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Importance of Gas Blender Maintenance by the End User

Posted by Rachel Stansel on Wed, Jul 20, 2011 @ 09:35 AM

Today we conclude our four part series, "Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders."  Tom wraps up by looking at the responsibility of the end user in maintaining the accuracy chain. 


The responsibility of maintaining accuracy lies with the end-user and owner of the gas blender. Input gases fed into any gas blender should be clean and free of moisture. Most anhydrous gases are inert to the standard materials used in a gas blender. However, in the presence of moisture, anhydrous gases can become corrosive. Contamination can develop over time and, if contaminants are carried downstream into the MFC, they can render the MFCs inaccurate. Contamination on the sensor tube walls reduces the effectiveness of the heat transfer between gas and sensor tube and may also block the small inner diameter of the tube, often rendering the MFC completely inoperable. NOTE: Make sure that all gas ports are capped when they are not in use. This ensures that moisture, particulates, and other airborne contaminants will not enter the system plumbing.


When possible, cylinders with 100 percent pure gases should be used. A premixed gas can be the first source of contaminants that lead to errors in the gas blending system.


When uncertified cylinders of premixed gases are used, very large errors can be introduced; a dynamic gas blender cannot compensate for these errors. When there is no choice other than to use a premixed gas, certified cylinders should be used for instrument calibration procedures, especially when guidelines or regulations mandate it.


The most important responsibility incumbent upon the end-user is that he follows all manufacturer-prescribed maintenance and calibration procedures stipulated for that equipment. Even if guidelines and regulations do not mandate calibration of the gas blender used to produce the calibration mixture, the dynamic blender should be calibrated on a periodic basis—typically once every year. Following all these simple guidelines will ensure a long and accurate life of an MFC-based dynamic gas blender.

 

We hope you found this series informative.  To download the full article, please click the button below.

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Tags: Thermal Mass Flow Controllers, Full Scale Error, Gas Flow, Environics Inc, gas mixing, ozone, Ambient Air calibration

Proper Development and Maintenance of an Accurate Dynamic Gas Blender

Posted by Rachel Stansel on Fri, Jul 15, 2011 @ 01:46 PM

In this installment of Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders, Tom discusses the proper development and maintenance of a Dynamic Gas Blender.

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Tags: Thermal Mass Flow Controllers, Full Scale Error, Gas Flow, Environics Inc, gas mixing, ozone, Ambient Air calibration

Selecting Mass Flow Controllers for a Dynamic Gas Blender

Posted by Rachel Stansel on Mon, Jul 11, 2011 @ 10:15 AM

Continuing our look at Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders, Tom discusses how to select the right Mass Flow Controllers (MFC).

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Tags: Thermal Mass Flow Controllers, Full Scale Error, Gas Flow, Environics Inc, gas mixing, ozone, Ambient Air calibration

Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders

Posted by Rachel Stansel on Thu, Jun 30, 2011 @ 02:38 PM

Mass flow devices, developed in the 1960’s, are critical components is systems that produce the specialty gas standards required to calibrate a wide variety of instruments that analyze gases.

Mass flow devices generate a signal that is proportional to the mass flow of a gas. Since the specific heat of a gas is unique to a particular gas, mass flow devices are unaffected by pressure and temperature changes. Thus, the signal they generate is very accurate and stable.

If the signal voltage from a mass flow device is used to indicate flow, it is classified as a mass flowmeter (MFM). If the signal from such a device is used in conjunction with a reference signal and a controlling valve, it is classified as a mass flow controller (MFC).

When several MFC’s are operated in parallel, and are electronically controlled within tight limits, the gas mixtures produced are extremely accurate. This is the principle behind a commonly used gas mixing technique called “Dynamic Blending.”

Dynamic Blending

When a certified gas mixture is ordered from a producer of calibration gases, lead time and price are important considerations. These factors can affect whether your analytical instrument is properly calibrated.

Where volume and urgency are major considerations in meeting calibration requirements, the purchase of a dynamic gas blender may be more cost-effective than purchasing the calibration gas. Dynamic gas blending is preferred to other types of systems that produce calibration standards since they can, in some cases, produce the end product more quickly and at a lower cost, and still ensure high accuracy.

Some dynamic gas blenders that are used to produce mixtures for calibrating ambient air pollutant monitors, also produce precision levels of ozone. This ozone can then be used to calibrate ambient ozone monitors and oxides of nitrogen (NOx) monitors that use gas phase titration with ozone.

Many factors can affect the overall accuracy of an MFC-based dynamic gas blender. These can range from the selection of the MFC to the care taken by the end-user in operating and maintaining the equipment.

To read more about selecting the right MFC, stay tuned for Part II next week!

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Tags: Thermal Mass Flow Controllers, Full Scale Error, Gas Flow, Environics Inc, gas mixing, ozone, Ambient Air calibration

The History and Accuracy of Thermal Mass Flow Controllers

Posted by Rachel Stansel on Mon, Mar 07, 2011 @ 10:50 AM

Thermal mass flow controllers are used in gas flow applications and utilize heat to measure flow. The heat is introduced into the flow and the dissipation is then measured using one or more temperature sensors.  The operation of thermal flow control is attributed to L.V. King (1).  King’s Law revealed how a heated wire in a fluid flow measured the mass velocity at a point in the flow.

A flow controller's accuracy is typically expressed as a percent of Full-Scale flow (%FS). This means that the flow controller is most accurate when operating at its rated full-scale flow rate. This also means that its accuracy, expressed as a percent of Setpoint, drops off significantly when operating at lower flow rates.

For example, a 1000 cc/min flow controller with a /- 1%FS accuracy, operating at a setpoint of 1000cc/min can be expected to be accurate to /- 1% of the set point. However, when operating at a setpoint of 100 cc/min, the accuracy expressed as a percent of setpoint drops to /- 10% of the setpoint.

Environics instruments use flow controllers calibrated to an accuracy of /- 1% of SETPOINT, NOT FULL-SCALE (2). This means that the accuracy of the flow controller is the same, regardless of the operating point of the flow controller. 

(1) L.V. King, On the Convection of Heat from Small Cylinders in a Stream of Fluid:  Determination of the Convection Constants of Small Platinum Wires with Application to Hot-Wire Anemometry, Phil Trans. Roy. Soc, A214, 373-432, 1914.

(2) http://www.environics.com/tech-support/technical-docs/full-scale-error-vs-setpoint-error/

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Tags: Thermal Mass Flow Controllers, Full Scale Error, Gas Flow