APPLICATION NOTE AN104
Using the LP Series Pressure Sensor
for Differential or Gauge Pressure
Introduction
The LP Series pressure sensor is a surface-mount device that is easy to solder to a PC board and connect to standard tubing. It canmeasure pressure ranges as low as 0 to 1 inH2O (0 to 250 Pa) with resolution better than 0.001 inH2O (<0.1 Pa). It performs withexcellent linearity, hysteresis, and stability.
The LP Series was designed to measure differential or gauge (also spelled gage) pressure, depending on the application. It contains
two pressure ports to which tubing can be connected, one tube directing pressure to the topside of the MEMS sensing element andthe other tube directing pressure to the backside of the MEMS sensing element.
Differential vs. Gauge Pressure
Differential pressure is determined by measuring pressure at
two distinct points (often labeled P1 and P2) and determining thedifference between the two measurements. Gauge pressure isdetermined by measuring pressure at one distinct point andreferencing that measurement against the ambient air pressure.
It is worth noting that absolute pressure is similar to gaugepressure with the exception that the pressure source forabsolute is referenced against a perfect, absolute vacuum(created by a glass constraint on the bottom of the MEMS
sensing element), rather than ambient air pressure.

MEMS sensing element
TopPressure Port

BottomPressure Port

For the measurement of differential pressure, tubing must be connected to both pressure ports of the LP Series. If the calibrated
differential pressure is symmetrical (±250 Pa, ±1 psi, etc.), then it is not critical which pressure port of the LP Series is connected
to P1 or P2. In either case the differential pressure value will be the same. In the case of a custom, non-symmetrical differential
pressure calibration (-500 to +1000 Pa, -5 to +10 psi, etc.), then it’s important to understand how each port was referenced
during calibration.
For the measurement of gauge pressure, where there is only
one pressure source, the positive pressure is applied to the top
pressure port of the LP Series. In this configuration pressure
from the application is applied to the topside of the MEMS
sensing element, and the backside of the MEMS sensing
element is vented to, or gauged against, ambient pressure
by way of the open pressure port. It is critical that thebottom pressure port be the open/vented port because
when the MEMS sensing element is calibrated for gaugepressure, it is done so with pressure applied to the topsideof the MEMS sensing element.

LP series - differentialpressure

LP Series - gaugepressure

One final note regarding very low-pressure gauge applications (typically less than 1 kPa): the MEMS sensing element is extremely
sensitive and may sense unintended variation or noise from the vented port. In these cases it’s recommended to also attach tubing to
the vented port to dampen the background variations in vent pressure and/or to directly control the vent pressure.
Merit Sensor’s In-House Expertise
With our own calibration equipment and experts on site, our company has been able to support a wide variety of market demands.
We have the experience and expertise to know when absolute, gauge, or differential is appropriate. We work with our customers toprovide the best product for each unique application. If you have technical questions about how our products might benefit yourbusiness, email us at sales@meritsensor.com.

Sales +1 801.208.4722 · Customer Service +1 801.208.4700 · Fax +1 801.208.4798 · sales@meritsensor.com · www.MeritSensor.com
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