Mass Flow Controller Measurement Technology: A Review
All mass flow controllers are made to achieve the same goal, but there are numerous techniques for measuring. Taming the flow rate of a gas stream requires two systems to work in tandem—a control valve, and a flow measurement element. Here are four main types of flow element technology used in today’s mass flow controllers:
Thermal dispersion is the Generation X of flow measurement technology; the 1960s and 70s saw the first thermal dispersion mass flow measurement technology, and it has since become a staple in the semiconductor industry. By measuring the transfer of heat between a heating element and a temperature sensor mounted parallel to gas flow direction on a bypass line, the mass flow of the gas in the line can be measured.
Thermal bypass flow measurement technology’s wide adoption in the semiconductor industry is a result of a few highly desirable features; wetted parts composed of 316L stainless steel, selectable elastomers, and even metal to metal seals provide necessary resistance to the highly corrosive chemicals used in semiconductor processes. Thermal bypass flow instruments are also capable of measuring a wide range of flow rates and pressures, from hundredths of a milliliter per minute up to thousands of liters per minute with various body sizes. Pressures of up to 700 bar are possible, but these devices are more typically used around 20 bar.
This incredible feat of human innovation is not without its shortcomings, however! These flow instruments need to be calibrated with the actual species of gas in the end use application – which are potentially dangerous and/or expensive. Otherwise, they need to have their flow values altered by a correction factor. Correction factors introduce a degree of uncertainty to the measurement, decreasing accuracy. Another drawback is that the most common turndown ratio is 50:1. While this is better than the 8:1 or 20:1 of prior technologies, compared to our 200:1, this ratio severely limits the usable range of these devices. One of the most inconvenient requirements of thermal bypass flow instruments is their lengthy warm-up time to reach thermal equilibrium: 30 minutes is not unusual (30 minutes of running gas!) When tight control of flow is critical, users may find the 500 to 1500 millisecond control times to be inadequate.
Through-flow Constant Temperature Anemometry
Through-flow constant temperature anemometry is a close cousin to thermal bypass technology; instead of a bypass, a heater and temperature sensor probe are inserted directly into the flow stream to measure the thermal dispersion through the flowing gas. A constant ΔT is maintained between the heater and sensor, and the difference in power required to maintain the ΔT at different flow rates is correlated with mass flow.
This kind of controller can be made with the same anti-corrosive materials as thermal bypass devices, but they have many of the same weaknesses as well. Among through-flow anemometry’s weaknesses are 50:1 turndown, 2000 millisecond settling time, 30 minute warmup period, 1.5-2% best accuracy, and a lower maximum pressure rating than thermal bypass units: 30 bar for stainless instruments.
MEMS and CMOS ‘Chip Flow’
These technologies are an application of thermal mass flow measurement in miniature chip form. MEMS and CMOS chips average the temperature change measured across a chip. The thermal load is created by a constant-power heater. Due to the size of the measurement element, chip flow devices can be very small and consume very little power. In contrast to through-flow constant temperature anemometry and thermal bypass technology, these tiny devices can have extraordinary response times when paired with a well-tuned control package, even as fast as 50 ms.
Alicat’s Basis OEM mass flow controllers employ this technology to give you fast, accurate mass flow control in a small, affordable package. With real-gas calibration you get a better accuracy than other thermal devices, 1.5% Reading + 0.5% Full Scale, with a fast 100 ms response time. Alicat’s Basis has turndown ratios as high as 200:1—for the 100 sccm model, it’s 100:1. And, thanks in part to the microscopic size of the sensor, warm-up times to full accuracy are less than a second.
Laminar Flow Differential Pressure
Laminar flow differential pressure technology uses a different physical parameter to fill a need in the industrial and the analytical worlds. Pressure sensors are built on diaphragms that are incredibly sensitive to changes, making them among the fastest sensors available. By laminarizing flow, Poiseuilles’ Equation can be used to determine mass flow from differential pressure, viscosity, temperature, and pressure.
Differential pressure sensors don’t require the same warm-up that thermal sensors do, and responses to changes in flow as fast as 10 ms are reasonable. Paired with a control valve the control settling time can be similarly fast, between 50 ms and 100 ms is common, and some applications achieve 20-50ms. Standard turndown for LFDP units is 200:1, allowing a lot lower controllable range than thermal units; thousandths of a standard cubic centimeter are readable on the instruments with lowest flow ranges. At the other end of the spectrum, we also offer one of the highest full scale rates offered for an inline flow controller—5000 SLPM. High accuracy is 0.4% of reading + 0.2% of full scale.
Alicat’s laminar flow meters and controllers take advantage of these benefits to build a reliable flow controller to meet your desired accuracy and speed for your project.
For guidance on how to optimize flow and instrumentation for your process, look at our guide.
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