Installation of Thermal Mass Flowmeters

As the following information makes clear, proper installation of the thermal mass flowmeter is very important in assuring accurate measurement of your gas flow.

The thermal mass flow meter must be installed at a location where the gas is dry or above the dew point temperature. Installations which allow large droplets of water to condense out and come in contact with the sensing element must be avoided. Such contact could cause sporadic “spikes” in the thermal mass flowmeter’s readings due to the dramatic cooling effect of the denser liquid.

Although our flow sensors are temperature compensated to remove gas temperature effects, rapid gas temperature changes can induce temporary inaccuracies in the flow readings. These effects will diminish as the temperature of the gas and the flow sensing element stabilize.

Optimum installation requires sufficient straight run to allow a uniform, non-swirling, fully-developed flow profile within the flow conduit. It is best to avoid installations which are immediately downstream of bends, abrupt cross-sectional area increases or decreases, fans, louvers, or other equipment installed in the line. These situations can cause non-uniform flow profiles and swirl which can result in signal errors. Problematic flow profiles require flow conditioning to improve meter performance. Consult the factory for additional information.

Effects of Reynolds Number in Closed Conduits

Reynolds Number (RD) defines whether laminar flow or turbulent flow are causing the flow profile within the flow conduit. Laminar flow has a RD of <2000 with a parabolic flow profile, while turbulent flow has a RD of >4000 with a more uniform (squared up) face velocity. The RD's between 2000 and 4000 are in the transition region allowing a change in profile from laminar to a turbulent face velocity profile. Turbulent flow profile is not the same as turbulence or swirl within the flow conduit. Changing Reynolds Numbers are caused by changing flow rates within the flow conduit.

Our insertion thermal mass flowmeters are calibrated for ANSI "point-of-average-flow" positioning in the process line. However, laminar flows often have a center velocity with is higher than the mean velocity. Therefore, you may need to make minor adjustments in the sensor position* or utilize the C-Factor of the Master-Touch thermal mass flowmeter's software for the most accurate flow readings.

*The design of our probe assembly is such that the active part of the flow sensor is always .75" from the bottom of the sensor's protective window. Please take into account the location of the active part of the sensing element when installing the flowmeter.

Other Considerations

For gas flows with entrained particulate matter, moisture, or very high velocities, EPI has developed the Particulate Shield (PS1). This shield blocks the gas flow immediately upstream of the RTDs, preventing contamination or coating of the sensor’s stainless steel sheath. The shield also reduces the bridge voltage generated by the gas flow, allowing our instruments to be used in an extended range of high velocity applications. The minimum flow velocity for effective use of the PS1 is 150 feet per minute. Consult our factory or a local sales representative for details.

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