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!