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 peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks occur in virtually all gas and liquid-filled pipelines. Those pressures arising in just a few milliseconds can exceed the overload pressure of the pressure transducers employed and also destroy them.

Pressure peaks, or very high pressures existing over a short timeframe, are usually noticed only when the damage has already been done. They are the result of pressure surges and also other physical phenomena (cavitation, micro-diesel effect) that occur wherever liquids or gases are transported through pipes. Pressure peaks, however, are less important among gases due to their high compressibility and thus only rarely represent a danger. In the context of water pipes, the term ‘water hammer’ is often used. With these terms, a dynamic pressure change of the liquid is ultimately implied. When, for example, a valve is quickly closed, water flow will stop instantaneously. This triggers a pressure wave, which flows through the medium against the direction of flow at the speed of sound and is then reflected back again. Within milliseconds, there is a sharp pressure increase which can cause damage to pressure sensors and other equipment (damage to pipe fittings and pipe clamps, as well as to pumps and their footings etc.). In the first line, however, it is the measuring devices that are affected, upon which we will be concentrating in the following. These damages can appear as a tiny “rupture” or a deformation (see Figures 1 and 2).

Figure 1: “Rupture” as a result of pressure spike

Figure 2: Deformations due to pressure peaks

If the pressure acting on the pressure transducer exceeds the overload pressure, then this will sustain permanent damage. There are two possible scenarios here: As paradoxical as it may sound, the complete destruction of the measuring instrument due to pressure peak is the mildest of consequences. Users, after all, do notice the damage immediately here. If the sensor is merely deformed as the result of a pressure peak, however, it will continue to operate, but deliver only inaccurate measurements. The financial consequences here are disproportionally higher than with a totally destroyed sensor.

How to prevent damage caused by pressure peaks

The golden path to preventing damage caused by pressure peaks lies in the integration of pulsation dampers or pressure chokes. Other means, such as the use of valves, would not lead to satisfactory results, because they are too slow to react to pressure peaks which actually arise in mere milliseconds.

The purpose of a choke is to dampen pressure peaks so that they no longer exceed the overload pressure of pressure transducers and then damage them. For this purpose, the choke is placed in the pressure channel in front of the sensor cell. As a result, pressure peaks will no longer reach the membrane directly and unchecked, since they must first pass through the choke itself:

Figure 3: Pressure channel with Pressure choke

Because of their very good protection from pressure peaks, the use of pressure chokes remains the best option. This variant, however, does have its pitfalls. It can lead to a blockage of the pressure channel due to calcification and deposits, especially in media with solid and suspended particles. This results in a slowing down of the measurement signal. If chokes are used in relevant applications, then regular maintenance should be carried out here.

A supplementary protection from pressure peaks can be achieved with a higher overpressure resistance, as opposed to the standard one. Whether this is advisable depends upon the particular application: If high accuracy readings are required, these can no longer be achieved in certain circumstances of very high overpressure resistance relative to the measurement range.

Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

Preventing Corrosion Caused by Aggressive Liquids in the Food Industry

When testing proportional pressure regulators as part of the development of complex hydraulic systems, high impulse capability and precision are required from the pressure measurement sensors employed.

Carbonic acid and alcohol can put a strain on measuring equipment. A manufacturer of automatic in-line and laboratory liquid analyzers has approached STS to find a durable and accurate pressure transmitter.

When exposed to aggressive fluids such as alcohol or carbonic acid, standard materials suffer from corrosion. For example, carbonic acid causes an increase in the hydron (H +) concentration and therefore leads to hydrogen corrosion. Once the corrosion eats through the membrane of the pressure sensor, it becomes unusable. That is why regular stainless steel will not suffice for applications with high levels of carbonic acid.

Other than being highly corrosion-resistant, the pressure sensor for this particular application in a bottling plant has to be able to deal with extremely low pressures close to a vacuum. As this application is part of the food industry, hygiene standards are very high. The near-vacuum conditions that the equipment is regularly exposed to is part of the sterilization process (similar, although not as extreme, as what happens in an autoclave). Low pressures below 0 bar can present a danger to the integrity of pressure sensors. The vacuum may cause the membrane to be sucked off from it position in the sensor. False measuring results or a completely broken sensor are the consequence.

Due to these requirements, we had to assemble a customized solution for this manufacturer of automatic in-line and laboratory liquid analyzers based on the pressure transmitter ATM.ECO. As material, we chose an extremely corrosion-resistant Hastelloy steel. To ensure membrane stability during low pressure conditions, we applied a special glue to fixate the membrane in place.

Since the pressure transmitter operates under room temperature conditions in this application, no special temperature compensation was necessary. The accuracy of 0,25 percent of the total scale is also more than enough for this particular application. The full scale ranges from 1 to 15,000 psi and is hence perfectly suitable for low pressure.

Gas distribution grid monitoring by continuous pressure measurement

Gas distribution grid monitoring by continuous pressure measurement

The autonomous process loggers from the firm AIRVALVE operate with pressure sensors from STS in monitoring critical points of the gas grid owned by SWK Netze GmbH. The principle applied here affords planning reliability at a comparatively low outlay in its implementation.

SWK Netze GmbH performs extensive measurements on its gas distribution grid for calibration of its pipeline program. To this end, continuous pressure measurements are to be made at fifteen critical points as part of its project “Grid Monitoring of the Gas Distribution Network.” Besides expectations of the most precise of measured values, it was also crucial upon realization of this project that the measurement instruments performed both reliably over a longer time span and simultaneously had sufficient signal strength to regularly transmit measurements even when mounted below ground. To reduce underground and pipe installation work to an absolute minimum, pressures were instead to be measured at already existing ventilation fittings. For this purpose, the measurement equipment was to be installed in size 3 street caps.

To fulfill this task, the selection went to process loggers of the type LS-42 produced by AIRVALVE. During extensive testing, it previously emerged that the products of this process logger series were the only to avail of an integrated high-performance antenna, which could provide for an undisturbed signal transmission even in underground shaft workings.

Long-term stability and user-friendliness are key factors

In addition, this measurement instrument, thanks to its high-performance, interchangeable battery, functions free from electrical and telephone connections over a duration of 10 years and more. This easily mounted process logger, which is also remotely configurable, ensures a secure transmission of the measured readings due to freely selectable SIM cards or multi-network with a private VPN tunnel (see Fig. 1 about design of the process logger). It is therefore perfectly suited to remote or poorly accessible facilities, which have to be monitored over a longer timeframe without arduous maintenance requirements.

Figure 1: Datalogger construction (Source: AIRVALVE)

These requirements in terms of durability and operational performance were, of course, also placed upon the sensors used for pressure measurement. AIRVALVE opted here for the ATM.ECO/N pressure transmitters from STS.  These 100 mbar sensors are provided with power from the interchangeable battery of the process logger, have a resilient stainless steel housing and deliver precise results to an accuracy of ≤ ± 0.70 % over a temperature range from -5 to 50°C. In terms of long-term stability, the ATM.ECO/N registers < 0.5 %.

 

Assembly of the measuring system on the gas distribution grid

The entire measuring system for monitoring the gas distribution grid is housed in street caps (see Fig. 2). By using already existing ventilation fittings, the work necessary could be performed without major outlay. To implement pressure measurements, the ventilation riser plug was replaced with a reducer fitting (1). Using a stainless steel ball valve, the measurement connection can be shut-off (2). Calibration of the pressure sensor is facilitated by a Minimess coupling (3). The pressure sensor (4) is connected via a pressure-equalizing junction box (5) to the AIRVALVE process logger (6). This is then fixed to a ground anchor (7) by a click fastener.

Figure 2: Overview of the measuring system (Source: AIRVALVE)

Measurements are performed every 5 minutes. This measuring interval is fundamentally selectable between one and sixty minutes. The measured values are transmitted several times daily to the control center. Transmission of the readings can take place over VPN-secured multi-network cards or basic agreement SIM cards. Communications are possible using internet control centers or also with SCADA systems. In this example application, SWK Netze GmbH opted for the “Web-LS” internet control center to manage the obtained data through highly secure servers.

Density measurement in gas flow meters

Density measurement in gas flow meters

Gas consumption is calculated using gas meters measuring the flow volume. Since the density of gas, and thus its volume also, is both pressure and temperature dependent, the measured quantity can deviate due to the prevailing  pressure or temperature. The gas volume, depending upon pressure and temperature, can be described by the formula p · V/T = Constant (p: pressure, V: volume, T: temperature).

Whilst the pressure with which gas flows through the pipes can be relatively easily controlled and monitored, this is not the case with the temperature. The resulting differences in density have an influence on the measured flow rate. What remains negligible here to the normal consumer due to relatively light usage becomes an important cost factor to those major consumers.

With the Measurement Instruments Directive (MID), an EU-wide guideline for measuring instruments was issued to establish a uniform approval procedure for all EU states and some other nations. Further objectives of the directive include a one-time and unified test for the approval of measuring instruments, as well as a uniform and transnational regulation for initial calibration. With these designated, transnational regulations an even better product quality is striven for and a level playing field ensured. Ten types of measuring instruments in the sphere of legal metrology are covered by the MID, with the requirements for gas meters and volume converters laid out in Annex MI-002.

Pressure and temperature must be taken into account when calculating exact gas quantities. And this requires appropriate sensors in the gas meters. Instead of the volume, the gas mass must be indicated, since this is the more precise measure in light of fluctuating density. To reliably determine this, it is necessary to measure both pressure and temperature and thus determine the density.

High precision through computational compensation

There are two types of pressure and temperature sensors to be connected to gas meters. In the first variant, the pressure transmitter is screwed onto the gas-delivery pipe and connected to the gas meter by means of a cable. In variant two, however, the sensor is installed directly into the device (the specific example below describes variant two).

The pressure ranges used for gas metering generally fall between 0.8 and 3.5 bar (absolute) and 2.5 to 10 bar (absolute). The requirements in terms of precision are enormous: Demanded is 0.2% of the measured value at temperatures from -20 °C to 60 °C. This figure, however, cannot be achieved with conventional pressure sensors. To maintain this level of accuracy, computational compensation must be applied. For this reason, STS supplies its pressure and temperature transmitters not only functionality-tested, but also parameterized (coefficients for polynomial compensation).