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Low-Level Differential Pressure

We use the term "low-level" for pressure transducers with full-scale ranges of about pascal (Pa) to 500 Pa. Differential pressure transducers are either uni-directional or bi-directional. Uni-directional models provide an output only when the "high" pressure input is at a higher pressure than the "low" pressure input. Bi-directional models can provide an output with either input at a higher pressure. The fittings on low-level pressure transducers are most often barbed fittings that are compatible with flexible tubing (or "hose") of the correct size. PVC and urethane are common tubing materials used in these applications.

Application Notes

Typical connections for differential pressure transducers used for building pressure, duct pressure, and water heater draft are shown at the bottom of the page.

Low-pressure differential pressure transducers in particular may be temperature sensitive, and the output may drift significantly as operating temperature changes. All manufacturers should provide specifications on the expected errors that can be introduced by temperature drift. Some designs include temperature compensation to reduce this effect. Some models of handheld differential pressure sensors include an automatic re-zeroing routine, in which valving takes the pressure transducer offline momentarily. Though not simple to implement, similar re-zeroing valving arrangements can be used in monitoring systems.

Orientation of the transducer may affect output in some cases. Run a trial, or check with the manufacturer if this could affect your results.

Pressure sampling tubes that are exposed to outdoor air may become occluded with rain water, snow, or condensation. The weight of even a small water drop (if it fills the tube) is enough to render building enclosure pressure data completely useless. To protect against this, the outside end of the tube should be protected from direct falling or wind-driven rain and snow, and far enough off the ground to prevent entry of splash water. Tubing exposed for prolonged periods should be checked for condensation. Clear tubing has an advantage in allowing visual inspection for water.

Wind can cause large swings when measuring outdoor-indoor differential pressures, and adequate wind shielding is important.

Static and dynamic pressure: Most duct pressure measurements are intended to be static pressure measurements. To measure static pressure, the orifice where pressure is sensed should face perpendicular to fluid flow. Dynamic pressure is used to estimate flow velocity of a fluid, and can be measured using a differential measurement of the pressure "looking into" the flow (i.e. with the sensor orifice facing the flow stream), with respect to (wrt) the static pressure in the same flow stream. The pitot tube is designed to perform exactly this function.

Typical connections for differential pressure transducers:

Three diagrams of differential pressure from outside air to inside air. The first diagram shows a differential pressure transducer on the left with high and low pressure tubes attached. The high pressure tube runs to the outside air on the right through the siding with a note to place the end of the tube facing down in a protected location. The low pressure tube vents to internal room air.

Differential Pressure Gauge

In Building America field tests, a differential pressure gauge is a very useful tool. Coupled with other gadgets, it can be used for precise differential pressure control required on blower door test, duct blaster test, exhaust fan air flow, supply fan air flow measurement tests, and similar testing. The differential pressure gauge normally comes with two independent pressure measurement channels (Channel A and Channel B).