Select the right Pressure Transducer for Critical Environments

Selecting a low differential pressure transducer (DPT) for critical care ventilation is one of the most important decisions mechanical designers, engineers and architects can make. These transducers perform precise monitoring and contamination control of isolation room pressures in critical environments. When properly selected and installed, they help prevent the spread of communicable diseases like tuberculosis (TB).

The positive pressure for protective isolation, or negative pressure for airborne infectious isolation control. Examples of positive pressure rooms include hospital patient rooms, hospital surgical suites, hospital pharmacies, pharmaceutical, and semiconductor manufacturing clean rooms, life science laboratories and animal vivaria. Instances of negative pressure rooms that form a protective space for the public include hospital rooms with TB, measles chickenpox or SARS patients and biological safety laboratories.

Why Low Differential Pressure Transducers in Critical Environments are Important?

Low DPTs accurately measure the very low differential pressure of a critical room space pressure relative to the adjacent space pressure, usually an adjacent corridor or anteroom.
The differential pressure gradient is used to prevent airborne infections or contaminants from moving from a protected space to contaminated space, or vice versa. Critical care environments are characterized by their extremely low-pressure measurement, on the order of 0.1” W.C. (25 Pa) FS range, or 0.0036 PSI. As energy consumption in buildings continues to become a more important factor, lower differential pressures are being specified, but not at the expense of safety.

Piezoresistive Pressure Sensors

There are three commonly used types of low DPTs: piezoresistive strain gauges, capacitive sensors, and Microelectromechanical Systems (MEMS) sensors. The piezoresistive strain gauge is typically made from silicon and joined to a steel substrate through metal-to-metal bonding. When a strain is applied, it causes material deformation to the single-crystal diaphragm. This deformation enables the resistors in the Wheatstone bridge circuit to detect pressure and produce an output voltage that is proportional to the sensed pressure. The piezoresistive strain gauge offers good resolution and bandwidth and is often chosen for cost-sensitive applications. However, there are limitations to this sensor. Their high sensitivity to temperature changes and the tendency to drift are disadvantages. The primary problem, however, is that this type of sensor cannot be made large enough for sensed pressures to deform the diaphragm effectively. Although the diaphragm could be made thinner, it would compromise its strength and integrity. Making the silicon diaphragm larger would make it cost-prohibitive. When used in low-pressure ranges, the result is noise and compromised long-term stability. Consequently, they are much better suited for high-pressure ranges. Transducers that use polysilicon thin film, bonded metal foil, thick film and sputtered thin film operate in much the same way and display very similar advantages and disadvantages. Fortunately, other sensor technologies are available that offer greater operational advantages.

Capacitive Sensors

Capacitance based pressure sensors have become a mainstay in pressure measurement applications. The principal advantages of capacitive pressure sensors over piezoresistive pressure sensors are increased pressure sensitivity and decreased temperature sensitivity. Other advantages include design simplicity, no requirement for exotic materials, low power consumption, tremendous resolution, and low cost. A capacitive transducer configuration consists of a compact housing that contains two parallel closely spaced, electrically isolated metallic plates. One plate is a diaphragm capable of slight flexing under applied pressure; the other is an insulated stainless steel electrode.
Differential pressure applied to the diaphragm causes a displacement of the diaphragm relative to the fixed electrode. The resulting change in capacitance is detected by a sensitive linear comparator circuit, which amplifies and outputs a proportional, high-level voltage signal. The extremely small deflection of the diaphragm helps minimize hysteresis and repeatability errors while providing fast response times.

MEMS Sensors

One popular capacitive option is the MEMS sensor. MEMS sensors integrate actuators, mechanical elements, and electronics onto a common silicon substrate through microfabrication technology. These sensors can be mass-produced very inexpensively. They’re also becoming increasingly small, where diaphragms are on the order of 0.02in2.
This miniaturization, however, presents performance limitations such as insensitivity, inaccuracy and signal drift. Since MEMS sensors are made from brittle materials, they can’t be loaded with large forces. That’s because brittle materials can be fractured under high stress.
The biggest restriction with capacitive MEMS sensors is that miniaturization limits their pressure sensitivity to high-pressure installations.

All pressure sensors work through the action of applied pressure on a diaphragm. However, the pressures within these applications are so low that there is not sufficient force to move the pressure sensing diaphragm unless it is sufficiently large, up to 6in2, for example.
Unfortunately, they are much smaller than that. There are some silicon diaphragm-based capacitive sensors that have excellent mechanical spring properties, but their diaphragm thickness is limited to 0.006”. Diaphragms made of stainless steel, on the other hand, can be 1/20th that size, giving them much greater sensitivity. MEMS and integrated circuit (IC) sensors are by nature extremely small and will have very limited signal outputs. Increasing the diaphragm size only makes the cost of the silicon prohibitive. Moreover, if their output signals are amplified the noise becomes a significant part of their performance. These technologies have yet to produce a <1” W.C. range sensor with enough signal-to-noise ratio to perform well in these installations.

Capacitive Macro Sensors

Therefore, the best option for critical applications is the capacitive macro sensor. This sensor offers all the advantages of the MEMS sensor, but without pressure sensitivity limitations.
What is extraordinary is their 2” diameter tension metal diaphragm. This relatively large diaphragm is the reason macro capacitive sensing has superior performance relative to any other low-pressure sensing technology. It can be resolved one million times.
Made of stainless steel, the diaphragm is 1/10th the thickness of a piece of paper. It has excellent elastic properties that limit its movement to the width of a single piece of paper.
That means that this sensor can be set up for different pressure ranges. For instance, the diaphragm can be configured with low radial tension for low-pressure applications of 0 to 0.1” W.C., and it can be configured with high radial tension for high-pressure applications of 0 to 100” W.C., The tensioned sensor should allow up to 2 PSI overpressure (range dependent) with no damage to the unit. Also, the parts that make up the sensor should have matched thermal coefficients, which promote improved temperature performance and excellent long-term stability.

Key Performance Specifications

The following are the specifications to be considered when selecting and purchasing capacitive macro sensors for a critical application.
Pressure range is the first thing mechanical designers, engineers and architects should look at. Once the pressure range is selected, the desired accuracy can be determined. Accuracy is the difference between the actual value and the readout indicated by the transducer. It is a percentage of the full-scale value achieved by calculating the combined error of linearity, hysteresis, and repeatability. For example, if a transducer has an accuracy of ±0.5% FS and output of ±0.1” W.C. FS, then the sensor will accurately measure pressure to within
±0.001” W.C. Although accuracy is an important consideration and often the first one, it should be noted that accuracy is insignificant unless it can be sustained. One great advantage of capacitance transducers is that they are designed and specified to maintain accuracy settings for extremely long periods. Unlike sensors such as bonded strain gauges, capacitive transducers exhibit extremely low creep, ageing effects, humidity effects and other common problems of output stability. Therefore, specify a transducer that can provide long-term stability between ±0.05 and ±0.5% FS/Yr.
Also, be sure to select a transducer with standard output levels that are compatible with energy management systems. The most commonly offered outputs are a high-level analogue 0 to 5 VDC, 0 to 10 VDC, or a 2-wire 4 to 20 mA current signal. Capacitive macro sensors require no signal amplification. As a result, they avoid problems common to low-level output devices,
such as piezoresistive (thin film and IC) types. These problems normally include poor long-term stability, thermal instability, high radio frequency interference (RFI) susceptibility and humidity effects.

SpecificationPiezoresistiveCapacitive MEMSCapacitive Macro Sensors
Accuracy321
Long-Term Stability121
Output Level131
Hysteresis2111
Creep211
Response Time213
Pressure Range231
Temp. Compensation231
EMI/RFI Susceptibility221

Table 1 1 = Best, 2 = Medium, 3 = Worst

There are many considerations when selecting a low DPT for a critical care application. The selection process is difficult since are so many sensors to choose from, ranging from basic transducers to sophisticated, state-of-the-art units (See Table 1 for sensor specification comparison).
The most important consideration for selecting a pressure sensor in all of these applications is to find a transducer with a high signal-to-noise ratio. The higher the intrinsic signal output is, the better the long-term stability, thermal error drift and electromagnetic interference (EMI)/RFI susceptibility.

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