Originally Published MPMN October 2004

PRODUCT UPDATE

When the Pressure Is On

Pressure sensors and transducers used in lifesaving applications must meet demanding requirements

Susan Wallace

Devices that measure various pressures in the human body have to be absolutely accurate. This means that all their components, but especially built-in pressure sensors, have to be able to withstand even harsh circumstances. The sensors have to perform even when overwhelmed by electro-magnetic interference from other equipment. To fit within the tight physical dimensions of a catheter, they have to be ultrasmall. If they are to be used in single-use products, they also need to be cost-effective.

FISO Technologies Inc. (Quebec, PC, Canada) has introduced fiber-optic pressure sensors that it says can meet all of these requirements. The company’s FOP-MIV units are suitable for minimally invasive pressure-sensing applications such as blood pressure monitoring, and intracranial and intrauterine urodynamic testing.

The sensors are based on the combination of micromechanical systems and Fabry-Perot interferometer technologies. The sensing element is a silicon diaphragm manufactured using processes derived from the semiconductor industry. The diaphragm forms a recessed cavity to be bonded on top of a glass capillary. A lead fiber-optic cable is pigtailed to the capillary.

The inner surface of the diaphragm and the surface of the glass substrate are coated with dielectric multilayer mirrors treated to partially reflect light. The space separating the mirrors forms a Fabry-Perot interferometer. Light reflected in the interferometer is wavelength-modulated in accordance with the cavity length.

As pressure rises, the diaphragm deflects and comes closer to the bottom mirror. The optical spectrum reflected by the interferometer is unique for each cavity length.

A white-light correlator is a spatially distributed Fabry-Perot cavity in which the cavity length varies along the lateral position. The conversion of the optical signal into an electronic representation is completed by a combination of a Fizeau interferometer and a linear CCD array. Light is transmitted maximally at the exact location along the Fizeau interferometer where its spacing equals the cavity length of the sensor. The interferometer makes a spontaneous correlation of the signal for all spacing values of the cavity.

Further processing detects the maximum peak response obtained by a pixel-decoded photodetector and determines precisely the absolute cavity.

The signal conditioner is designed to be integrated into an existing system, or can be externally interfaced to a monitor.
The company believes its fiber-optic technology has several advantages over liquid-filled, piezoelectric, or intensity-backed optical sensors. One of these is intrinsic immunity. Because of the optical nature of the sensors and transmission line, a monitoring system can deliver high-fidelity pressure measurements even in the presence of strong electromagnetic, microwave, or radio-frequency fields.

The sensors’ ultraminiature size is also an advantage. They are small enough to fit inside a needle or at the tip of a catheter. The nominal diameter is of 0.55 mm.

Insensitivity to light loss is another benefit. In case of light loss due to fiber bending, cable length, or light-source fluctuations, the light signal detected by the CCD array may vary but the position of the maximum peak is maintained.

No calibration is needed. An optical intelligent connector allows the user to change one sensor to another as a plug-and-play procedure. The intelligent connector has a top-mounted EEPROM that contains all the calibration parameters of each sensor.

Transducer Is Suitable for Process Systems

A pressure transducer uses CVD strain gauge technology. The Model 512 from Setra Systems Inc. (Boxborough, MA) has a thick diaphragm to withstand pressure spikes, shock, and vibration caused by the harsh physical and environmental conditions of industrial applications.

The unit’s design is resistant to aging and virtually insensitive to thermal transients and pressure cycling. Users can expect less than 0.2% drift per year.

The Model 512 offers 0.5% full-scale accuracy, a compensated temperature range of –5° to 180°F, and gauge or compound pressure ranges from –14.7 to 6000 psi. The transducer’s proof pressure specification is 43 full scale, with less than a 1.0% zero shift. All wetted parts are constructed of corrosion-resistant 17–4-pH stainless steel.

A variety of voltage or current outputs are available over almost any pressure range. Choices of pressure fitting and cable electrical termination are offered, enabling the unit to be custom configured to any OEM application.

Force Sensor Measures Blockages in Tubing

A thin-film force sensor from SMD Sensors (Meriden, CT) is used in ambulatory infusion pumps to measure occlusions in silicone or PVC tubes. The tube with associated fittings is known as a disposable set. The SMD100 sensor is pressed against the outside of the tube and detects an increase in force as downstream blockage causes pressure in the tube to increase.

An advantage of the sensor is that it can be used with an inexpensive standard tube, reducing the cost of the disposable set, thereby saving money. The SMD100 is made using a 10,000-W bridge, giving 1-mV output per volt on input power. It is available in custom shapes for different placements.

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