Gas & Air Flow Measurement: FAQs

Gas flow is defined as the volume of gas per unit time. Mesa’s DryCal® technology measures gas flow directly by using a precisely known volume – the measurement cell -and the measurement of time from an internal clock triggered by the movement of the piston in the measurement cell.

Using this proprietary DryCal® technology, our meters directly measure gas flow. Many other gas flow meters measure flow through indirect means of either a pressure drop across a flow restriction or through the transfer of heat from the gas flow.

Because Mesa’s flow meters measure gas flow directly with volume and time, they are largely immune to the effects of the gas species being measured. Gas humidity and gas temperature can degrade the accuracy of other instruments. Also, while some other flow meter accuracy specifications are given as percent of full scale, Mesa flow meter accuracy specifications are given as percent of reading.

The Defender 510 measures volumetric gas flow.

The Defender 520 measures volumetric gas flow with displayed gas temperature and pressure measurements. This allows for manual calculation to standardized readings with a standard formula.

The Defender 530+ measures volumetric and standardized gas flow readings with a higher accuracy than the Defender 510 and 520 with Swagelok® inlet and outlet fittings.

ISO 17025 is the international quality standard for calibration laboratories, set forth by the International Organization for Standardization (ISO).

The requirements of ISO 17025 encompass all aspects of laboratory management, including calibration procedures, analytical testing proficiency, report generation and record keeping, and ensure calibrations are performed by properly-trained personnel using controlled test methods and procedures. ISO 17025 is to laboratory measurements as ISO 9000 is to products. Certification to ISO 9000 alone does not demonstrate a lab’s ability to produce technically valid data and results, and all ISO 9000 elements relevant to the testing and calibration services within a laboratory’s quality system are incorporated in ISO 17025.

Yes, Mesa Labs is accredited to ISO 17025 by The National Voluntary Laboratory Accreditation Program (NVLAP). NVLAP, administered by The National Institute of Standards and Technology (NIST), provides third-party accreditation to public and private laboratories based on evaluation of their technical qualifications and ability to perform specific tests and calibrations compliant with ISO.

We are proud to state that Mesa Labs’ scope of accreditation appears to be the greatest of any ISO 17025-accredited flow facility in the world, at ±.08% standardized accuracy. A copy is available upon request.

Volumetric flow rate is defined as the volume of gas being transported across a defined boundary per unit time, i.e. liters/minute, cubic feet/hour, etc.

However, gas is compressible and gas volume will change as the gas temperature or gas pressure changes. Compressibility of gas for a hypothetical ideal gas is defined by the ideal gas law equation of state:

PV=nRT where:
P is the absolute pressure of the gas
V is the volume of gas
n is the number of moles of the given gas
R is the universal gas constant
T is the absolute temperature of the gas

Since gas changes volume with temperature and pressure, gas flow is commonly reported as standard-ized flow. The definition of a standardized flow rate is the volume of gas transported per unit time across a boundary with the measured gas volume converted to the volume the gas will occupy at a defined pressure and temperature. No single universal standardizing temperature exists, however common temperatures used are 0 centigrade and 21.1 centigrade. A standardization pressure of 760mmHg1 is universally used. For any gas flow measurement, it must be clearly stated if the gas flow measurement is volumetric flow or volumetric flow that has been standardized with the standardization temperature and pressure clearly stated.

760 mmHg

Gas compatibility should be considered when using a Mesa primary standard to calibrate corrosive or hazardous gases. Our products are not intrinsically safe and are not recommended for use with explosive gases. Also, materials within the wetted flow stream (the path of the gas being calibrated) may be damaged by corrosive gases. The Mesa warranty does not cover parts or service labor related to use with corrosive gases.
Here is a list of materials within the wetted flow stream of each Mesa primary standard:

DryCal® DC-Lite
Aluminum
Black Chromate over Zinc
Borosilicate glass
Epoxy potting compound (3M Scotch-Weld Item #: DP-270)
Graphite
Mylar
Nylon
Nylon mesh inlet filter material
Silicone
Solenoid valve (magnets, windings, frame, cover, shaft)
Stainless steel
Teflon
Viton A

DryCal® DC-2
Acrylic
Aluminum
Borosilicate glass
Brass
Epoxy potting compound (3M Scotch-Weld Item #: DP-270)
Graphite
Mylar
Nylon
Nylon mesh inlet filter material
Polycarbonate
Silicone
Solenoid valve (magnets, windings, frame, cover, shaft)
Stainless steel
Teflon
Viton A

Definer 220 and Defender Series
Borosilicate glass
Graphite
Viton
Mylar
Stainless steel
Zinc plated steel
Tin
Ceramic
Epoxy
Nickel
Polycarbonate

Metrology Series
Aluminum
Anodize
Black Chromate over Zinc
Borosilicate glass
Epoxy potting compound (3M Scotch-Weld Item #: DP-270)
Graphite
Mylar
Nickel-plated brass
Nylon
Nylon mesh inlet filter material
Pressure transducer
Solenoid valve (magnets, windings, frame, cover, shaft)
Stainless steel
Temperature sensor
Viton

DryCal 1020
Borosilicate glass
Graphite
Aluminum
Stainless steel
Epoxy
Viton
Nickel
PolyCarbonate
ML One
Borosilicate glass
Graphite
316 Stainless steel
Teflon
AFLAS

DryCal instruments read gas pressure in the measurement tube when taking a gas flow reading. Typically this pressure is higher than atmospheric pressure by 1 to 2 mmHg. Do not compare this pressure reading to atmospheric pressure reported by your local weather service.

Atmospheric pressure as reported by weather services is corrected to a theoretical pressure if the station was at sea level. Since actual atmospheric pressure decreases with elevation, customers at elevations above sea level will find the DryCal instruments pressure reading lower than the atmospheric pressure reported by their local weather service.

Mesa primary standards, featuring our DryCal® technology, are not intrinsically safe and are not for use with explosive or flammable gases, or for use in explosive environments. If you choose to calibrate explosive or flammable gases with your Mesa instrument, please follow your organization’s laboratory safety procedures, which typically require operation within an inert atmosphere. To enable use in an inert atmosphere, some of our models do provide gas Purge fittings.

Manufacturers use different methods to calculate flow rates for multiple gas MFCs. The accuracy of these MFCs can vary greatly dependent upon the MFC or equipment manufacturer and the variety of gases used. Understanding these methods can help you, the user, identify problems in the process caused by inaccurate flow measurements.

Know the Sensor Factor

All MFC manufacturers will provide a sensor factor to users. Commonly, this is a ratio of the specific heat of the process gas to the calibration gas; however, these factors are not universal across MFC manufacturers for each individual gas. Different bypass sensor designs will utilize different flow rates. Heating elements also operate at different temperatures, and a gas’s specific heat will change with the temperature. You should only use the sensor factors provided by the MFC manufacturer.

Ask About Different Methods with Multiple Gases

Some MFC manufacturers also use different methods to support multiple gases in a single MFC. Some have tested and developed physics-based models in an effort to provide a more accurate measurement across gases. Some condition the flow entering the MFC in an effort to reduce the difference in flow response between gases as they pass through the sensor.

Ask the manufacturer to describe the method conversion used for alternate gases and to define the accuracy specification for each alternate gas. This should be a different specification than the accuracy with calibration gas. Claims that “gas expansions are linear” means that a linear conversion factor is used.

Note that flow accuracy will vary between different gases, and may vary at different flow rates even when using the same gas. The importance of this will depend on the nature of the process that the MFC is used in and how critical the flow measurement provided by the MFC is. Purchasing a process-gas calibrated MFC for each individual gas is an option when accuracy improvements are needed.

If the gas flow source connected to your equipment (such as a sampling pump) exceeds its rated flow range of the display will read “Over Range” and the piston may become “stuck” at the top of the flow cell. For example, Defender 510-H is only to 30 liters per minute. If you attempt to use the Defender 510-H to calibrate beyond 30 liters, “Over Range” may appear in the display.

When the gas flow source is disconnected from the flow meter the piston should return to the down position and the “Over Range” display should clear. In most cases, the meter will be unharmed, although it is not recommended to use it above its rated flow range, and Mesa is not responsible for damage caused by exceeding the meter’s rated flow range.

DryCal’s viscous-sealed prover uses a piston and cylinder fitted so closely that the viscosity of the gas under test results in a leakage small enough to be insignificant. The leakage that does occur is tared out during the measurement interval (see Piston Tare Value for additional information). The piston and cylinder are of materials with matched temperature coefficients of expansion and low friction so that DryCal flow meters can operate under a wide temperature range without a change to their accuracy.

Here is a list of several things to check. If you still have difficulty obtaining accuracy, feel free to contact Mesa Labs and talk to our application engineers.

• Verify that the flow source is connected to the pressure port of your meter for pressure sources and to the suction port for verifying suction pumps. The unused port should be at atmospheric pressure with any cap or plug removed. If you are calibrating a gas that requires an exhaust line to vent the measurement gas, ensure that the tubing is of sufficient diameter not to create a pressure drop greater than .3PSI.
• Ensure that hose and tube fittings are tight and leak free.
• The tubing connecting your flow source (pump, mass flow controller, needle valve, sonic nozzle or restrictor) to the meter should be kept as short as possible. See the FAQ entry on “Dead Volume”.
• Verify that you are measuring the correct type of flow and that the meter is set properly – volumetric or standardized (Note: Does not apply to Defender 510 and 520 series, the DC-1 or the DC-lite, which only measure volumetric flow).
• If you are measuring standardized flow, check that the correct standardization temperature is set on your meter (Note: Does not apply to Defender 510 and 520 series, the DC-1 or the DC-lite, which only measure volumetric flow).
• (Verify that the Sensor Factor on your meter is set to one, unless you specifically want it set to a different value (Note: this applies only to definer and ML models).
• Verify that the PTVM value for the meter is set to one (Note: this applies only to ML models).
• When calibrating MFCs or other flow generators, the gas pressure above the MFC or other flow generator should be 30 PSI or greater.
• Temperature variations in the measurement environment should be minimized and the DryCal® instrument should be thermally equalized to the environment for best accuracy.

Our flow meters are made for gas flow use only, not for the measurement of liquids.

If you have drawn liquid into your flow meter, you should immediately send it to Mesa for service, called “recertification.” This service will include full product refurbishment, which will restore the instrument to as-new condition in most cases. In the rare event that liquid damage is permanent, additional repair costs may apply. The need for repair can only be determined during the course of product testing, which is a normal part of the recertification process.

PTV stands for Piston Tare Value; this is the amount of gas that passes around the piston during measurement. All Mesa calibration equipment has a factory set Piston Tare Value that is stored in the memory of the DryCal® cell. The value is typically very small 0.1 ccm for low flow cells, 0.2 ccm for medium flow cells and 1.4 ccm for the high flow cells. We adjust for this leakage by adding the PTV to the measurements.

On our highest accuracy instruments we allow for the adjustment of the Piston Tare Value with the Piston Tare Value Multiplier (PTVM). When using the instrument with gas species other than air or nitrogen, the molecular behaviors of these gases may degrade the Piston Tare Value. For highest accuracy, the instrument’s PTV can be adjusted. Adjusting the Piston Tare Value is accomplished by entering a new PTVM. The Piston Tare Value Multiplier is multiplied to the Piston Tare Value and used to adjust the measurement; the default value for air and nitrogen is 1.000. The PTVM can be set to any value from 3.000 to 0.2000.

For flows above 20 ccm, a new a PTVM value can be calculated by using the viscosity of the gas being measured and accurate results will be obtained. Calculate the PTVM by taking the ratio of the viscosity of nitrogen to the viscosity of the gas under test. For example, to calibrate hydrogen consider the following: at 0° C, the viscosity of nitrogen is 165.31 microPoise, and the viscosity of hydrogen is 83.21 microPoise. Express these as 165.31/83.21, or 1.987, and enter 1.987 as the PTVM for this cell.

When measuring alternate gases at flows below 20 ccm, or for absolute best accuracy, it may be necessary to perform a dynamic leak test using the gas under test. Contact Mesa for information on this test.

Connecting Volume is the gas volume between a flow generator and the instrument taking the measurement. Since gas is compressible, this gas can act as a spring between the flow source and the measurement instrument. For best accuracy this volume should be kept to a minimum.

We recommend keeping the tubing volume between the gas flow generator and the instrument below the values listed below.

Sensor factor is a number that can be entered into some models that multiplies the measured flow to scale the reading for certain types of calibrations. The Sensor Factor is a convenience feature that allows customers who are calibrating Mass Flow Controllers or other instruments with an alternate gas and need their readings scaled to compensate for calibration with an alternative gas. Care should be exercised to always verify that if the scaling factor is set correctly and we recommend always returning the scaling factor to one after completing a calibration.

STP Corrections used by DryCal® Units

Vs = Vf x (Pg/760) x ((273.15+Tk)/ (273.15+Tc))

Our built-in self leak test is a quality control measure. It is not, however, a full product diagnostic, so your flow meter may pass the leak test, but still require service by Mesa to restore the product to specification.

Essentially, the leak test shows whether the flow measuring cell has maintained its basic integrity. In other words, a leak test failure usually means that something is wrong with the instrument, causing unusual leakage of the gas being measured. On the other hand, a passed leak test is not “proof” of perfect performance.

Properly performing the leak test takes time, and should be done on a stable, vibration-free surface. If the leak test fails, you may want to try it again on a more secure surface.

The leak test is simply an easy way for you to check whether your instrument has a leak that requires immediate attention, Mesa recommends that you perform the leak test only once or twice annually, in between sending the instrument to Mesa Labs ISO 17025 facility for service.

It is normal for your Metrology Series Instrument to display a temperature that differs from your laboratory’s temperature. Our calibrators measure the temperature of the actual gas entering the flow cylinder. This is the temperature to which the volumetric reading must be standardized in order to give accurate readings. Room temperature or the gas’ original temperature is not important, but the temperature of the actual gas in the cylinder is.

In normal operation in a very stable lab, the instrument may indicate a temperature slightly higher than the lab temperature due to its internal heating. This is a normal effect, and can be minimized by not charging the battery during critical readings. The battery should be charged overnight prior to using the instrument so that extra heat from the charger is not introduced during the flow measurements. If already fully-charged, the battery can still be left connected to the power line. For the most critical flow readings (beyond specifications), the instrument can be operated on battery alone.

There are several possible reasons why your Mesa primary standard’s piston won’t rise within the flow cell, or seems to “stick” within the flow cell (won’t drop to the bottom of the flow cell after a flow measurement):

Possibility #1: Battery
Your Mesa product’s battery may be too weak to open the internal valve, which releases the piston from its top position within the flow cell once a flow measurement is completed.

If recharging the battery does not resolve the issue your battery may need to be replaced. Please return your Mesa product to Mesa for battery replacement. Or, if it’s time for your Mesa primary standard to be recalibrated, we’ll replace the battery free-of-charge as part of our Recertification service.

Possibility #2: Particulates or Corrosion
The interior of your Mesa product’s flow cell may become “dirty” and particulates, such as dust, can affect the piston’s free movement within the flow cell, Or, if your Mesa primary standard has been subjected to a corrosive gas, any number of interior parts or mechanisms may be corroded.

Disconnect your Mesa product from its gas flow source and then turn it upside down and then right side up a few times. If the piston doesn’t move freely and smoothly within the flow cell, then the piston and/or the flow cell interior may be affected by particles or corrosive gas. Return your Mesa product to Mesa for factory recertification.

Possibility #3: Sunlight
If your Mesa product is used outdoors in direct sunlight, the flow cell’s internal infra-red sensors may be affected, causing the piston to appear to “stick” within the flow cell, or not move at all.

Simply covering the flow cell with paper or your hand during flow measurements in direct sunlight will solve the problem.

Please note that the Definer 220 is often used to perform field verifications of environmental monitors, and therefore this product comes standard with “sunlight film” adhered to the interior of the flow cell.

However, other Mesa primary standards, such as the Defender, Met Lab Series, or our discontinued DC-Lite, DC-2, and DC-1, were not intended for use outdoors, and do not have sunlight film protection. If you routinely use one of these products outdoors in direct sunlight, check with Mesa as to the possibility of having sunlight film installed during your next annual Recertification.

Possibility #4: Valve
The valve may not be functioning properly. When this is the issue, often the normal “clunking” sound of the valve is no longer audible (this “clunking” of the valve is not to be confused with the quiet “clicking” sound of the electronic solenoid). This may be due to a weak or dead battery, or it may be a mechanical problem. Return your Mesa primary standard to Mesa for our Recertification service, or for basic repair if it’s been less than a year since your last recertification (note the “Battery” section above).

Possibility #5: Application
Is a flow source attached to your Mesa product? For a suction application, your Mesa primary standard’s Inlet fitting (Pressure fitting) should be open to ambient air, with its Outlet fitting attached to the flow source through tubing. For a pressure application, its Outlet (Suction fitting) should be open to ambient air, with its Inlet fitting attached to the flow source through tubing.