow measurement for water and gas

13th December 2017
Posted By : Anna Flockett
Clever conduits: smart flow measurement for water and gas

 

Avnet Abacus helps engineers to develop smart flow meters for monitoring water and gas consumption. The company’s VP of Marketing, Alan Jermyn, here takes a look at what’s driving demand for smart meters, how they work and how they’re built.

In a bid to improve energy conservation and cut carbon-dioxide emissions, the European Union set out in the 2009 Third Energy Package a requirement for EU member states to implement smart meters wherever it is cost-effective to do so, with the goal of replacing 80% of electricity meters with smart meters by 2020. While progress from member states has been variable, the environmental drivers for smart meters are undisputable, as are the potential cost-savings to consumers and utility companies from being able to more accurately monitor and manage their inputs and outputs.  

Utilities such as water, gas and electricity can all benefit from becoming more ‘intelligent’. Whether it is automatic monitoring of electricity and water usage in remote locations or throughout a ‘smart city’, smart metering can prove helpful at a micro level for leak detection – in gas metering, the safety benefits are primordial – and at a macro level, providing more accurate billing and consumption patterns for local areas and districts.

Whilst measuring electricity consumption is relatively simple, thanks to mature current sensing technology (you know the voltage and can apply power factor correction), it’s a little more complicated for fluids. Until relatively recently most liquid and gas meters were mechanical, requiring manual readout and in some cases, flow calibration during installation. Smart meters need to offer significant advantages over their mechanical counterparts, including higher accuracy, greater robustness and remote features that cut the need for regular ‘manual’ readings. In addition, as many smart meters are being installed where a grid power supply is not available, there’s a requirement for very low power consumption so that they can operate for long periods from small batteries.

Over the last few years, a number of semiconductor companies have revolutionised the market for remote metering. In addition to modules that add automated meter reading (AMR) capabilities to existing mechanical water meters, thus avoiding the expensive replacement of deployed meters, dedicated integrated circuits for ultrasonic meters have been introduced that eliminate mechanical flow meters for liquid, gas and heat measurement. Transparency Market Research forecasts that the water meter segment will grow from a value of $3.5bn in 2015 to $5.2bn by the end of 2024, driven primarily by smart water meters that leverage cutting edge ultrasonic and electromagnetic technologies.

There are two distinct engineering approaches to ultrasonic flow measurement: time-of-flight (TOF) and Doppler Effect. Some smart meters are configurable to use either.

The time-of-flight (TOF) approach
The principles underpinning utrasonic TOF measurement can be traced back to a book entitled ‘The Theory of Sound’, written by Nobel Laureate physicist, J.W.S. (Lord) Rayleigh, back in 1877. The book describes how sound behaves when flowing through solids and fluids.

TOF flow measurement in pipes involves applying two (or more) piezoelectric transducers to a pipe at a specified linear distance apart. Piezoelectric transducers of a particular size and type generate an ultrasonic signal from an applied alternating electrical voltage. Conversely, when they are subjected to an ultrasonic signal, they produce an alternating electric voltage in response.

In TOF measurement, ultrasonic signals are transmitted from one transducer received by the other. If there is no flow in the pipe, the transit time of the signals is the same in both directions. When there is a flow of gas or liquid in the pipe, the signal transit time is reduced when it travels in the direction of flow and extended in the other. The time difference, or phase shift, is directly proportional to the rate of flow.

Using the Doppler Effect
Proposed by Austrian physicist, Christian Doppler, in 1842, the Doppler Effect is the observed change in frequency of a wave as it moves relative to an observer. It appears to increase in frequency as it approaches the observer and decrease in frequency as it moves away. The effect can be observed for any type of wave, including ultrasonic ones.

With an appropriate arrangement of transducers, transmitted and received ultrasonic signals can be compared in the frequency domain, rather than in the time domain used in TOF measurement. When there is no flow in a pipe, the frequencies are the same. Any flow of water or gas produces a Doppler Effect and the frequency difference observed between the transmitter and receiver increases with flow rate. This frequency difference can then be converted into a DC level and then, using an ADC, into a digital signal for subsequent processing, transmission and display.

The Doppler Effect may be particularly valuable when measuring low flow rates, below 1 litre per hour for water, and the measuring installation can sometimes be smaller than that needed for TOF measurement. However, the most appropriate technology depends on the final application. Doppler flow meters are said to work best in dirty or aerated liquids like wastewater and slurries, whilst TOF flow meters are more suitable for ‘clean’ liquids like water, oils and chemicals.

How to make a smart flow meter
As illustrated in Figure 1, the main elements of an ultrasonic water or gas flow meter are piezoelectric transducers with appropriate housing and connectivity, a transducer interface with host microcontroller to compute the flow rate, a power source that is independent of the grid and, in a growing number of applications, some form of low power wireless communications to transmit measurement data. 

Figure 1: The building blocks of a smart water or gas flow meter

Piezoelectric transducer construction
A typical piezoelectric transducer for flow measurement is shown in Figure 1. The piezoelectric elements are held within a housing, which is hermetically sealed at the point at which the signal cable emerges from the assembly. The cable and connector assembly is usually of a custom design, tailored to the specific application. 

Figure 2: Simplified construction of a piezoelectric transducer assembly for water and gas flow meters

The analogue front end and digital processing
Among the wide choice of analogue front ends and microcontrollers for flow measurement applications is the popular Maxim MAX35103. Offering low power consumption and a high degree of functional integration, including temperature measurement, this is a single-chip, time-to-digital converter with 20ps measurement accuracy. It is capable of 2-16Hz measurement frequency in event timing mode and has a current consumption for TOF measurement of 5.5µA. A typical application circuit is shown in Figure 2.

Figure 3: The MAX35103 is a highly accurate time-to-digital converter for use in water and gas time-of-flight flow meters

Adding wireless connectivity
Most new smart meters now have integrated wireless communications. Numerous wireless protocols are deployed with none yet achieving the status of a de-facto standard, at least not in Europe. SIGFOX, ZigBee, Wireless M-Bus, and 868MHz ISM and cellular radios are all being used and there’s growing interest in Narrowband IoT (NB-IoT) – a protocol that’s designed to transmit short bursts of limited amounts of data over existing 2G, 3G or 4G cellular radio networks. What all these radios have in common is the need for an effective antenna. The more efficient the antenna, the lower the transmitter power needed to achieve an acceptable signal path. The fundamental choices are PCB antennas made from copper traces, chip antennas or external whip antennas. Whip antennas are the largest and may not be physically or economically the best option in every application but they do offer the highest performance. Figure 4 gives a brief summary of the pros and cons of each antenna type and there is a more detailed analysis here.

Figure 4: Size, performance and cost are the key factors in antenna selection for wireless smart meters.

Powering smart meters
As only electricity meters have a guaranteed grid connection nearby, so many remote smart meters need to be able to operate for a long time on an independent power source. Whilst the industry waits for viable energy harvesting devices, most current meters rely on disposable batteries, with the expectation that they will run for 10 to 20 years between battery changes. However, it’s not just the total capacity of the primary battery that needs to be considered. The profile of how power will be drawn matters too, as do the environmental conditions under which the battery is required to operate. Lithium-based primary batteries with low self discharge are the preferred power sources for most metering applications but this is a broad category – there are several different chemistries to choose from. Experience suggests the lithium thionyl chloride batteries are best suited to most metering applications, millions have already been proven in the field since the 1990s, but it is vital to ensure that battery vendors can provide comprehensive and verifiable test results for the products on offer and that there is full traceability, right back to the raw materials used. Caveat emptor definitely applies to the battery industry.

The radios in smart meters create a demand for infrequent, short bursts of power, typically at currents of up 2 Amps, in order to transmit metering data to the wireless network. A secondary, rechargeable battery can be used to deliver the required power pulses but that creates a maintenance issue. Electrical double layer capacitors – known as either EDLCs or ‘supercapacitors’, charged from the primary battery, have proved to be a popular solution, but it’s one with some limitations. Many supercapacitors, particularly early generations, suffer from self-discharge, gradually losing charge over time. In some applications, the effect may not be of concern, but where it is, a new generation of lithium ion capacitors from Taiyo Yuden promises a solution. Figure 5 compares the self-discharge curve of a 40F lithium ion capacitor with that of a similar (50F) supercapacitor, or EDLC.  This particular limitation of supercapacitors is all but eliminated.

Summary
Building a cost-effective and reliable flow meter for liquids or gases involves a lot of component choices. Understanding the design trade-offs, ensuring that each component is the best for the application and, most importantly, that the individual parts work well together, is at the heart of creating a successful metering device.

For more information, click here.


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