What type of flow measuring device should I use? This is a recurring question. The choice is bounded by customer needs, product variables and different technologies available on the market today. It’s important to find a vendor with extensive knowledge in flow technology to guide you in your purchase. Selecting the wrong flow measuring instrument can result in inaccurate data and increased the cost from maintenance/replacements and upgrades. This blog focuses on two specific types of measuring devices: Rotameters (a variable area flow meter) and Thermal Mass Flow devices. We will outline how both devices work and highlight important specifications to look for when purchasing. This information will help guide Original Equipment Manufacturers (OEMs) and instrument manufacturers in selecting the right device when challenged with measuring gas flow in a range of industries from life science markets to industrial manufacturing.
Rotameters are widely used in a variable area flow meter due to low cost, simple installation, versatility, low-pressure drop, wide rangeability, and visual output. This device measures volumetric flow rate and is the best choice for measuring and controlling the gas volumetric flow under actual process conditions. A rotameter uses the variable area principle, which uses a very simple design, consisting of a tapered glass tube. In the tube is a weighted “float” made of a corrosion resistant material (sapphire, glass, stainless steel, carboloy™ or tantalum), that rises in the tapered tube with the increasing flow until the force from the fluid velocity matches the weight of the float. Because the float position relies on gravity, the rotameter must be vertically mounted. Most manufacturers offer rotameters with a glass tube in a metal housing in varying sizes. The rotameter can be the solution for many applications with a simple design, low cost, easy installation, optional manual controls, visual outputs and no required external power. The rotameter is less accurate when temperature and pressure changes occur. For many applications when high accuracy and high-pressure capabilities aren’t necessary and low cost is a priority, this device is a good choice. Mass flow devices are a more advanced technology but fundamentally both concepts are easy to understand.
Above is a diagram of a thermal mass flow system, one of the more popular mass flow systems. At a no-flow condition, the heat distribution along the sensor tube is balanced. With flow, the temperature profile (heat distribution) moves downstream as the upstream sensor is cooled, the gas carries heat from the upstream sensor (T1) towards the downstream sensor (T2). Only a small amount of the total gas flow is diverted through the sensor; the sensor is a stainless-steel tube wrapped in two heating sensors. Measurements are taken at both heat sensors, (T1) and (T2) (see diagram). The temperature difference between the two points will be zero when no flow is measured. As flow increases, the temperature profile shifts down the sensor tube, the gas molecules passing the first heated sensor (T1) will carry the heat away from the upstream sensor towards the downstream sensor (T2). The increased flow will result in a greater temperature differential as the center of the heat distribution moves downstream. These distinct heat profiles are measured as temperatures at the sensors. The magnitude of the shift is proportional to the mass flow within the sensor. In a mass flow controller, the customer’s flow requirement (setpoint) is electronically input into the digital controller (as either an analog or digital signal). The processor compares the customer’s digitized setpoint with the meter’s digital output and controls the proportional valve, to adjust flow to match meter output with the customer’s setpoint as quickly as possible. Mass flow measuring devices offer exceptional accuracy, high-pressure rating, automatic controls and electronic output — but these features mean an increase in cost. The initial investment can be justified if the integration into electronic systems, accuracy and durability mean cost savings in the future that outweigh the present cost.
Volumetric flow vs. mass flow
When comparing volumetric flow rate or mass flow rate, you need to know how accurate the measurement needs to be. The mass flow is best when accuracy is needed with a high-pressure rating. Volumetric flow is best when measuring the volumetric flow of the gas and high accuracy isn’t necessary. Compared to volumetric flow, mass flow shows virtually zero fluctuation with pressure and temperature. The cost between the two technologies is significant. However, new products, like Parker’s X-Flow Mass flow controller, are offering mass flow technology at a more economical price that is comparable to the higher end rotameter. Again, the initial investment, now less with Parker's X-Flow, can be justified if the integration into electronic systems, accuracy and durability mean cost savings in the future that outweigh the present cost.
Thermal mass flow devices
The thermal mass flow controller and thermal mass flow meter have become the leaders in flow accuracy and repeatability. Now, with products like Parker’s X-Flow, a more economical, compact, flexible and easy to use thermal mass flow instrument is available. Parker’s X-Flow offers a compact design, made of non-corrosive, chemically inert, chemically clean stainless steel and improves productivity and reduces costs in many analytical, industrial and OEM applications. The thermal mass flow controllers can monitor and automatically control flow while integrating into electronic systems. The results are accurate, repeatable, fast and reliable. In using the bypass technology, the series has a laminar flow element and turbulence filter, allowing for Parker to easily tune to a customer’s requirement and rapidly produce a specific flow range and maintain an accurate measurement.
Talking to experts is the best way to know what type of flow measuring device is best for you. Make sure the supplier has a history with volumetric and mass flow science to select the right product for your needs. Becoming an expert in these fields takes many years to develop and experienced engineers to design and produce.
Our applications engineering team is always available to provide recommendations and customize equipment to customer specifications. With over 30 years in both volumetric and mass flow technology, we have the knowledge to answer any questions. To speak with an engineer, call 603-595-1500.
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This article was contributed by David P Sheffield, product engineer, Parker Precision Fluidics. David has been a mechanical engineer for almost 30 years and has 10 years of experience in mass flow and mechanical flow measurement technology.
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