Test fixture pressure sensors – Pressure measurement in the aircraft engine compartment

Test fixture pressure sensors – Pressure measurement in the aircraft engine compartment

As many engineers have found to their chagrin, dealing with pressure measurements in the engine compartment of an aircraft can be a delicate and frustrating experience. The heat, vibrations, orientation, and a multitude of other factors come into play. So how can we hope to develop a method for consistent and accurate readings? Well naturally, we’re left with hours, days, and most likely months of testing! However, we still need a test sensor that can rise to the occasion, function through all these changing conditions, and continually produce correct and repeatable results. We are engineers after all, and repeatable results are an occupational necessity. Thankfully for us, STS has stepped up to the plate to provide a complete series of pressure sensors to meet all of our testing needs. Where those needs can range from specific temperature requirements, size constraints, sealing material, and electrical output signals. All of these requirements will be covered in the following article as we address STS pressure transmitter usage for our testing needs.

Continuing with our engine compartment example, let’s zero in on the oil pressure. One of the first concerns while selecting a pressure sensor for this test is temperature resistance. Naturally, it gets quite warm next to an aircraft engine; therefore, we must ask ourselves, can the sensor be mounted alone or does it need a heat shield? More importantly, will the sensor even function properly when the components begin to heat up? Erratic oil pressure readings are very low on the wish list for a pilot! Therefore, both are valid points; but don’t fret too much. The STS line of pressure sensors include excellent temperature resistance, up to 125° C. This, in most cases, takes care of our initial temperature concerns and allows the sensor to be mounted it the most logical position in the engine compartment without the need to worry about temperature interference. Furthermore, we can fiddle, finagle, and fine-tune the test sensor’s location without constantly looking over our shoulder to see if the increased temperature will manipulate our results.  This provides us with a great deal of flexibility when constructing our test plan. 

Along the same subject of mounting locations, the size of the sensor is also crucial. Trying to wedge an ungainly box next to your sleek engine for a series of oil pressure tests would undoubtedly result in a few raised eyebrows amongst all those concerned. Additionally, space in this area is constantly at a premium. However, that is one bridge you don’t have to cross as STS has produced a very compact and low profile pressure sensor that makes for convenient mounting throughout your testing area of operations. Thanks to the advanced customization options, which we’ll discuss later, the exact dimensions vary from sensor to sensor. However, they tend to fall within the 50-60mm (2.0-2.4”) range. This small size allows for easy fixturing using common Adel clamps or any other off the shelf bracket without spending the time to design a custom mounting scheme, or trying to dream up a new overly complicated fixturing method every time the sensor has to be relocated to find the optimal position for oil pressure readings. All in all, this is certainly a time saver when we are focused on a timely and efficient series of tests.  

The last factor that we’ll touch on that can be invaluable for our pressure testing is customization. More often than not, the pressure sensors that are readily available on the market for such a test have a well-defined scope that they operate in. A single configuration that works best in ‘this’ pressure range, for ‘that’ frequency of collection, and it all comes with just ‘this’ product design. However, STS pressure sensors offer several options and customizations that give us the freedom to not limit our test based on our sensor’s individual capabilities.  

For our example, we of course must have a sealing material that with neither contaminate the oils, nor degrades with constant exposure. Well, we have several options for the sensor seals that can accomplish just that, including EPDM and Viton to ensure that the sensor is operating at peak performance for the entire test. Or, conversely, we can opt for a metallic sealing option to ensure proper test results. What’s more, perhaps we need a frontal diaphragm connection, with a PUR cable, along with 20 mA output signal. STS can deliver exactly that, along with any number of other combinations to ensure that the process connection, electrical and output signals, pressure connection, and seals are exactly what we need. In essence the sensor is cherry picked for our test and not simply some component we need to shoehorn into the test setup.  

To recap, we are required to design a series of oil pressure tests; and as with most tests, many of the factors will be manipulated. The heat, mounting method, pressure range and a mind-numbingly large number of other issues will all be changing constantly over the course of the test. To cap it all off, we need a test pressure transmitter that can fit into this envelope and consistently produce accurate results. Well we can at least nip that problem in the bud straight away by incorporating an STS pressure transmitter for our testing regimen. The high temperature and pressure ranges, combined with custom seals, process connections, electrical and signal outputs, and overall design ensure that this is a sensor that can pre-configured to slide seamlessly into your testing apparatus, and not require that your entire system by reconfigured to suit the sensor.

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!

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!

Pressure measurement on injection molding machines

Pressure measurement on injection molding machines

Injection molding machines operate at the very highest of precision. The Swiss company Netstal-Maschinen AG offers high-performance and high-precision injection molding machines and system solutions for the beverages, packaging and medical technology industries. Pressure sensors manufactured by STS are installed inside these sophisticated devices.

With a plastics injection molding machine, finished plastic components are produced from plastics originally in granular form. A device such as this, in simplest terms, consists of two components: the injection unit and the clamping unit. The raw material is prepared inside the injection unit. As a general rule, this is heated and homogenized within a worm barrel positioned inside a hydraulic cylinder. Inside the clamping unit is a tool which represents the negative profile of the plastic component to be finished. The molding compound prepared inside the worm is then injected under pressure into this negative form.

Monitoring of the required pressure ratios is indispensable in guaranteeing a flawless injection molding process. Sensors for this purpose are thus mounted in the hydraulic circuit of the injection axis. The specific melt pressure can be calculated on the basis of the measured chamber pressure during the injection procedure itself. It is especially important here that sensor measurement error is extremely low, since the plastic pressure would otherwise then be calculated either too low or too high.

When the melt pressure is too high or too low,

  • this influences the injected filling volume,
  • the finished plastic component may be defective,
  • it can lead to loss of material or tool damage,
  • it can result in a standstill of the unit.

High-precision devices like the injection molding machines from Netstal-Maschinen AG require pressure transmitters delivering totally reliable output across the required measurement range. To find the best solution to such high demands, extensive tests were conducted using instruments from various manufacturers. Not only the precision of the measuring instruments was to be tested here, but also their long-term stability at high temperatures. The following measurement intervals were performed on the test bench:

Figure 1: Standardized testing procedure for evaluating a suitable pressure transmitter. Following four, six and eight million pressure cycles, the pressure sensors were each subjected to temperature stress (artificial aging).

The high precision ATM.1ST pressure sensor from STS achieved the best ratings during this thorough test in terms of tolerance, long-term stability, and accuracy and precision across the entire pressure and temperature range. Particularly decisive, above all, was that the pressure sensor, even over an extended period, had no issues with high temperatures and, in the lower pressure range, impressed with its extremely high precision.

Figure 2: Analysis of an STS pressure transmitter over time and temperature. OZ (Original Status – in red, dotted line) was applied as the starting point, the extended lines each after a fixed interval and the dashed lines take into account the aging process according to the testing procedure in Figure 1.  The value Tolerance Range Sensor relates to manufacturer specification (data sheet), with the solid lines Tolerance Range NM representing the target values of the analysis.    

A further advantage of the ATM.1ST is that it can be easily adapted to individual applications due to its modular construction. The data in summary:

  • Pressure measurement range: 100 mbar … 1,000 bar
  • Relative and absolute measuring ranges
  • Accuracy: ≤ ± 0.10 / 0.05 % FS
  • Operating temperature: -40 … 125°C
  • Total error: ≤ ± 0.30 %FS (0 … 70°C)
  • Materials: Stainless steel, titanium
Pressure measurement in abrasive media using Vulkollan® membranes

Pressure measurement in abrasive media using Vulkollan® membranes

Typically, pressure sensors are available in either stainless steel or titanium finishes. In this way, all of the common test bench applications or monitoring tasks are covered. But when it comes to contact with particularly abrasive media, then additional protection is required. An added Vulkollan® membrane can often meet the requirements here.

Before turning to two specific application examples, first a few introductory words on the material itself: Vulkollan® is the trade name for a polyester-urethane rubber, a polyurethane plastic with elastic properties, as well as good chemical and mechanical resistance. This rubber-elastic material is used in different variants including foam, cellular soft plastic and also as a solid plastic. While the first two variants are predominantly used in pigging technology, the solid plastic one is processed into wheels, rollers and coatings. The operating temperature range here lies between -20 to +80 degrees Celsius.

Concrete as contact media

A market leader in the field of specialist civil engineering contacted STS in the search for a pressure sensor, which can be used without hesitation in a flowing, abrasive medium. In this particular instance, concrete was the case. The specialist engineer makes hydraulic equipment that drills holes in the ground and then fills them with concrete to form piles.

To ensure that these concrete piles have a stable structure, a continuous flow of concrete must be ensured. The concrete is filled into the hole via a pipe. After the pipe has been inserted into the hole, it can happen that concrete blocks the pipe inner, leading to an interruption of the process.

To prevent this, a pressure sensor was to be inserted into the interior of the pipe. Since the concrete is delivered through the pipe into the drilled hole by means of a pump, a blockage can be easily recognized by high pressure inside the pipe. For this task, a stainless steel pressure sensor was out of the question, since it would survive in concrete only for a short time.

To meet this challenge, STS proposed to furnish a flange sensor fitted with an additional Vulkollan® membrane. With this added protection, the sensor employed achieves a lifespan of one year at a five percent accuracy. The mechanical construction, as well as the electrical connections, are custom designs, which could be supplied within a short time.

Fill-level measurement in trimming tanks

A manufacturer of ship control systems looking for a reliable solution to water-level measurement inside trimming tanks approached STS.

Trimming tanks are used to bias the position of the center of gravity within a vessel. Cargo ships, for example, are constructed in such a way that the design waterline coincides with the actual waterline when fully loaded. But if these are put to sea without cargo, the hull lifts out of the water so much that the bow usually towers way above the water. Because of the weight of the engine, the hull sits deeper, but potentially not deep enough that the propellers are still sufficiently immersed in the water – the ship becomes unable to maneuver in this scenario. To counteract this, the trimming tanks are filled with water.

The pressure transmitters for monitoring the filling level, however, come into contact not only with salt water (where titanium housings are sufficient), but also with sand, small stones or even shells. To optimize the lifetime of the sensor, its membrane was coated with a Vulkollan® film.

Image 1: Example of a pressure transmitter with Vulkollan® foil

It is because of Vulkollan® that pressure sensors can be optimized for use within abrasive media. This does not apply, however, to explosive substances or acids. Read more about the media compatibility of peizoresistive transducers here. In addition, users must remember that this additional Vulkollan® protection adversely affects the precision of the sensor. The thermal characteristics also become more unstable.

As a result, nothing beats the comprehensive and qualified advice of experts in the search for a suitable pressure-measurement solution in abrasive media.