Differential pressure-based laminar flow measurements are faster than thermal

Mass flow controllers need to be accurate and reliable. And thermal-capacitance based instruments can meet those requirements. But when a process or experiment needs high accuracy, faster response times and flexibility, thermal can come up short. At Alicat, our MC-series mass flow controllers use differential pressure-based laminar flow measurement. By forcing the flow to be in a laminar state, we can use Poiusuelle’s equation to calculate the volumetric flow rate as long as the temperature, pressure and gas, measured by the device in real time, are known. Allowing the user to enter in the standard (or normal) temperature and pressure, an Alicat translates volumetric flow into mass flow rate and even a true mass flow rate in mass per unit of time (e.g., kg/s). If the gas’s viscosity as a function of pressure and temperature is known, then Alicat instruments can measure it.

The Slow Way

Compared to an Alicat Laminar Flow Differential Pressure instrument, thermal bypass flow instruments are slow. One of the most inconvenient requirements of thermal bypass flow instruments is the lengthy warm-up time to reach thermal equilibrium: 30 minutes is not unheard of, during which time the flowing gas is running and wasted. Once running at thermal equilibrium, the device’s ability to respond to changes in flow is subject to thermal hysteresis, which is a delayed response of the sensor, as compared to a pressure differential sensor. Thermal bypass flow instruments need to be calibrated with the actual gas in the end use application. Because every gas has a different thermal capacity, if they aren’t calibrated to the gas in use, they will need to have their flow values altered by a correction factor or K-factor. K- 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 a decent turndown, it severely limits the usable range of the device.

The Alicat Way

Pressure sensors are built on diaphragms that are incredibly sensitive to changes, making them among the fastest sensors available. Alicat mass flow controllers use this different type of sensor to fill a need for speed and responsiveness in the industrial and the analytical worlds. By laminarizing flow, Poiseuille’s equation determines mass flow from differential pressure, viscosity, temperature and pressure.  We’ve preprogrammed every Alicat mass flow controller with viscosity and compressibility data for up to 130 gases and gas mixtures, to give you accuracy without K-factor uncertainties.

Because pressure equilibrates faster than temperature, differential pressure sensors don’t require the same warm-up that thermal sensors do. Differential pressure equilibrates so quickly that responses to changes in flow as fast as 10ms can be expected. Paired with a control valve, the control settling time can be similarly fast, between 50ms and 100ms is common and some applications achieve 20-50ms.

Standard turndown for Alicat units is 200:1, allowing a greater controllable range than thermal units; thousandths of a standard cubic centimeter per minute are readable on the instruments with the 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—5000SLPM.

Using differential pressure-based laminar flow measurement, an Alicat mass flow controller can change gases accurately, accommodate a wider flow range, and operate faster than thermal technology.

  • Turndown ratio: 200:1
  • Standard accuracy calibration, NIST-traceable: +/- (0.8% of the reading + 0.2% of the full-scale range)
  • Repeatability: 0.2% of the full-scale range
  • Typical control response time constant: 50-100ms (as low as 25 ms with PID tuning)
  • No warm-up time required

One Device to Rule Them All

Alicat devices are flexible, not only in the lab where you can control and change settings directly from the device display, but on the factory floor where many instruments can be controlled simultaneously from one central PLC. Each device is made custom for the customer’s needs. And, as those needs change and evolve, we can recalibrate and even reconfigure existing devices, with no need to replace a device every time a new update comes out or the process changes.

Alicat’s unique way of measuring flow makes it gas independent and faster than thermal-based instrumentation. Pressure measurement also leads to our 200:1 turndown ratio, allowing for a larger range of flow rates in a single device. And device flexibility means that an investment in an Alicat device, with its lifetime warranty, will keep measuring a variety of flowed gasses to a high degree of accuracy over time and across evolving experiments processes.

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