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How Biogas Generators Work with Air Flow Meters


Aug 28, 2012
immersible thermal flow meter for wastewater biomass energy applications

SteelMass Flow Meter


For several years, Sierra has been working with the Santa Cruz Wastewater Treatment Facility near Monterey, Calif., to improve the rate of biogas cogeneration. Santa Cruz produces energy – enough to power 3,000 homes – by blending digester gas (a by-product of its sewage decontamination) with natural gas. This type of biomass helps create electricity using internal-combustion engines that are linked to a generator set. As you can imagine, accurate gas measurement is a key part of the whole process. Learn more about our cogeneration applications… 

The digester gas itself is produced in four large domed anaerobic digesters, which function like a stomach that breaks down solid waste. In the digesters, which are heated to an optimal 98 degrees, methane-forming bacteria break down digester acids and create large amounts of anaerobic digester gas (60% methane and 40% CO2).

Here’s where a quality, low-flow air flow meter like the SteelMass Model 640S comes in. Thermal technology:


  • Measures true mass flow directly with one instrument

  • Has excellent turndown and accuracy at low flows

  • Has a much lower cost-of-ownership


thermal flow meter for wastewater treatment digesters

FlatTrak Flow Meter



Facility operators at Santa Cruz connect the flowmeter with their monitoring system to determine if the right amount of digester gas is being produced. If readings are low, they know to check factors such as PH levels and alkalinity ratios, temperature, and feed rates.

As I mentioned above, thermal flow meter technology is ideal for biogas cogeneration applications because digester applications operate at relatively low pressures. The air flowmeter causes virtually no pressure drop, accurately measuring these low flow rates. You can’t even use an orifice plate in this type of application. The SteelMass also offers 2% reading accuracy and 1% repeatability over a 40:1 turndown range, essential for digesters.

Santa Cruz also uses our inline mass flow meter, FlatTrak 780S. Both the SteelMass and the FlatTrak are immersible thermal mass flow meters and feature an insertion probe with two small diameter sensors to measure gas temperature and velocity and nominal pressure drop.

Scott Rouse, Product Line Director
Written By:
Scott Rouse, Product Line Director
Sierra Instruments

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