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Using WEBENCH for Designing Pressure Sensing Circuits

Welcome to All Sensors “Put the Pressure on Us” blog. This blog brings out pressure sensor aspects in a variety of applications inspired by headlines, consumer and industry requirements, market research, government activities and you. In this blog we’ll look at WEBENCH and how it can help design a pressure sensing circuit.

Using WEBENCH for Designing Pressure Sensing Circuits

The WEBENCH Design Center from Texas Instruments provides several online techniques to simplify interfacing pressure sensors and other products. By selecting the Sensors option from eight possibilities on the homepage, then the SENSOR AFE (analog front end) from the pull down Sensor Tool menu and then Pressure from the eight sensors options, you access a list of pressure sensor manufacturers and part numbers with over a dozen parameters identified.

Selecting a specific sensor such as the 1 INCH-G-BASIC sensors from All Sensors Corporation leads to a screen with the LMP90100 and the selected sensor. With this screen you can select from nine application parameters and see the performance of the design. The LMP90100 is a highly integrated, multichannel, low-power, 24-bit Sensor AFE. Estimated device performance of the sensor and AFE combination is indicated by Input Referred Noise, ENOB (effective number of bits), NFR (noise-free resolution), Current, and Device Error. Pulldown menus and tutorials allow you to modify several aspects and hone in on the right design.

Another design path starts by selecting the Sensors option from eight possibilities on the WEBENCH Design Center homepage, then Sensor Designer from the pulldown Sensor Tool menu and then Pressure Sensor Amplifier Design from the three options. When you press start design, you access the WEBENCH®Sensor Designer. Once again, you can select the sensor supplier and the sensor for a specific application. For this tool, you can narrow the search to a specific sensor supplier from four suppliers. For example, selecting All Sensors Corporation provides a list of 21 products ranging from 1-inch of water to 15 psi to select from.

Selecting a specific sensor provides a list of key parameters for a standard product and you have the option of modifying parameters to create a custom sensor. For ease of availability, the best choice is the standard device. This leads to screen with an ADC and amplifier selected for the sensor and additional information as shown below. However, you have the ability to modify many of the design parameters. Once you are satisfied with your choices, you even have the ability to obtain documentation and a prototyping kit.

WEBENCH provides sensor system designers a great starting point, design options and a simplified path to evaluating system performance.

What do you think/Comments?
Do you have a pressure sensing question? Let me know and I’ll address it in an upcoming blog.
-Dan DeFalco, Marketing Manager, All Sensors Corporation ([email protected])

Standards for Pressure Sensing Applications

Several standards exist for sensing pressure in automotive, medical, industrial, military and other applications.

In 1981, the Society of Automotive Engineers, now SAE International, published SAEJ1346 “Guide to Manifold Absolute Pressure Transducer Representative Test Method” and SAE J1347 “Guide to Manifold Absolute Pressure Transducer Representative Specification.” These documents use the manifold absolute pressure (MAP) sensor to provide guidelines for specifying and testing sensors in the recently developed engine control systems.

The Association for the Advancement of Medical Instrumentation (AAMI) developed standards for blood pressure transducers in sphygmomanometers (SP10, 1987) and disposable blood pressure (BP22) applications. SP10 and BP22 are now American National Standards Institute (ANSI) standards as well: ANSI/AAMI SP10-1992 and ANSI/AAMI BP22:1994/(R)2006.

In 1993, the Institute of Electrical and Electronics Engineers (IEEE) and National Institute of Standards and Technology (NIST) initiated a standards creating activity that has led to seven accepted and proposed standards addressing several aspects of smart sensors for industrial applications: IEEE Std 1451.1 to 1451.7. Pressure sensors are among the sensors covered in these documents.

In 2000, the U.S. Congress passed the Transportation Recall Enhancement Accountability and Documentation (TREAD) Act. The National Highway Traffic Safety Administration (NHTSA) of Department of Transportation (DOT) “Tire Pressure Monitoring System” FMVSS No. 138 addresses the requirements of this act.

These are some of the more well-known standards for pressure sensors. Additional standards that indicate requirements that a customer or government could impose on a pressure sensor used for a specific application include (but are by no means limited to):

MIL-STD 202G Method 105C Barometric Pressure (9/12/63) describes test procedures for barometric sensors used in high altitude aircraft.

The International Standards Organization (ISO) has several standards under ISO/TC 30/SC 2 – Pressure differential devices, as well as ISO 21750:2006, Road vehicles – “Safety enhancement in conjunction with the tyre inflation pressure monitoring” and others. ISO 15500-2:2012(en) Road vehicles — “Compressed natural gas (CNG) fuel system components” has two parts that specifically involve sensing pressure: Part 2: Performance and general test methods and Part 8: Pressure indicator.

NSF International has a certification program specifying safety and quality requirements for automotive in wheel tire pressure monitoring sensors for the aftermarkets parts industry.

ASTM International, formerly the American Society for Testing and Materials, has issued “Standard Specification for Transducers, Pressure and Differential, Pressure, Electrical and Fiber-Optic, Active Standard” ASTM F2070 that covers the requirements for pressure and differential pressure transducers for general applications.

The U.S Federal Drug Administration has issued “Non-Invasive Blood Pressure (NIBP) Monitor Guidance,” most recently updated in 2014.

Microsoft’s Object Linking and Embedding (OLE) standard is used in the OLE for Process Control (OPC) standards by the OPC Foundation to define requirements for interoperability in industrial automation systems.

Pressure’s Role in Predicting Weather

With all the harsh winter weather occurring recently, knowing what to expect in your area is important knowledge for short term planning and, in some areas such as the northeast, for long term planning. Sensing barometric pressure was one of the earliest forecasting tools and continues to be important today.

One website provides the barometric pressure history from numerous major cities in the U.S. from Friday, Feb 20 to Thursday, Feb 26. The high and low readings only tell part of the story. The intensity of the weather depends on how quickly the front develops and other factors.

City	Highest pressure in period (In. Hg)	Lowest pressure in period (In. Hg)
Boston	30.55	29.55
Minneapolis	30.8	29.65
Denver	30.35	29.75
San Francisco	30.35	29.85
Chicago	30.7	29.7

In stormy weather, the barometric pressure tends to be lower and a lower reading is one sign of approaching inclement weather. During fair weather, the barometric pressure is typically higher and if the pressure begins to rise, it is a sign of tranquil weather.

According to the webpage CHANGES IN ATMOSPHERIC PRESSURE, the barometric pressure is reduced through several processes:

The approach of a low pressure trough
The deepening of a low pressure trough
A reduction of mass caused by upper level divergence (vorticity, jet streaks)
Moisture advection (moist air is less dense than dry air)
Warm air advection (warm air is less dense than cold air)
Rising air (such as near a frontal boundary or any process that causes rising air)

In the U.S., the Federal government uses pressure as part of several methods available to predict weather. On Feb26, the weather in many places around the U.S. was much less severe than it had been in recent weeks with at least one exception.

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF NORTH AND CENTRAL NEW MEXICO.

Humidity 63%
Wind Speed SE 20 mph
Barometer 30.04 in. Hg. (1017.6 mb)
Dew point 17°F (-8°C)
Visibility 9.00 mi
Wind Chill 15°F (-9°C)

For weather forecasting and other sensing applications, accuracy and other factors make the measurement acceptable for reliable use. That is where the expertise of the user comes into play.

Another PSI Sighting: Pressure Shows Up Everywhere

While I could be more sensitive than many other people, any indication of the need to measure and/or control pressure in everyday situations usually catches my attention. A recent observation was the number on a neighborhood fire hydrant – in big letters it stated 200 psi. It turns that this is a common working pressure design criteria for residential fire hydrants.

Conducting flow and pressure testing measurements require a pitot gauge and a fire hydrant cap gauge. Pressure measurements on fire hydrants are performed primarily using analog gauges with a 0 to 300 psi range although digital instruments do exist with one digital gauge specifying 0.5% accuracy. Static pressure is the normal pressure existing on a system before the hydrant flow valve is opened. Local requirements vary but in one case, normal minimum water pressure in a distribution system cannot be below 35 psi when a fire hydrant is opened downstream and the minimum water pressure (residual psi) cannot be below 20 psi. Observed pressure requirements include: 75 psi for larger cities and 50 psi for smaller cities.

In addition to the pressure range and accuracy, environmental aspects for a fire hydrant pressure sensor include the ability to withstand the contact of water and possibly other materials. Properly specifying the “designed for” and other operating pressures and environmental requirements are just the beginning of getting the right pressure sensors for testing fire hydrants or measuring the pressure of any flowing/static liquid.