A common problem associated with traditional two-sensor mass flowmeters is that they do not account for the heat lost via stem conduction, according to supplier Sierra Instruments. The Monterey, Calif.-based company says this can introduce measurement errors as high as 20 percent, depending on the gradient between the gas temperature and the temperature outside the pipe. It has developed a four-sensor design in which two additional temperature sensors are used to measure and account for stem conduction, eliminating unwanted heat-transfer components.
A combination of three platinum temperature sensors and a no-drift mass velocity sensor delivers gas mass flow rate measurement accuracy of +/-0.5 percent of readings above 50 percent of the full scale (air) in the supplier’s new QuadraTherm 640i/780i Thermal Mass Flow Meter Series. This compares with an accuracy limited to between 1 and 5 percent of reading for most traditional thermal dispersion flow meters because they do not account for stem conduction, and even this is true only under calibration conditions, according to Sierra. It claims that QuadraTherm brings flow measurement accuracy to the level of Coriolis flowmeters at much lower cost.
QuadraTherm isolates forced convection (the critical variable for measuring gas mass flow rate) by calculating and then eliminating unwanted heat-transfer components, like sensor stem conduction, one of the main causes of false flow readings. The primary difference between QuadraTherm and a traditional thermal mass flowmeter is that there is an additional temperature sensor in each probe — one above the gas temperature sensor and the other above the velocity sensor — for a total of three temperature sensors. The extra temperature sensor in the stem of each probe accounts for stem conduction.
Real-time measurement of stem conduction is critical to flowmeter accuracy for two reasons, according to Sierra. First, stem conduction is a function of the total heat transfer budget and must be accounted for. Second, stem conduction depends on the ambient temperature outside of the pipe. For example, if the outside temperature drops at night and rises during the day, the amount of stem conduction varies accordingly. This heat lost via stem conduction looks like flow unless it is measured and accounted for, the supplier notes.
“The additional sensors in QuadraTherm do exactly that,” says Matthew Olin, president of Sierra. “With the two additional sensors, you can measure the heat flux going down the stem. It more or less acts like a heat flux-gauge.”
Another inherent weakness of traditional flowmeters is change in the “skin resistance,” which is a given material’s resistance to heat flow, in the velocity sensor. Sierra incorporates a velocity sensor technology, called DrySense, using a proprietary swaging process that eliminates all air gaps between the heated velocity sensor and the tubular probe sheath, which in turn eliminates the need for any potting compounds. This provides the no-drift sensor with maximum sensitivity, reproducibility, immunity to cracking and shifting over time, and ultimately greatly improved accuracy, Sierra claims.
Traditionally, the heated velocity sensor is inserted into the tip of a tubular probe sheath and surrounded by an organic potting compound such as epoxy, ceramic cement, thermal grease, or alumina powder. Aging and cracking caused by differential thermal expansion between the parts of the heated velocity sensor result in measurement errors over time.
While the additional sensors provide the added capability of stem-conduction measurement, the data must be crunched to derive the true mass flow rate. The combination of measuring all heat transfer and a proprietary, dedicated algorithm set embedded in the microprocessor solves the first law of thermodynamics (for thermal dispersion technology) for each mass flow data point. The algorithm set calculates stem conduction and all other unwanted heat loss components, subtracts them, and computes the gas mass flow rate from the remaining forced flow convection (gas mass flow) component.
The QuadraTherm is available in two models: the 640i insertion probe version and the 780i inline flow conditioned version. The no-drift sensor with lifetime warranty has multivariable output: mass flow, temperature, pressure (optional); measures all inert and all non-condensing clean gases as well as flammable gases (methane, propane, hydrogen, and digester gas); and has a repeatability for mass flow rate of +/- 0.15 percent.
Sierra enables the qTherm to switch the flowmeter’s capability with changes in gas and pipe size, allowing gas updates to the instrument via the Internet. The instrument connects to a server at Sierra that houses a proprietary gas library. Currently, the library has mapped 18 gases and mixtures, but will continue to expand, according to Olin.
Moreover, the millions of data points collected over time in Sierra’s metrology laboratories are used to tune the instrument for performance and accuracy. End-users of qTherm, according to Sierra, can expect hundreds of data sets on gases and gas mixture combinations in the future that can be downloaded from the library to the flowmeter for upgrades and improvements.