Cabin Pressure Testing

Cabin Pressure Testing

Proper cabin pressure is crucial in the aerospace industry. After all, a pilot rendered unconscious from lack of oxygen will not be overly helpful at the controls of a complex aircraft. Therefore, it falls to the engineers to develop a stellar cabin pressure system that will withstand even the most extreme conditions. To do that, we will of course be spending a great deal of time at the bench testing and re-testing every manifold, valve, and pressure vessel. So what do we need to create an effective and resilient cabin pressurization system? An effective and resilient pressure transmitter of course! In the following article we will cover many of the possible options and applications of the STS pressure sensors and how we can use them in this situation.

As we piece together our master plan for the cabin pressure test we will want to focus our attention on two critical factors; temperature resistance and overall accuracy. For our example, let’s move forward with a turbofan aircraft. As the air enters the engine, it is compressed by a series of rotors and a portion of this compressed air is diverted towards the cabin air system for the pressurization process. Now is the time to remember the compressible flow equations. As the incoming air is compressed, the temperature will also increase very quickly. Immediately after this initial compression the cabin air is transferred to a preliminary intercooler to shed a certain amount of that heat to the ambient air.  

As you can imagine, there is a great deal heat going into this area of our system. So naturally, if we wish to install a test pressure transmitter in this space to fine-tune, or verify, our cabin pressurization process, we’ll need one that includes an exceptionally high temperature resistance. Well, the STS line of pressure sensors offers us just that with a temperature limit of 150˚ C (302°F), where the sensor will continue to the function and transmit accurate data even in these  warm conditions. Furthermore, STS has adapted a fully customizable and modular approach to their design process to give us access to many more features in addition to superb temperature tolerance.  

Once the pressurized air has been cooled sufficiently, and its pressure recorded by our test sensor, the air can proceed to the primary manifold where the still warm air is mixed with colder atmospheric air to achieve a comfortable environment for the pilot. This is yet another crucial link in our cabin pressurization process, and it is therefore very likely to be equipped with a test sensor throughout the course of system testing. However, the conditions here are vastly different from those seen in the intercooler. Will the same pressure sensor even work here? The answer from STS is, YES! The wonderfully adaptable modular approach to the STS line of pressure sensors ensures that we will always be able to order a sensor to fit our needs.  

For our purposes, the manifold is one of the last stops for the air before it is passed along to the cabin. Therefore, accurate pressure measurements are crucial to ensure that the cabin is kept at standard ground level atmospheric pressure. With that in mind, we have the capability to select the most accurate variation of the sensor at ≤± 0.05% FS. This highly precise transmitter, the ATM.1ST model, will ensure that we the engineers have reliable and consistent data for this particular stage in our cabin pressurization sequence. 

While we’re on the subject of options and modules, STS also gives us the flexibility to select from a long list of possible electrical connectors and output signal types to ensure that each sensor is precisely assembled to our needs. This saves us from the painstaking process of redesigning a test fixture to the sensor’s needs. The standard connectors that we can readily choose from include PUR, FEP, and 5-pin M16 connectors. However, if this is not exactly what we need, STS does have the capacity to work with us to create an entirely custom connector, so there’s nothing to worry about!  

The last stop in our cabin pressure system that could do with a sensor during our testing project is the outflow valve. It is here that excess air is bled off into the atmosphere if we approach the point of over-pressurizing the cabin. Just like a test sensor in the manifold, accuracy is pivotal to ensure that we are maintaining the exact desired pressure in the cabin at all times, so once again the high precision ATM.1ST line would seem a logical starting point.  

Let us briefly reiterate the stops we made along our test plan. First, we have the intercooler which serves a fundamental role as the air moves towards the passenger compartment. Therefore, this location is also fundamental for our testing and requires a sensor that can register highly accurate data while at the same time resisting the high rate of temperature exchange in that particular area. Can the options available to us with the STS sensor accomplish this? Check. Next we moved to the manifold, or air mixing box, where accuracy and consistency are paramount. What’s more, a temperature transmitter would not go amiss in this area. Can we tackle this task through STS? Check. Last stop, the outflow value, where we once again need to precisely measure and record pressure data for our test, and again we can put a big check mark next to STS pressure sensors being able to keep up. All in all, the ATM.1ST pressure sensor has the potential to fulfill all our diverse testing needs throughout a dynamic and complex aircraft system, so stride forward confidently into the world of cabin air pressure!

Pressure measurement of fuels – Material selection is decisive

Pressure measurement of fuels – Material selection is decisive

Aggressive liquids and gases pose a particular challenge to the pressure-sensing technology employed. For this reason, sensors are required which can be flexibly adjusted to the particular requirements. With the ATM.1ST product series, you will always remain on the safe side. 

A significant product characteristic of pressure transmitters is their modular construction. A variety of mechanical and electrical components can be co-assembled, according to application, to:

  1. optimally maintain the usage of matched pressure transmitters, and
  2. ensure a rapid implementation of the measurement setup.

Figure 1: Assembly of a pressure sensor with O-ring measuring cell

The basis for this are high-quality measuring cells of a piezoresistive nature, which are sealed using O-rings. This construction allows for a multitude of combinations. Dependent upon usage within the pressurized medium, various O-ring materials are employed (Viton, EPDM or Kalrez) to optimally tailor the pressure sensors to that particular application.

Figure 2: Example of a metal-seating pressure measurement cell

For application in aggressive media such as fuels (diesel, gasoline, …) or in high-pressure operations, however, sealing with O-rings becomes unsuitable. In such environments, the measuring cell has to be welded together with the pressure port. For this reason, an elastomer-free metal-sealing variant was developed for applications in fuels: The ATM.1ST product range.

These elastomer-free (metal-sealing) versions can be offered in the most diverse of mechanical designs. In the accuracy class of 0.05% FS, the pressure transmitter is available in nominal pressure ranges from 0…20  bar up to 0…100 bar and with an output signal from 4 – 20 mA.

In the 0.1% accuracy class, the pressure sensors are offered in nominal pressure ranges from 0…20 bar up to 0…700 bar and in versions of 4-20 mA or 0 – 5/10 V.

The analog transmitters are calibrated in two temperature ranges, -25…125°C (standard) or -40…125°C (optional). Across both temperature ranges, a Total Error Band of < 0.4% FS is guaranteed.

Featuring a shortened form, robust housing and a very high flexibility, the ATM.1ST product range allows end users to configure these pressure sensors according to the prevailing requirements. Regardless of pressure port or electrical connection, a broad range of possibilities for mechanical mounting are available.

With this convincing technical specification, these pressure sensors are ideally suited to various fields of application in measurement technology or plant and mechanical engineering, as well as in the equipping of test beds or calibration facilities.

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.