The difference between Mass and Volume Flow Measurement
Confusion often arises over the difference between mass and volume flow measurement and when a particular measurement should be applied. Though both technologies will deliver almost identical results under certain conditions, the deviations that can occur where a process is subject to pressure and temperature changes makes it crucial to make the right choice from the outset. Mark Allinson, Process Flow Specialist for ABB’s UK instrumentation business, explains the differences between mass and volume flow measurement and highlights the best applications for each.
Understanding how to correctly select the right flow variable can lead to significant improvements in process performance and cost effectiveness.
Put simply, mass can be defined as the amount of matter that something contains. As such, it is directly related to weight and is measured in units such as grammes, kilogrammes or tonnes. The volume of something, on the other hand, is the amount of space it takes up, and is usually expressed in units such as cubic metres, cubic decimetres, cubic centimetres or litres.
Where measurement of the flow of a liquid, gas or steam is concerned, the relationship is always governed by the equation D=M/V, where D is density, M is mass and V represents volume.
You can measure flowrate by using the two basic units of mass and volume, this is expressed as either the mass flowrate qm (e.g. g/sec or kg/hr) or the volume flowrate, qv (e.g. l/sec or m3/hr).
The nature of the relationship between density, mass and volume is such that a change in one will have an impact on another. Examples of this include entrained gases in products such as whipped cream or carbonated drinks and mixtures of dissimilar density liquids, such as oil and water.
As an example, allowing liquids with different densities to flow through a volumetric flowmeter at the same velocity will have no impact on the flow rate, as the result will be expressed in terms of how much space the liquid is occupying.
But, repeating this with a mass flowmeter will result in differences between the two flows, as the difference in densities will have an impact on the mass measurement. For example, a high density liquid will give a high mass flow rate compared to a low density liquid flowing at the same volumetric flowrate. As an illustration, one litre per second of vegetable oil may give a mass flow rate of 950 grammes per second, whereas a 20% sodium chloride in water solution would give a mass flow rate of around 1.1 kilogrammes per second.
A good example is a yogurt filling application. A yogurt pot, when full, should contain a specific volume of yogurt, which will always be the same, assuming a repetable product density. However, the density of the yogurt will vary according to the amount of fruit in it and the specific recipe of the yogurt itself. In this case, you need to know not how much yogurt is in the pot, but whether the yogurt mix contains sufficient fruit and the correct amount of other ingredients. For this, you need to know the mass of the yogurt, which will tell you whether the yogurt mix is the correct weight for the volume, since yoghurt is usually solid by weight.
The situation becomes slightly more complicated in applications measuring gas or steam, as gases and steam can be compressed, resulting in a shift in density. Having access to proper information about steam and hot water flows around a site is a tremendously powerful tool for monitoring and controlling energy use. Strategically positioned meters form the front line in energy management systems. In these situations, the measurement of steam flow will be affected by variations in temperature and/or pressure. As you compress a gas, the volume measurement changes, but its mass remains the same, assuming there are no leaks or losses in the system. Mixing gases will also have an effect on density.
Accurate metering is the key to energy management. But it’s not the volume of steam that’s the critical measure of the amount of energy moving around the system. What you really need to know is the mass. Traditional differential pressure meters such as orifice plates require ancillary measurements including line pressure transmitters, temperature sensors and a flow computer to produce mass readings for steam, all of which adds up to a high-maintenance headache and additional cost.
Vortex and swirl meters provide a superior alternative, with virtually zero maintenance requirements and greater accuracy – especially in applications where the flow varies over a significant range. Rather than an accuracy of two percent of the upper flow range, which is the best traditional orifice plate can provide, vortex and swirl meters offer an accuracy class as good as +/-0.5% of reading over the entire flow range. Furthermore, the turndown is up to ten times greater than that of a traditional orifice plate. However, the modern generation of integrated orifice meters, such as the ABB OriMaster range offer significant advantages over traditional vortex installations, offering mass flow measurement of steam with integrated multi-variable transmitters, and enhanced accuracy due to accurate centring arrangements.
Volumetric or mass flow?
As with most issues relating to the selection of flow measurement technology, there are no hard and fast rules favouring the definite selection of volumetric technology over mass technology, or vice versa. Instead, there are a number of different factors that need to be considered.
Foremost amongst these is to decide what you actually need. What is your product, process or business based on – volume or mass measurement? If you’re buying or selling by volume, then volumetric flowmeters may provide the best solution. Conversely, if you’re using weight as your final measurement, or to derive the value of a product, such as fuel, then mass flow will provide the most accurate measurement for your requirements.
Other aspects to consider include the cost of the flowmeters on offer and the level of accuracy required.
Although both liquids and gases can be measured using both volumetric and mass flow measurement, mass flowmeters are increasingly finding favour for high accuracy applications in particular, as the measurement remains unaffected by the effects of temperature or pressure.
There are three main types of mass flow measurement technology. Coriolis mass flowmeters use the momentum of the fluid to directly determine the mass flow.
Although comparatively more expensive than other mass flow methods, Coriolis flowmeters are highly accurate, and have an extremely wide turndown. They also offer long term benefits in terms of increased process efficiency, production cost savings and reduced cost of ownership.The high accuracy of Coriolis mass flowmeters means they are ideal for liquid mass flow measurement applications, especially those subject to variations in product density or where a product is priced by weight. In addition coriolis meters also provide direct density measurement which can be invaluable for product quality assurance purposes.
Thermal mass flowmeters work by measuring the amount of heat transfer a gas carries produces as it flows past a heating element. A reference probe checks the ambient temperature of the surrounding gas, while a measurement probe senses the heat transfer from the heating element. The amount of energy required to keep the measurement system in equilibrium depends directly on the mass of the passing gas or gas mixture.
This is a direct measurement of the mass flow so it is more straightforward, and hence easier (and often cheaper) to implement than techniques that derive the mass flowrate of gases indirectly. For example, a volumetric flowmeter on gas duty would also need to know the temperature and pressure of a gas in order to compute its mass flow, which means buying, installing and maintaining extra instrumentation.
In addition, thermal mass flowmeters take measurements using two small probes on the end of an insert. This causes only a minor obstruction in the surrounding flow, so that correctly sized thermal mass flowmeters offer an extremely small pressure drop, typically between one and two millibars. Vortex meters for example, can produce a pressure drop of anywhere between low tens to low hundreds of millibars for a similar measurement system, while the pressure drop across an equivalent orifice plate can be even higher.
The third type of mass flow technology is the multivariable DP flowmeter. These devices measure the temperature and pressure of the gas or liquid as well as its flow. This information is then used to assess the density as well as the volumetric flow, from which a mass value can be derived. In contrast to coriolis and thermal mass flowmeters, which are in direct contact with the gas or liquid, multivariable flowmeters are considered an indirect method of measurement, as the mass flow information is inferred using temperature and pressure values.
Express yourself
It is not unusual for engineers to express mass flow measurements in volumetric units. However, to be able to compare volumetric-based flowrates for gases, it becomes necessary to factor in standard or normalised conditions for temperature and pressure. In practice, ‘normalised’ volume units and ‘standardised’ volume units are the same, with only the reference temperatures and pressures being set at different amounts. Normalised units are referenced to 1013 mbar a, and 0 degrees C. Standardised units are referenced to 14.7 psi a and 70 degrees F.
To allow accurate comparison of flowrates to be made, it is essential to ascertain whether the gas measurements in question are being expressed in normalised units, standardised units, such as standard cubic feet (scf) or as actual units, these being the actual temperature and pressure conditions that exist in the plant.
The importance of this is demonstrated by the following comparison between normalised volume and actual volume:
ACTUAL NORMALISED
0.1m3 at 0°C and 10.13 bar a = 1m3 at 0°C and 1.013 bar a
If both of these volumes were run through a system, the actual volume flow of 0.1m3 per second is equal to 1Nm3 per second. Although the volume flow in both cases is different, the mass flow rate is identical.
A key point to remember when using actual volumetric measurements is that accurate measurement of different flows will only be possible where those flows are subject to identical temperature and pressure conditions.
Summary
Put simply then, choosing whether you need to measure volume or mass flow depends on what you’re trying to measure and why. For an application filling a tank, for example, where the tank holds a specific volume, volumetric flow measurement may provide the most straightforward answer.
Where you wish to measure the quantity of something referenced to weight , then mass flow will provide the best solution.
The interlinked relationship between volume, mass and density measurement, coupled with the greater sophistication of mass flow measurement instruments over volumetric devices, particularly for gas flow measurement, has seen users increasingly adopting mass flowmeters, particularly in energy anagement and ‘mass balance’ applications.
For liquid mass flow measurements, coriolis mass flowmeters should always be considered. These flowmeters are ideal for applications subject to variations in product density or where a product is priced by weight. Their high accuracy and repeatability can deliver excellent cost savings. Furthermore, because these flowmeters enable measurement of product density, the quality of the product and even its identity can be readily assessed.
For gas flow measurements, consider either thermal mass flowmeters or perhaps differential pressure flowmeters, such as ABB’s OriMaster M. Both of these technologies offer an economical solution, particularly in large pipe sizes and across wide measurement ranges, enabling accurate measurement even at very low flows. They also offer a low pressure drop, helping to minimise energy losses.
Steam mass flow can be measured with OriMaster, with vortex meters or by using ABB’s unique Swirl flowmeter design, offering excellent low flow performance and minimal installation length.
As a leading manufacturer and supplier of a comprehensive range of flowmeter technologies, ABB is well placed to offer advice and guidance on the best flowmeter for your application. For more information, please call 0870 600 6122 or E-mail: moreinstrumentation@gb.abb.com
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