The Art of Low Pressure Sensing and Finding the Right Sensor

July 1, 2019 | 11:55
By Ian Bentley, Engineering Fellow, Honeywell Sensing and IoT

Don’t let the wide range of available low-pressure sensors be intimidating. Because of the range, there will likely be a sensor, or combination of sensors, to meet most application requirements. The key is to know your requirements and constraints first.

Consider just two industries, and see the wide range of possible applications:
  • Medical: Airflow monitors, anesthesia machines, blood analysis machines, gas chromatography, gas flow instrumentation, hospital room air pressure, kidney dialysis machines, nebulizers, pneumatic controls, respiratory machines, sleep apnea equipment, spirometers, ventilators.
  • Industrial: Barometry, drones, flow calibrators, gas chromatography, gas flow instrumentation, HVAC clogged filter detection, HVAC systems, HVAC transmitters, indoor air quality, life sciences, pneumatic control, VAV (Variable Air Volume) control, weather balloons; energy management systems

Recently when Surrey Sensors Ltd. needed more accurate, smaller, and cost-competitive pressure sensors for use in their pressure measuring systems, they found that the Honeywell TruStability™ RSC Series and HSC Series Digital Board Mount Pressure Sensors were the only solution that met their list of strict requirements, especially at ultra-low pressures.

Honeywell low- and ultra-low pressure sensors are manufactured and rated to help account for application design considerations that affect output readings:
  • Overpressure – how high a pressure can go without damaging a sensor.
  • Burst Pressure – how high a pressure can go without bursting the sensor diaphragm.
  • Resolution – how small a pressure change can be measured by the sensor.
  • Stability – innate characteristic of sensor readings and output to drift over time.
  • Total Error Band – a measure that combines all of these considerations into a value that helps you apply sensors for your application.

All Honeywell pressure sensors bring high accuracy, durability, and design flexibility required by engineers. For medical applications alone, measuring tiny changes in the pressure of human breath can be lifesaving.
A good place to start when applying pressure sensors is to be aware of even unexpected, sources of pressure changes in your application. Being aware of application constraints can remove confusion when selecting appropriate sensors. That allows you to accommodate the constraints with additional sensors, or even change the application to remove the constraints.
High pressures are easier to measure and require less robust sensors. On the other hand, measuring lower pressure ranges require more robust and sensitive sensors that are also more susceptible to unexpected higher pressure bursts. A good cough in a breath monitor can suddenly cause 80 times or more the expected pressure, and damage a sensor.
Measuring small pressure changes in a small diameter plastic tube may seem easy. But also consider small how pinches in that tube, caused by tube movements, or mechanical wear at connection points, can throw readings off.
One solution is to measure for anomalies before they cause problems. You could measure for higher pressure bursts upstream before they reach, and potentially ruin, sensitive low-pressure sensors downstream. Have additional sensors before and after potential constrictions to identify anomalies before they become problems and contaminate results.
Be aware of potential issues in your manufacturing process to avoid problems. Vacuum pick-up tools, used when placing sensors on circuit boards, creates low pressures that can negatively affect, or even rupture, sensitive pressure sensors. Inadvertently blocking sensor inlet or outlet ports with debris can cause major problems when sensors make it to the end user application. Even post-manufacture cleaning done incorrectly can leave debris that clogs or enters sensors and contaminates results.
Even unexpected sound waves cause differences in pressure. HVAC or box cooling fans turning on, or doors opening and closing, can cause unanticipated pressure changes by the noise alone.
If you can’t eliminate the possibility of noise, you can at least mitigate the impact by using various techniques. Keep both sides of your pressure sensor physically close to each other. Minimize pressure tube runs to minimize noise on one side. Use pneumatic filters to remove physical contaminants, and dampen noise contaminants. Just remember, any physical filter can also affect the pressure difference between the ports and must be cleaned or replaced as they clog so they don’t affect readings. You can use additional pressure sensors to proactively measure the condition of those filters.
You can also filter out errors by digitally accounting for drift over time. Good sensors minimize long-term drift - most of which occurs over years rather than days. In fact you may decide your application is not affected by drift.
If drift is important in your application, you can apply offset correction by taking a reading when you are in a known zero condition, like at application start-up in the morning or on a weekend.
Tube runs
Then subtract that value from real-time sensor readings until the next time you can officially provide a zero condition. You can also obtain that zero value by directly connecting input and output ports and take that as a zero reading for the calculation. This process can provide a correction for offset to within a quarter percent error, and provide a major improvement in the quality of the output.
In summary, know your design considerations and constraints first. That makes applying the correct sensors easier. Honeywell helps by providing Total Error Band values for each sensor that combines the considerations and constraint characteristics in one value that makes it easier for you to apply the right sensors.
 

The Author

Ian Bentley is an Engineering Fellow at Honeywell with 34 years of experience in sensor design and manufacturing. Over the years, Ian has lead design teams working with several different pressure sensing technologies, airflow sensors, humidity sensors, magnetic sensors and accelerometers.

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