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.

 

Reference: click here.

D&D Engineering – “W’ere YOUR Sensor Guys”
3835 E. Thousand Oaks Blvd. Suite 464 Westlake Village, CA 91362  – Voice: (818) 772-8720 Fax: (818) 772-2477 Toll Free: (888) 333-6474
Email: sales@sensorguys.com Website: www.sensorguys.com

The RS-232 Communication Protocol and your Alicat instrument

Posted by: Joe Ancona

The 55-year-old RS-232 protocol is the default method of talking to Alicat devices. Aside from RS-232, you could configure your instrument with RS-485, Profibus DPV1, ModBus RTU, DeviceNet, and Ethernet IP protocols. For the bulk of our customers however, the tried and true RS-232C communication standard will remain the go-to method for commanding and reading Alicat devices.

The Not-So-Standard RS-232 Standard

One of the main reasons that RS-232 has survived for over five decades is that it is a useful, but low level and rudimentary signal, with fairly loose operational guidelines. In 1962, the sole application for the RS-232 communication standard was connecting electromechanical typewriters and their host mainframes/modems, commonly known as ‘Teletype’ systems.

When more advanced electronic machines were developed subsequently, proprietary adaptations led to nonstandard pin assignments, connectors and signal voltage levels. For example the original specification called for a DB-25 connector, but in the last 30 years most RS-232 products have adopted a DB-9 connector (technically called DE-9M).

The ‘data’ being sent on RS232 lines are simply positive (+) and negative (-) voltage pulses relative to a ground reference. A group of +/- pulses sent by one device are carefully timed by the receiving device and decoded into whatever the hardware settings deem to be data bit packages. In other words, the RS-232 standard only defines a relatively loose general electrical framework to transmit and receive electrical pulses. What one does with all these pulses is ultimately up to the connected hardware. Things like character encoding, spacing, start bits, stop bits, bit order, error detection, bit transmission rate, etc. are not the responsibility of the RS-232 scope, and are established by the user’s connected circuitry, usually in the form of a serial communication port and its associated chips and transistors.

db9-pins

It’s the COM Port’s job to make sense of the pulses on behalf of the attached computer or peripheral. For reference, an RS-232 system must transmit from one device (sent on its Tx pin), to a receiving device (received on its Rx pin), and vice-versa. Do not try to connect Tx to Tx or Rx to Rx in an RS-232 three wire system! The only pin that is connected directly is the ground pin, which gives both ends a common reference point to measure the pulses from. Each RS-232 driver uses inversion logic and employs a single ended, bi-polar output voltage to feed to a UART (Universal Asynchronous Receiver/Transmitter). Because the system has three wires and two distinct channels of communication, it is considered a “Full Duplex” system. Data can be transmitted at the same time as it is being received.

RS-232, the Alicat Way

Understanding how loose the RS232 “standard” really is, you might be wondering how Alicat uses it. Alicat does offer the ubiquitous DB-9 or the “standard” DB-25 connector, but we can provide RS232 communication on any connector that is offered, such as DB-15, 6 pin industrial locking connectors, and of course the default 8 pin miniDIN jack.

Alicat RS-232 Specifications

However, our real departure from the standard, is how Alicat has exploited signal levels and allowed for multiple units to work on the same COM Port. Because Alicat devices neither accept nor produce negative voltages, a traditional +/- 15V RS232 is not possible. Fortunately, a positive only pulse of +5V can be made to replicate an RS232 waveform (logic high ‘mark vs. logic low ‘space’), readable by 99% of all UARTs used today.

Once UART serial ports went out of fashion in the early 2000’s, USB to serial converters took their place; most today use the FTDI chipset to replicate the COM Port. Alicat’s unique signal profile is also fully compatible with these devices.

In addition to bending the rules for signal level and polarity, Alicat has also designed a clever work-around for being able to use up to 26 units at once on a single serial COM Port. The technical term for this capability is called ‘Multi-Drop’ communication, and is supported by all Alicat units equipped with serial communication (whether RS-232, or the differential signal based RS-485). Through multi-drop communication, every device on the line is configured to have a unique identifying letter (A-Z), and every unit listens to the commands that have been sent. However, even though each device ‘listens’ to each command, a particular unit will only accept and respond to the command if the instruction begins with that instrument’s unique ID letter.

So you can read the current flow rate on unit “A”, give a new setpoint to the MFC “B”, and reset the totalizer on unit “C”; all while being hooked together on the same three wires (electrically in parallel).

Even though the RS-232 communication ‘standard’ itself is old enough for a place at the Smithsonian Institution, it is still heavily used today for all types of computer based systems that talk to various peripheral components. Employing the universal ASCII (American Standard Code for Information Interchange) character set as our language, Alicat instruments will continue to be sold with the robust, universal, and reliable RS-232 system for the foreseeable future. For basic connection and terminal examples please see our video and instructional here:

Reference: click here.

D&D Engineering – “W’ere YOUR Sensor Guys”
3835 E. Thousand Oaks Blvd. Suite 464 Westlake Village, CA 91362  – Voice: (818) 772-8720 Fax: (818) 772-2477 Toll Free: (888) 333-6474
Email: sales@sensorguys.com Website: www.sensorguys.com