There are several technology options in precision measurement devices but the two that have frequently been compared against each other are thermal mass flow and differential pressure mass flow. These technologies are widely used in high purity applications including semiconductor, analytical and medical. Manufacturers in these industries tend to focus on one technology or both. This white paper discusses the importance of thermal mass flow and laminar flow differential pressure mass flow instruments, the pros and cons of each, and a guide to help direct consumers toward the product that best suits their needs.
Learn the pros and cons of thermal vs. differential pressure mass flow and get recommendations on choosing the right technology for your application. Download a copy of our white paper "Mass Flow Explained: Thermal Versus Differential Pressure".
Figure 1 below shows a typical diagram of a thermal mass flow system. The temperature sensors used to measure the mass flow are wrapped around a stainless-steel flow sensor tube. Temperature measurements are taken at both heat sensors, heater 1 and heater 2 (see diagram). With no flow, the heat distribution along the sensor tube is balanced and the temperature difference between the two sensors is zero indicating no mass flow.
As fluid flow increases through the instrument, a portion of the flow is forced through the sensor tube by the pressure drop caused by the laminar flow element. Within the sensor itself, the temperature profile (heat distribution) moves downstream. The upstream sensor is cooled by the fresh gas entering the sensor, the gas carries heat from the upstream sensor element (heater 1) towards the downstream sensor element (heater 2). 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.
The heat profiles are measured as changes in resistance with the individual windings wired across a wheathouse bridge to acquire a sensitive differential temperature measurement. The bridge's analog output is measured digitally and scaled to the 32,000-count digital output signal. In this way, the laminar flow element is used as a ranging element allowing gas and flow range flexibility so a unit can be scaled for each specific application. The laminar flow element device reduces the flow velocity to a laminar flow condition (Reynolds number <2000) by splitting the flow up into many equivalent parallel flow channels. The total number of channels in the laminar flow element determines the total gas mass flow through the metering section.
The volumetric flow rate is calculated from this drop and the properties of the gas flowing through the unit. To calculate mass flow, the volumetric flow is converted to mass flow using gas properties and independent absolute pressure measurement, and an absolute temperature measurement. Once the three measurements are made and the analog signals are converted into digital signals the processor looks up the gas properties and applies them to the readings acquired to calculate the mass flow rate. The pressure-based mass flow device must accurately measure and convert 2 additional fluid properties (absolute pressure and absolute temperature) and maintain the metrology of 3 separate measurements over time. The ability to read the temperature, the common-mode pressure, the volumetric flow rate, and the mass flow rate can be beneficial in certain applications
Thermal mass flow devices using a bypass tube typically need a 30-minute warm-up time to achieve the full stated accuracy of ±0.5% of reading plus ±0.1% of full scale but the unit has relatively high accuracy prior to the 30-minute warmup being complete. Typically, the device can achieve an accuracy of ±2% of the reading almost immediately. Figure 3 shows the typical error for a thermal mass flow controller over time at room temperature, from a cold start-up. The graph shows that within 5.5 minutes the accuracy achieved by the unit is within the ±0.5% of the reading value. This is significantly shorter than the maximum recommended warm-up time of 30 minutes.
Compare this to a pressure-based laminar flowmeter that requires almost no warmup time but can only produce accuracies to ±0.8% reading plus ±0.2% of full scale. If the application calls for instantaneous full accuracy in seconds the pressure mass flow measuring device may be the better choice but if long term accuracy is the main objective, thermal mass flow is the best choice, especially if measurements can wait a few minutes.
Typically thermal mass flow devices are produced with a high level of particulate and chemical cleanliness. They are suitable for high purity applications including semiconductor, analytical, and medical applications. A material like 316L stainless steel or equivalent can handle many corrosive gases; this increases the cost but also ensures the product will function well over its working life. When purchasing a device look for the materials of construction (wetted parts) and compare their chemical compatibility to the gases they will be in contact with. Some thermal mass flow devices that use a bypass system offers a unique way of handling corrosive materials. The heat sensors are on the outside of the stainless-steel sensory tube (see figure 1), which provides a barrier between the sensor and the material being measured. Pressure differential measuring devices frequently have RTV-coated silicon-based pressure sensors inside the wetted flow path, which makes them very susceptible to corrosion, hydrogen embrittlement, or introduce unwanted chemistry to the flow stream. This can result in inaccurate readings or potential nonfunctioning parts of the device. If the application calls for the measuring of corrosive gases, look for the wetted materials to be a 316L stainless steel or equivalent and a thermal mass flow devices that use the thermal bypass system; this device is better equipped to handle corrosive gases and provide a cleaner output stream than a pressure-based mass flow device. Some differential pressure devices are available with stainless steel wetted materials to allow for compatibility with corrosive gases, but upgrading to that configuration can add significant cost. Pressure based mass flow devices compromise on materials for speed and sensitivity; this extra speed and sensitivity is desired in certain applications but may not be suitable for analytical and high purity applications flow devices.
With the above breakdowns the biggest takeaways are:
When high accuracy, high-pressure capability, and compatibility with corrosive gases are the primary needs of your flow measure, thermal mass flow devices are the best solution. When it is critical that your device warmup time in seconds and accuracy and pressure is less of a concern, pressure mass flow devices are the better choice. This can vary with manufacturers, so be sure to do research and speak with the manufacturer’s application engineers.
Parker Precision Fluidics offers a wide variety of thermal mass flow meters and controllers for all your application needs. The new X-Flow™ is a new simplified mass flow controller for your instrument, lab, or process needs. X-Flow™ delivers fast, repeatable, and reliable high accuracy flow control through constant thermal by-pass mass measurement technology coupled with digital communication protocols. X-Flow™ is calibrated to your specific conditions and includes asset calibration management software. Other features include:
With over 30 years of experience with the mass flow technology, Parker Precision Fluidics offers guaranteed high quality, reputable products and market-driven innovation, helping our customers improve equipment accuracy and repeatability. Contact us today at firstname.lastname@example.org or call 603-595-1500 to speak to an expert applications engineer about your fluidic circuit needs.
This article was contributed by David P. Sheffield, senior product engineer at 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.