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Pressure Sensors: Stability, Sensitivity, Accuracy, and Other Key Specifications



Pressure sensors are used in a variety of applications, ranging from medical equipment to portable devices. As a designer, there are a few general specifications to consider before selecting a sensor for your product's design, including pressure range, environmental conditions, packaging, and output type.

Also, there are several key performance specifications that need to be considered when selecting a sensor solution. In the case of pressure sensors, these include stability and accuracy requirements, depending on the application. In addition, designers should take into account specifications such as total error band (TEB) and integrated amplification, compensation and calibration. All of these factors can impact cost and product performance.

Some sensor manufacturers, such as Honeywell Sensing and Control, have developed product platforms that simplify the way designers can select and buy these devices, taking all of these factors into account.

As an example, Honeywell's TruStability platform, initially launched in 2009, is said to be a highly flexible and configurable solution that provides more than 500,000 user-configured products. This means designers can select from a host of specs including package style, porting options, pressure ranges and types (absolute, differential, or gauge), supply voltages, and signal output types. In other words, designers can virtually design a customized product by selecting from the standard capabilities within the platform.

In addition, the TruStability platform offers fully amplified, compensated, and calibrated over temperature and pressure solutions. This allows designers to eliminate components associated with signal conditioning on the board. And by using a platform approach, suppliers are reusing proven technology, design and processes, reducing both design and purchasing risks.

Stability is paramount

Sensor stability is extremely important to maintain performance over time, and is considered by many sensor manufacturers as the number one specification to pay attention to when selecting a sensor. In some applications, the sensor offset, or sensor null, is an absolute reference point in the system so it is important to know exactly what that offset is at all times, or that it stays in a very tight range.

Why? If the sensor is not very stable – meaning that the offset can change over time, temperature or humidity, or any other environmental condition – the customer has to implement an auto null or zeroing algorithm to compensate for null drift.

It may also require customers to implement an extra valve in order to shut off all pressure that can be applied to the sensor so they can guarantee that it is in a zero or null condition. Hence, if a sensor is not very stable or changes over time, it adds cost and complexity.

As an example, the long-term stability of the TruStability platform allows customers to minimize additional processing and/or equipment that might be required in their factory or in the equipment itself. It also minimizes calibration requirements and zeroing algorithms. In some cases, according to the supplier, customers have completely eliminated the valve and auto-zeroing algorithm altogether, reducing their total cost of ownership.

Pay attention to total error band (TEB)

Sensor manufacturers may play a bit of gamesmanship when it comes to accuracy specifications. Some suppliers provide a total error band (TEB) specification, which indicates a sensor's true accuracy over the compensated temperature range, and thus whether or not the sensor meets the requirements of a design.

In other cases, sensor manufacturers only provide accuracy, which is part of the overall TEB specification, on the datasheet. Ask the supplier for additional information if the details are not provided, otherwise you will not be able to make an apples-to-apples comparison when evaluating sensors from different suppliers.

Looking again at our TruStability platform example, Honeywell specifies parts with a TEB that includes all possible errors for offset, full-scale span, pressure non-linearity, pressure hysteresis, pressure non-repeatability, thermal effect on offset, thermal effect on span and thermal hysteresis. This means that designers do not have to calculate the total effect of individual errors. Additionally, the parts meet the IPC JEDEC J-STD-020D.1 standard that indicates the part will still work within the total error band after reflow.

Figure 1: Honeywell's TEB specification eliminates individually testing and calibrating every sensor, which helps reduce manufacturing time and process. (Courtesy of Honeywell Sensing and Control)

Sensitivity vs. burst pressure

Look closely at sensitivity, overpressure and burst pressure specifications. Sensor manufacturers maintain three performance factors – high sensitivity, high overpressure, and high burst pressure – are difficult to achieve simultaneously in the same product, which translates into performance trade-offs. As an example, if a part exhibits higher sensitivity, it will typically offer lower burst pressure.

Honeywell appears to have solved this problem with the introduction of its TruStability ultra-low pressure (ULP) sensors that combine high sensitivity with high overpressure and burst pressure for medical and industrial applications.

These sensors provide an amplified compensated digital or analog output for reading pressure over the full scale pressure span in the ultra-low pressure range of ±2.5 to ±40 mbar (±1 to ±30 in. H2O).

Figure 2: Honeywell's ULP HSC and SSC sensors can be used in industrial HVAC and medical control (including ventilators, anesthesia machines, spirometers, nebulizers, and hospital room air pressure) applications. (Courtesy of Honeywell Sensing and Control)

The sensors also provide high burst pressures above 1,034 mbar (415 in. H2O), which allows the sensor to withstand a wide range of conditions while maintaining a high level of sensitivity, and high working pressure ranges above 336 mbar (135 in. H2O), which allow the devices to be used continuously well above the calibrated pressure range.

The TEB for the HSC (high accuracy silicon ceramic) Series varies between ±1 percent FSS and ±3 percent FSS depending on the pressure range. The TEB for the SSC (standard accuracy silicon ceramic) Series varies between ±2 percent FSS and ±5 percent FSS (depending on the pressure range).

A few other advantages of these ULP sensors include their small 10 x 10-mm (0.39 x 0.39-in.) package, helping to save board real estate, and low-power consumption of typically less than 10 mW, which reduces power consumption and provides extended battery life. The sensors also feature accuracy of ±0.25 percent FSS BFSL (best fit straight line).

Summary

Once you have determined the pressure range in which your device will operate, as explained above, the next steps will be to look at the type of device to choose (absolute, differential, or gauge), followed by an examination of key performance specifications such as stability, total error band (accuracy), sensitivity, overpressure, and burst pressure. In addition, the decision-making process should include platform benefits such as speed to market, total cost of ownership, and product/supplier quality. Armed with supplier datasheets and using the links provided here to obtain additional information, you will be well on your way to successfully finding the right pressure sensor for your application.

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