Measuring Ultra-Low Air and Gas Pressures with LDE/LME/LMI Series

The LDE/LME/LMI differential pressure sensors from First Sensor are designed to measure ultra-low air or gas pressures from 25 Pa (0.1 in H2O) full scale.

The sensors are based on a new and advanced MEMS technology, which combines a microflow channel inside the silicon sensor chip. At the same time, the sensors use the well-established principle of inferring differential pressure from a thermal mass flow measurement as shown in Figure 1. A heating element is placed between two temperature sensitive resistors. A gas flow transfers heat from the upstream to the downstream resistor causing a temperature difference between them and consequently, a voltage signal proportional to mass flow is produced. As the flow is the result of the pressure difference between the two sensor ports, the output signal is also a measure of the applied differential pressure.

Shown in Figure 2 is the unamplified output of a standard LDE/LME/LMI sensing element as a function of differential pressure. The LDE/LME/ LMI technology features high sensitivities and high dynamic ranges for low pressures particularly around zero. The sensors provide digital signal conditioning for temperature compensation, calibration, and amplification. They can be optimized to different application needs based on whether a high dynamic range, a high sensitivity, or a linear output signal is required.

Figure 1. Principle of thermal mass flow measurement

Figure 2. Characteristic curve of an unamplified basic LDE/LME/LMI sensor element

2. Sensor Construction

The LDE/LME/LMI sensors are based on a silicon sensor chip, measuring approximately 4 mm2 (0.006 in2) in size, which contains the sensing element and the micro-flow channel. By combining the miniaturized flow channel on the sensor chip level as seen in Figure 3, the LDE/LME/LMI pressure sensors from First Sensor can achieve very high pneumatic impedances up to 200,000 Pa/(ml/s), which is up to 1000 times higher than comparable sensors. This decreases the gas flow through the sensor to an absolute minimum and provides unique application benefits in humid and dusty environments as well as when using long connection filters or tubes.

In traditional flow-based differential pressure sensors, the flow channel and gas flow through the sensor is established by the geometry of the plastic housing. The micro-flow channel of the LDE/LME/LMI devices on the other hand is defined on the silicon chip level.

This allows benefits in the building of the sensor housing such as very small and stable packages, high design flexibility, as well as reduced manufacturing costs. Additionally, the semiconductor technology used for the silicon sensor chip allows very low production tolerances along with cost-effective mass production.

Figure 3. Principle construction of the LDE/LME/LMI differential pressure sensors (cross section)

Figure 4. Typical volumetric flow measurement set-up with differential pressure sensor

3. Flow Measurement with Differential Pressure Sensors

Differential pressure sensors with very low pressure ranges of just a few millibar (a few inches of water column) are frequently used for volumetric flow measurement in pipes and tubes. Examples are respiration flow measurement in medical devices as well as filter control or air flow measurement in HVAC applications. An artificial flow restriction, for example by means of an orifice, baffle, or laminar flow element induces a pressure drop to the flow which is a measure of the volumetric flow rate (V̇) and can be detected with a differential pressure sensor as shown in Figure 4.

First Sensor’s flow-based LDE/LME/LMI sensors are calibrated to differential pressure and can sense the pressure drop across a flow element in a bypass configuration. As a result of its very high pneumatic impedance, the flow through the sensor is limited to maximum 120-180 µl/min. This means that the LDE/LME/LMI sensor virtually acts like a membrane-based differential pressure sensor which does not allow any flow-through. However, it still displays the high measuring sensitivity for low pressures which is characteristic for the thermal mass flow sensing principle.

4. Immunity to Long Connection Tubes and Input Filters

To measure volumetric flow, the flow-based differential pressure sensor has to be linked to the main flow channel, for example via tubes (Figure 4). Occasionally, extra filters will be used in the bypass channel to safeguard the sensor against humidity, dust, or bacterial contamination. However, any pneumatic element between the bypass and the main flow channel signifies an extra flow resistance which results in a pressure drop. The pressure sensor will thus measure a differential pressure which is lesser than the one caused by the flow restricting element in the main channel. The result is an erroneous measurement of the volumetric flow rate in the main flow channel. The higher the flow impedance of the connecting tubes and extra filters compared to the sensor, the more dominant is this effect. Thus, for conventional flow-based differential pressure sensors, a maximum allowed tube length to the sensor is proposed or respectively a correction formula is specified to offset the pressure drop in the bypass.

A tube of 1 m (40”) length with an inner diameter of 1.6 mm (1/16 in) causes a pneumatic impedance of approximately 120 Pa/(ml/s). The LDE/ LME/LMI sensors from First Sensor display pneumatic impedances of up to 200,000 Pa/ (ml/s), meaning the bypass flow is nearly exclusively established by the very high flow impedance of the LDE/LME/LMI device, and influences of extra components with resistance to flow can be ignored. Therefore, LDE/LME/LMI differential pressure sensors can be used with filters, long tubing, or other pneumatic elements without losing its calibration. Even if these elements alter their resistance over time, such as a clogging filter, there will be no negative impact on the measurement accuracy.

5. Immunity to Dust

If flow-based pressure sensors are used for volumetric flow measurement in dusty environments such as for example HVAC applications, there is the hazard that dust particles might enter the sensor and get deposited on the walls of the inner flow channel. This would increase the pneumatic impedance of the sensor and thus lead to loss of calibration and a decrease in the sensor output signal. In a worst case scenario, the flow channel will be totally blocked which results in a complete failure of the sensor. Moreover, dust can cover the sensitive measuring elements which also degrade the sensor signal.

First Sensor’s LDE/LME/LMI pressure sensors are extremely immune to applications in dusty environments. Due to its extremely high pneumatic impedance, the air flow via the sensor is very small. This means that the total amount of dust-laden gas which streams via the bypass channel in a volumetric flow measurement set-up is decreased to an absolute minimum compared to standard flow-based pressure sensors. Moreover, the flow velocity is significantly reduced so that the remaining dust quantity can usually settle out before it enters the sensor’s input. In this way, the LDE/LME/LMI flow-based pressure sensors stop the ingress of dust into the sensor and guarantee highly accurate measurements and lengthy sensor lifetimes.

6. Immunity to Humidity

In many medical devices such as spirometers, respirators, oxygen conservers and sleep diagnostic equipment, the patient’s breathing is regulated with the aid of flow-based differential pressure sensors. Since the respiratory flow has a substantial amount of humidity and is also warmer than the environment, this can cause condensation within the device. Water droplets can condense in the sensor itself or onto the tubing walls in the bypass line. If the droplets surpass a certain size or build up to larger droplets, this can change the pneumatic properties of the sensor and the connecting tubes resulting in an erroneous sensor output signal and a loss of sensor calibration. In a worst case scenario, the flow channels will be totally blocked, leading to a total failure of the sensor.

First Sensor’s LDE/LME/LMI pressure sensors are highly resistant to humid environments. In view of its very high pneumatic impedance, the air or gas flow via the sensor and its connecting tubes is very small. This means that the total quantity of humid air which streams via the bypass channel in a volumetric flow measurement, and which can potentially condense, is lowered to an absolute minimum compared to conventional flow-based pressure sensors. Thus, LDE/ LME/LMI flow-based pressure sensors guarantee highly accurate measurements and extended sensor lifetimes in typical high humidity applications.

This information has been sourced, reviewed and adapted from materials provided by First Sensor AG.

For more information on this source, please visit First Sensor AG.

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