Selecting your pressure sensor: A how-to guide for the aerospace engineer

Selecting your pressure sensor: A how-to guide for the aerospace engineer

Devising and creating an aircraft is a daunting task, and no small feat by any means. The endless calculations, designing, simulations, and re-designing seems to be a perpetual process; however, we will eventually reach the milestone of intensive testing! This is a very exciting process, all the 3D parts you’ve designed, the systems you’ve pieced together, and all the components are now sitting right in front of you. It is time to prove to yourself, and your managers, that everything will operate flawlessly, but don’t get ahead of yourself! To do that, we need top-notch data recording equipment to verify our system’s performance. What’s more, we need test sensors that can function in the most extreme conditions both inside and outside the aircraft. Well, that is why STS is here, to furnish us with reliable pressure measurement transmitters to ensure that our rounds of pressure testing work just as smoothly as the system we designed. We’ll spend the rest of this article presenting a step by step guide to fully acquaint you with the full range of options that STS offers and how to integrate those into our system.

Accuracy

Step one, we need to take a close look at the aircraft system we’re testing, and determine the precision required for our data collection. For example, the hydraulic system that controls the aircraft’s brakes often operates within a specific pressure range, and this range is large enough that extraordinary precision is not a requirement when selecting a test sensor. Therefore the STS option of ± 0.25% FS would be a suitable option. On the other end of the spectrum, the oil pressure must be monitored much more judiciously when compared to the brake hydraulics. With that in mind, we can select the STS option for a high precision pressure transmitter with the highest degree of accuracy available, namely ± 0.05% FS to ensure that the oil pressure remains at its peak level throughout the engine system. 

Temperature  

Now that we’ve established the required accuracy for our application, let’s move on to integrating the pressure sensor into our test aircraft system. Naturally, the pressure oriented systems on an aircraft are exceptionally diverse in terms of size, operating temperature, and pressure medium; consequently, we need the freedom to cherry-pick every one of these features for our sensor. 

For the next step in the selection process, let us turn our attention to the operating temperature. In an aircraft, your test pressure sensor could potentially be recording data within the sweltering confines of the engine compartment. Conversely, it could be located externally, measuring the Pitot pressure or perhaps the de-icing fluid pressure in which case the operating temperature will be drastically lower than the engine compartment. Never fear, STS offers an impressive range of operating temperatures from -25 to 125° C. This base range will by and large cover the majority of our aerospace pressure needs. To sweeten the deal, all STS sensors are manufactured to include a compensated temperature range, meaning the inherent measurement error is drastically lower within the limits specified above. This is an exceptionally beneficial feature when completing intensive testing on our pressure systems! 

The aforementioned temperature range is by no means set in stone. When the need arises, we can opt to have our sensor outfitted with cooling fins to boost to max temperature to 150° C. Such a need might arise if the sensor was to be located next to the engine exhaust system which can radiate a significantly large amount of heat. Furthermore, we can choose for our sensor’s minimum temperature to be lowered -40° C if the sensor was to be exposed to a particularly high altitude. That covers the selection process for your sensor’s temperature resistance; always keep your operating environment in mind!

Process Connection

As previously mentioned, the sizes and gauges of the different pressure systems within an aircraft are far from constant. Therefore, the next step in our selection process is to determine the optimal location for the sensor, and select a connector that will allow the sensor to fit in that particular location. For example, take an aircraft brake system. The hydraulic system will consist of various tube sizes and components, but once you have selected the exact location for your sensor, the process connection can be chosen. STS offers a range of sizes and diaphragms including G ¼ M and G ½ M with the additional choice for Hastelloy and frontal diaphragms, amongst other choices. This wide range of possible selections ensures that we can order a sensor that will slide into our test system perfect without any special retrofitting in order to install, which lowers the workload for us!  

Seals 

The final major component of our test sensor that we’ll cover is the sealing materials that are available to us. As with the process connector, the material to select to seal your sensor is highly dependent on the fluid that makes up your pressure system. Luckily for us in the aerospace field, our pressure systems will seldom experience corrosive, acidic, or other unsavory fluids. Nevertheless, we still must give some thought to our seals. In the case of our hydraulic system for landing gear, the standard choice is Nitrile (NBR) as our seal. This rubber-like material is ideally suited for this application in addition to being resistant to oils and other lubrication materials. However, if we’re expecting high temperatures or other harsh conditions that are present in an engine compartment then Viton would be a much more suitable choice with its improved temperature resistance and durability. Last but not least, EPDM rubber has a proven track record when dealing with brake fluids. These are only three of the many sealing options that STS offers, with the main takeaway being that not all seals are interchangeable. Research your system, the options available, and make the best choice to ensure optimal sensor results! 

 

Now you are fully prepared to begin the pressure sensor selection process for your aerospace testing! We’ve covered the level of accuracy required for your sensor, which is dependent on the exact system in which the sensor is located. We then moved on to determining the correct level of temperature resistance required for our individual applications. Followed by the process connection where we can select various sizes and diaphragms to ensure that the sensor is always tailored to our exact needs. Our last point was to explain the primary differences between the many seal options that are available to you, and the ideal application of each one. With this information, you can look at the primary components of your test pressure sensor and make the best selections to ensure that your sensor is quite literally made just for your use!

Landing gear hydraulic pressure testing

Landing gear hydraulic pressure testing

Imagine, you’re a pilot in your own plane, cruising on a beautiful day. You line up your approach to the airstrip and flip the switch to actuate the hydraulics for the landing gear to deploy. Suddenly, a low pressure warning appears and the landing gear won’t deploy. Now you have a problem! Amongst the grumblings under your breath will likely be a few unsavory comments about the engineers that designed that hydraulics system. Well, that’s us; and that pilot we’re imaging, that’s our client. They deserve to have a reliable and flawless landing, don’t they? Therefore, it’s on us to design a hydraulic system that can achieve just that, but how? Well the specifics, components, and design are across the board from plane to plane; however, there is one point that is guaranteed to be universal for our endeavors, hydraulic pressure testing! We will all test and test, then try to damage the system and test again! So naturally, we need a pressure sensor that can consistently and accurately record the conditions in our hydraulics system as we fine-tune all the details. Well we have a sensor that can do exactly that and for the remainder of this article we will explore the capabilities of the STS high precision pressure transmitter ATM.1ST.

As we begin to develop our hydraulic pressure measurement regimen, we must first determine the exact data we wish to collect. As we all know, ‘pressure’ is a very broad term to use in a hydraulic system and has very little meaning on its own. Are we referring to the accumulator pressure, pump delivery pressure, the regulator pressure, or perhaps the relief pressure? That decision is up to you, but thankfully STS has developed a series of pressure transmitters that can collect data in any of these sub-components. What do we mean by that? Well, the STS high-precision pressure transmitter ATM.1ST is designed with a modular and adaptable approach. We, as the engineers, can cherry-pick the features and capabilities for every sub-component of the sensor to ensure that every last one is perfectly suited for the environment that it will encounter over the course of the test.  

Let us now break down these sensor modules for a moment. First, we have our choice of materials for nearly every part of the sensor to ensure strength and durability. For example, the housing and transducer can be constructed from stainless steel or titanium depending on the burst pressure we must withstand amongst other factors, and this will in turn be determined from your particular hydraulic setup.  

However, our material selection is not limited to the housing. We also have the power to pick and choose the seal material for our sensor. The selections in this department include Viton, EPDM, Kalrez, and NBR. Naturally, the hydraulic fluid will be consistent throughout the landing gear system; therefore, once we determine the seal material that will have the best interaction with the fluid that particular material can be guaranteed to function throughout the system. Another factor to be kept consistent throughout our test setup is the overall accuracy of our pressure sensors. Luckily, STS grants us leverage over that characteristic as well with the high precision sensors of the ATM.1ST line. We have 0.25%, 0.1%, and 0.05% FS to ensure that our data collection is both accurate and consistent throughout the entirety of the test.  

The last two modular selections that are prudent to our landing gear testing are the electrical and process connections. In the electrical world we have FEP, PUR, and PE cables to select from, along with a range of different connectors. As for process connections, our diaphragm, DIN, and other specifications are entirely at our discretion. While the sheer volume of different combinations may seem slightly overwhelming at face value, they grant us the capability to piece together a pressure sensor that will slide into our test setup seamlessly without driving any special setup or design changes.  

Now let us return to our landing gear testing. As we develop and test the hydraulic system to achieve flawless landing gear operation we are going to need data from several locations within the system. As mentioned above, we have the accumulator that acts as a damping device of sorts to smooth out any pressure variations within the system. Naturally, we as the engineers need to know what those variations are exactly. Therefore this seems to be a perfect location for a test sensor!  

On the subject of pressure maintenance, the regulator also falls squarely into this category. As the pressure fluctuates due to valves opening and closing or any irregularities in the system, the regulator kicks in to ensure that the system pressure remains within the specified range. Once again, this is another crucial component to keep an eye on as we develop our landing gear, and we now have the resources to select a perfectly customized pressure sensor to slide into the system and accomplish just that with precise measurements and easy to install electrical connectors. 

In review, we are tasked to develop a reliable landing gear system via a rigorous curriculum of testing. However, the hydraulic system of such a mechanism is extremely diverse in terms of components and potential locations for sensors. Thankfully for us, STS has produced a reliable little powerhouse in the ATM.1ST pressure sensor that allows us to have nearly complete jurisdiction over all aspects of the sensor; including materials, accuracy, sealing, and electrical connectors. Long story short ladies and gentlemen, this high-accuracy sensor allows us to devise a streamlined and robust testing process where our test sensors complement our setup, and do not dictate it.