Designers face a big challenge when selecting pressure sensors for medical equipment applications that must withstand condensing humidity. Often, they have to deal with tradeoffs in functionality, packaging, and cost. In addition, there are few sensing options for medical devices that require liquid compatibility outside of large, heavy media-isolated pressure transducers. These sensing devices were once the only choice for designers before the introduction of liquid-compatible, board-mounted sensors.
When used with applications with low or medium pressure requirements—including gas or liquid chromatography, chemistry analyzers, ventilators, and blood analysis and diagnostic equipment—pressure sensors must interface directly with liquid media or deal with humid environments while providing highly accurate pressure measurements for a variety of media including expired air, reagents, samples, and cleaning fluids. In applications with a pressure range of less than 150 psi and size constraints, a board-mounted pressure sensor is often the preferred solution for these applications, offering the right balance of functionality, package size, reliability, and cost.
For the most accurate pressure reading, sensors should be positioned in close proximity to the media being measured. However, not all pressure sensors are liquid compatible, creating a key challenge for medical device designers. In many cases, they need to design in additional components such as a bypass or a filter to address humidity issues, or rethink the positioning of components, particularly the tubing, in the system to keep any fluids from contacting the sensor. This can result in additional design time, and cost.
Liquid, humidity protection
Today, sensor manufacturers are developing board-mounted products that protect against liquid media and high humidity environments for medical-device applications that require low to medium pressure measurements. These sensors also help simplify designs by eliminating the need for additional protection features or product redesigns so the fluids won’t contact with the sensor.
In addition to providing accuracy and repeatability at low pressures in chemistry analyzer applications, pressure sensors need to withstand higher pressures during flushing and cleaning.
As an example, a board-mounted, liquid media-compatible pressure sensor is a good choice for a chemistry analyzer because it requires the precise measurement of fluid passing through the system repeatedly. Typically, in a chemistry analyzer, a pipette draws the sample fluid and deposits it into another vial or container for mixing or analysis. Pressure sensors are used to ensure that the right amount of fluid is being measured while the sample is being drawn, and they can also be used to detect obstructions or misplacement of the pipette in the sample vial.
These pressure sensors need to provide high accuracy and repeatability at low pressure levels. However, in the same systems, they also need to withstand higher pressures that are produced when the fluid pathway is flushed or cleaned. All of this is done while in direct contact with the liquid media being measured.
Examples of such sensors include the Honeywell Sensing and Control’s 24PC/26PC board-mounted pressure sensors, which can detect such small changes in pressure from one reading to the next. They also provide precise measurement of a specific amount of fluid passing through the system at the same rate, repeatedly, which is a key requirement for analytical equipment. In addition to gage sensors that can contact the liquid media on one side of the sensor, the differential sensors uniquely offer liquid media compatibility on both pressure ports.
Honeywell’s TruStability pressure sensors provide a wide range of configuration options to speed up design and development time.
Board-mounted pressure sensors can be also used in a ventilator application where it measures the airflow to and from the patient. To ensure that the air and oxygen pressure does not exceed the level set by the medical professional, the sensor needs to provide high stability and accuracy. As an example, the tight accuracy of Honeywell’s TruStabilit HSC/SSC pressure sensors allows them to be used in a ventilator, detecting pressure at ranges down to 250 Pa (an inch of water column). These sensors offer I2C or SPI digital outputs, allowing them to be directly interfaced to microprocessors or microcontrollers, which simplifies the design while also enhancing system performance.
The TruStabilit sensors offer a liquid media option that allows them to withstand direct liquid media contact or condensing humidity. This option should be a key consideration when the sensors are placed close to the patient, particularly within the expiratory portion of the ventilator. This is where the sensor is subjected to high humidity air from the patient factors such as coughing and breathing.
The 24PC/26PC and TruStabilit HSC/SSC pressure sensors allow the designer of a chemical analyzer, ventilator, or other similar type medical devices to position the pressure sensor as close as possible to fluids or humid gases to achieve the most accurate pressure reading. These sensors can handle a wide range of noncorrosive, nonionic liquids or gases. They provide design flexibility through options for specifications such as pressure range, package styles, and digital/analog outputs.
The TruStabilit pressure sensors offer several seal options to meet requirements in a wide range of liquid media and humid environments.
Because many medical device applications have the potential to be in contact with fluids and gases, designers should always consider pressure sensors with liquid media compatibility to simplify their designs. Designers should understand what types of fluids may be in contact with the pressure sensor to ensure that it is compatible with those fluids.
Designers also should evaluate pressure sensors that are fully compensated, amplified, and calibrated for easier implementation. These sensors can eliminate components from the printed-circuit board related to signal conditioning. Advantages range from space savings and higher reliability to lower costs and faster implementation.
Matching sensors with application requirements
Keep in mind, though, that selecting the right pressure sensor depends on the application’s requirements. As an example, a low-cost, unamplified, uncompensated pressure sensor with liquid media compatibility may be all that’s required for an alarm circuit.
Pressure sensors should be easy to integrate into a design by offering flexible configurations through options for port styles, packaging, output types (analog and digital), and other features. Prevalidated sensors are also available to help accelerate design, development and manufacturing cycles.