Theory

Dual Absolute ®

In the field of leakage measurement, two types of pressure-decay instrumentation are currently known and widely used: those with absolute decay, or relative, and those with differential decay per sample piece.

Although the quality of the measurement of the two types of equipment is described and catalogued in the standard, the field of application of one or the other method is not always clear and well defined, so much so that in most practical cases the two systems overlap.
This is also because over time, technologies and components have evolved; the basic schematics have been enriched becoming more and more performing, both in the management software and in terms of variants, options and pneumatic modules not foreseen in the original circuits. Sample volumes, electronic regulators, capacimeters, vacuum generators, main folds, isobaric or coaxial houses, have been added to the equipment in order to improve its effectiveness and reliability.

One of the circuit variants that is now beginning to spread is the dual technology.

This new system, also called Dual Absolute Leak Tester, is not placed in the list of options and even less in a middle area between the two previous types of measurement, but rather opens up new horizons, improving the quality of the measurement and simplifying the two previous basic types already existing.

If historically differential systems are born with the dual purpose of increasing the resolution of the pressure decay and to compensate for the thermal trend of the part in measure, it is also true that over time the technology has improved exponentially in terms of acquisition of strain gauge and electronic transducers, while increasing the quality of systems with absolute decay.

A technological parallelism is possible in the field of weight measurement by comparing thrust or dynamometric scales with standard weight stadera type rod scales. Although comparative scales may seem infallible in precision, over time these systems have left room for strain gauge bridge measurement, which, thanks to the electronics, for several decades now has surpassed for both precision and ease of use any other previous mechanical system, reducing costs, maintenance, mechanical parts and increasing efficiency and reliability.

 

The resolution of Δp/Δt

Akin to a gravimetric dosing system, even in a pressure decay loss measurement system the measuring solution factor plays a key role in application quality, perhaps even more so than the long-term overall accuracy characteristic.
In numerical terms, referring to the technical data on the web declared by the various manufacturers of leak testers, it can be seen that the standard now widespread is to ensure a resolution of 0.1 Pa up to 16 bar for differential systems, since it is reduced to 3 bar for absolute decay meters.

This means that by installing a transducer, for example with a scale of +/- 50 millibars, in a differential system and comparing it with the direct pressure of an absolute decay meter with a maximum scale of 3000 millibars, the two measurements in terms of resolution of Δp/Δt work in the same way. If instead we installed a 6 bar transducer in the absolute system, the resolution would worsen by twice, i.e. by doubling to about 0.2 Pa per division.

Considering the limited benefit in terms of acquisition, it is clear that differential systems, in light of the greater mechanical complexity, contribute to bring benefits especially on the compensation of the measurement in terms of temperature and mechanical stress of the piece, rather than in terms of accuracy of the pressure decay.

Technical considerations

Detailed on this resolution aspect, some purely technical considerations are necessary.

The first is that, in practice of the thousands of applications solved by ForTest, about 90% is within the pressure of 6 bar, while 60% is within the 3 bar of testing. So, even if the difference in benefits is already minimal, in more than one case out of two there is no benefit, not even theoretical, in terms of resolution in the use of a differential gauge.

In addition, in applications over 6 bar it is not advisable to set a loss threshold near the end of the meter resolution. On the contrary, it is evident that the higher the test pressure, the more the decay to be measured increases proportionally. In reality, at pressures greater than 8 bar it is strongly discouraged to set Δp below 100 Pa; in this type of regimen having a resolution of 0.1 or 0.2 Pa does not change anything, except advertisingly.

The last consideration is that these resolution data are usually measured at zero, i.e. compared with primaries at ambient pressure. In practice, little is known, except with laboratory trials performed with leaks and sample volumes at real test pressure, of the actual behavior in terms of resolution hysteresis. That is to say that any membrane, both absolute and differential, subjected to a pressure of offset (or common mode in the case of differential) is inevitably subjected to a mechanical noise and stress that in the conditions of certification are not considered.

Obviously the quality of the differential transducer, being the “heart” of these systems, largely defines the metrological quality of the devices on the market as well as the reliability in terms of robustness to pressure peaks and compatibility with wet or contaminated air or test pieces present in the test.

 

Temperature compensation

If over the years the technology of transduction and digitisation of force and pressure signals has considerably brought the characteristics of the various measurement systems closer together, at least in terms of resolution, it is equally true that the problems in terms of compensation for thermal and mechanical variations have remained the same. In this scenario, differential systems still play a major role.

Analyzing the pressure decay measurements symmetrical differentials, compared to this sample piece, we still have two cases, opposite each other, where absolute leak testers are disadvantaged. These are the cases of tests on very small volume parts with very high productivity (tyre valve, fittings, biomedical components, etc..), where the cycle time is the master and the measurement speed is a preponderant parameter, and the cases of large volumes of tests, where the drifts and temperature elastic affect too clearly not to be compensated.

In fact, in both cases, alternative measuring systems have shown more suitable solutions than differential meters. For example, systems in compliance recovery or interception in the bell for small volumes and mass-flow for large pieces.

The current research is being conducted in the area of improving the application of differential systems. In this vision, sample part balancing is what we are focusing on. Also because the concepts of absolute, differential and dual measurement are actually transversal to the various types and applicable in different ways to the physical principles of the transducers used.

Measures on small volume pieces

With regard to measurements on small volume parts, with a view to containing test times (for example 1.2s total start-to-end cycle, 1cc test, leakage = 10 cc/h @ 2bar) and although the absolute measurement enjoys a very high dynamic range and does not require the long stabilisation times required by the differentials, we note that the mechanical balancing of the differential transducer is actually even more immediate and rapid than the dual systems.

This means that with such a short time (typically 100/200 ms) of decay acquisition duration Δp/Δt, even small signal phase shifts of the two membranes, or their resonances, result in large overall measurement errors. Such errors are actually non-existent when the bandwidth is contained below 100 Hz on the signal slopes, i.e. with times of Δp/Δt greater than half a second. However, in these cases “ultra fast” in micro-volume applications traditional differential pneumatic circuits are certainly preferred, although revisited with micro-valves, transducers and small tubes as much as possible.

Whereas in these particular micro-volume conditions the pressure decay that occurs in the event of a leak is always of great magnitude, the application of transducers also of low dead volume such as MEMS or solid-state bridges, instead of capacitive transducers, simplifies the problems of breakage and reliability as well as ensuring a very high dynamic measurement scale, albeit with a more limited resolution.

This is the opposite case to the measurement of large volume pieces, where it is necessary to have measurements as stable and immune from all types of noise and drifts, even at the expense of the bandwidth. Here the resolution and stability in measurements up to 60/120 seconds are the special features.
In all cases of direct measurement, it should be remembered that the ratio to leakage is always inversely proportional to the pressure decay Δp/Δt. In these cases, it is better to have designed all the possible conversion bits that AD components offer, as well as greater filters and EMC immunities.

Measures on large volume pieces

Regardless of the conditions of small parts, the physical and pneumatic scenario of measuring parts with large volumes is very different, i.e. in cases where sensitivity is required, already driven by sizes greater than 250 cc. It is in this context that all manufacturers of measuring equipment, including ForTest, have researched systems to assist in the use of reference sample parts.

Large part of the technology based on software algorithms provides for the characterization of the tests considered "good" or within a band of extreme security, so as to recreate in an "anti-transient" way a dynamic offset and to be able to continuously adjust a Dynamic Offset Compensation on the measurement (DOC). All systems already widely used as auto-zero algorithms of the most popular weighing systems, which in fact only partially adapt to the wider problems of complex leak testing processes.

Measures on large volume pieces

Regardless of the conditions of small parts, the physical and pneumatic scenario of measuring parts with large volumes is very different, i.e. in cases where sensitivity is required, already driven by sizes greater than 250 cc. It is in this context that all manufacturers of measuring equipment, including ForTest, have researched systems to assist in the use of reference sample parts.

Large part of the technology based on software algorithms provides for the characterization of the tests considered "good" or within a band of extreme security, so as to recreate in an "anti-transient" way a dynamic offset and to be able to continuously adjust a Dynamic Offset Compensation on the measurement (DOC). All systems already widely used as auto-zero algorithms of the most popular weighing systems, which in fact only partially adapt to the wider problems of complex leak testing processes.

The disadvantage of these systems and alternative solutions

The main disadvantage of these offset correction systems is the inability to split and correct various errors one by one. As effective as these automatic compensations are, they can provide help if used in small percentages of the set point as they are used only for the purpose of chasing slow / very slow variations of error. In general, however, the overall measurement is corrupted by various spurious phenomena due to the overlapping of several factors such as mechanical movements, stress of materials, elasticity of the fittings connecting to the pieces and only in part by the variation in ambient temperature.

Other widely used systems allow to sample through temperature probes the trend of environmental factors, creating a compensation of the offset in terms of Pa/C° (DOCT). In this mode, after a period of analysis of practical tests in production, ie acquisition in Excel format of the measurements correlating them with the measured temperatures, we introduce a correction factor to the measurement in order to compensate for temperature fluctuations.
Although more laborious in the development phase, these algorithms have the advantage of only compensating for the thermal phenomenon and therefore to prevent an excessive accumulation of phenomena to be corrected.

In all cases, a balance via a sample piece or a reference emule greatly helps the stability and repeatability of measurement, if only in terms of acquiring the environmental thermal conditions.

 

Differential and repeatability meters

It should be borne in mind that differential leakage meters are commonly used in three practical configurations, which can generically be summarized as:

  • Asymmetrical differential, i.e. with reference side blocked by a cap. It is a simplification in the installation phase so as to make it equivalent to an absolute system.

  • Center-zero differential, designed to measure two pieces at a time.

  • Symmetrical differential, the true balanced comparator, where the reference side is connected to an airtight sample piece.

We now analyze the benefits of using reference sample parts in various ways.

of these three layouts of use, the symmetrical one with sample piece is the method that provides the best answers in terms of accuracy, repeatability and especially rejection to noise generated by temperature and mechanical stress.

Microvolume applications

In micro-volume applications, where therefore the preponderant thermal mass and the dilating elements are in practice the tubes connecting to the piece, the use of a reference circuit as similar as possible to the measuring side allows to perfectly balance the system and correct, in addition to the temperature, also the expansion of the two sides of the circuit (Test and Reference), since small test pieces are generally rigid. In these cases, a simple identical tube sealed on the reference side and of the same length as the one connected to the Test is more than enough to obtain both excellent repeatability and drastic reductions in settlement times. In the case of metal parts, a blind fitting as a cap at the end of the reference tube ensures a temperature capture function, further improving the application.

Higher volume applications

This is no longer true in the second case of using a differential gauge, i.e. in the most frequent applications of testing parts with volumes already larger than the dead volumes of the connecting pipes. To complicate the scenario, which is already complex in itself, problems arise linked to the mechanical stress of the parts and the endogenous generation of parasitic temperatures when tests are repeated on the same part.

In fact, in the practical use of sample parts and in contrast to the desired measurement compensation, it is noted that the volume variance due to the expansion of the two parts under test in turn introduces errors in the measurement. Consider that in a differential pressure decay system, commonly intended for industrial production with a high operational rate, the mechanical expansion of the part under test will be limited to the measurement operation only, while the mechanical stress on the reference sample part will accumulate for the entire time the apparatus is in use to an indefinite number of times, leading to all effects of a continuous drift in the behavior of the two parts already after 15/30 minutes of work at constant regime.

In these cases the expansion of the tubes or circuits inside the instrumentation is no longer predominant, as in the case of applications on micro volumes, but it is the pieces themselves that create the repeatability error.

Analogously, due to the continuous pressurization and emptying of only the reference sample piece, there is a growing thermal accumulation such as to trigger endogenous phenomena that largely frustrate the compensation of the measurement, creating unwanted drifts.
In practice, empirical surveys have shown that a 300cc metal part with a volume of 300cc subjected to a pressure of 2bar relative to it needs at least 20 minutes to re-establish the conditions of elasticity and temperature of stillness, i.e. to reappear within a margin of repeatability of 10% compared to the first test performed.

For this reason, the concept of apparent repeatability has been introduced over time in the use of differential pressure decay loss gauges, that is, the phenomenon of good repeatability in performing repeated measurements on the same part, measurement stability per gram which is then not maintained during practical use in production.

The birth of dual absolute systems

To overcome all these problems of drift and stress of the reference pieces, dual absolute systems were born. In a first version, or rather during the experimentation phases, these systems were presented as simple kits for expansion and modification of normal absolute and differential devices. Through a three-way pneumatic valve, a sampling procedure was then introduced, i.e. "self-learning" of DOCs in automatic form, with time frequencies fast enough to follow the evolution of the ambient temperature, but leaving enough time to rest on the reference side to return to the initial elasticity condition, i.e. the real elasticity condition to be compared with the production pieces being tested. The same systems are sporadically used by various manufacturers to sample environmental factors (Tamb and Pamb) using sample nozzles and to conveniently compensate for volumetric flow measurements.

In the case of pressure measurement, over time it has been found that, thanks to a successful combination of positive factors and all in the same direction of product improvement and economy, the creation of two symmetrical branches of absolute measurement independent of each other but governed by different modes of software has led to an unparalleled improvement of all types of measurement.
As you can guess, in addition to improving symmetrical measurement, you have discovered the possibility, thanks to different methods of test management, to significantly improve both the central zero measurement and the asymmetrical type.

 

Absolute decay meters

Since ever considered the most“poor” system, thanks to the improvements in acquisition and transduction already exposed, absolute decay meters have reached an increasing popularity, now commonly flanked by both differentials and mass flow.
This success is largely due not only to the real quality of the measurement, but also to an enormous simplicity, robustness and reliability of maintenance and use compared to any other leak tester in the industrial field. Far from the basic concept of plc, valve and pressure transducer, through the methodical development of hardware and firmware over time we have managed to obtain precise and versatile machines, with a more immediate approach to the leak testing procedure.

It is necessary to always remember the scope of use of these devices (which is not generally the ideal laboratory and with sterile conditions) in which even simple things are often complicated with enormous ease.
Although apparently less sensitive on small scales than other systems, the high dynamics during the phases of settlement and measurement of absolute decline and the absence of limits in high pressures have consecrated its application in fields not recommended for differential meters and mass-flow. For example, in the biomedical field where, in addition to the reliability of pneumatics and the need for sterility and non-contamination of the parts under test, the high oscillation of elastic materials used as bags or transfusion sets have defined as standard these systems to the detriment of others.

Obviously, having a complete range of technological solutions and different measurement methods, ranging from tracer gases to micro-flows, from recovery systems to pressure decays, the approach with the application always provides the most suitable solution, first of all in terms of purpose and scope of use, then sensitivity and lastly the cycle time required.

Advantages of absolute type gauges

Remains the fact that the application of an absolute decay system, where possible, always has the charm of “install and forget” while any other dual-sensor method requires some more attention in the metrology field, because of the double measurement. Periodically, in fact, a more careful verification and control of drift is required, as well as, as in any case, a double certification. For example, in the case of mass-flow flow meters (which have however reduced and simplified the cases of interventions related to capillary systems) it is always necessary to check the quality of the air used and the state of cleanliness or degradation of the measurement sensor.
In particular, in differential decay systems, wear and dirt in the equalisation valves is unavoidable due to the exhaust necessary to preserve the life of the measuring transducer, while pneumatics is much more sensitive and sophisticated than any other system in comparison.

As much as both pneumatic and mechanical engineering and the periodic verification and calibration procedures of all systems have evolved dramatically over time, it is evident even at a glance that all these technologies are more complicated when compared to absolute type gauges.

In this type of meter, the only transducer used is of excellent quality and covers the entire measuring range. It is therefore very robust, does not require forcibly an exhaust at the end of the test and can withstand water hammering caused by discharges not synchronized from the outside of the instrument, is not particularly affected by dirt and is insensitive to the dielectric capacity of the gas used and, within certain limits, to its humidity.
In addition, simple pneumatics involves the use of mostly commercial components, oil and silicon free, if necessary supplied with certifications for food, packaging and pharmaceutical applications. The pneumatics is therefore easy to maintain and, if properly designed, intrinsically safe, or always at a loss in case of malfunction. All these characteristics are difficult to obtain in pneumatics for differential systems, both with symmetrical scheme and in master less axis, and with isobaric cavities. For this reason, this second type of device requires more frequent maintenance and more accurate periodic checks.

In our T8960 differentials, for example, the opportunity to use commercial valves has been studied in order to take advantage of the benefits of interchangeability and versatility of absolute decay models, leaving the functions of equalization and protection of the transducer no longer to mechanical components, but to software procedures and high speed PWM signals.

In practice it is difficult to define which system is most practical to use. How, for example, to understand whether it is better to use a diesel or a gasoline. That the future is in the hybrid?

 

Dual technology

As already mentioned, the new dual systems are not born from the assumption of placing themselves in a middle position between the meters now known, but of flanking and improving them where possible.
Drawing on the characteristics of both types now known, they basically aim to merge their functions, simplifying and enriching the measurement cycles. On the one hand, the reliability and safety of absolute systems, on the other hand, the “amplifier effect of the loss” of differential decay systems.

The main distinctive elements

Although it is still too early to define standards, as most of the research and development of software in the various modes, you can still draw up a brief description of the dual absolute systems.

The most obvious distinguishing element is in comparison to the use of a symmetrical differential with sample piece. In this case the strategy is to sample the reference piece in any case during the test phase, as well as in a differential, but only at intervals of time such as to be able to make a correct comparison to the piece under test, although not distorting the elastic and thermal characteristics of the reference piece. In turn, these samples are stored and compared in vector mode to the tests in progress, creating to all intents and purposes a virtual comparison until a new sampling.

The evidence of improvement is then even stronger if used in symmetrical differential mode at central zero, where this is now completely abandoned the current differential systems, being considered unreliable due to the uncertainty of the measurement in case of loss on both sides. In this mode, the power of the dual system is fully expressed, thus being able to take advantage of the benefits of symmetrical compensation but making the system safe. In practice, the measurement cycle in this mode provides for an extension of the test time only in the case of deviation of absolute values by detecting a low differential factor. In other words, by doing so it is possible to benefit both from the high immunity to environmental noise of mechanical stress and temperature drift made by the real symmetrical balance, and from the reliable simplicity of the absolute decay.

In asymmetric differential mode the software concentrates instead on the ability to discharge the air only when necessary. Due to the fact that there is no need to safeguard the transducer, it is no longer necessary to generate an exhaust phase at the end of the test, as required for differential meters. This allows the two measuring sides to be kept under pressure as much as possible, settling them and avoiding the complicated isobaric mechanics, coaxial tubes and other anti-expansion devices aimed at reducing the phenomena of elasticity inside the instruments. In practice, where possible, the unloading phase takes place at the beginning of the test, no longer at the end and piloting takes place by intercepting by the software when the operator or the bench is about to empty the piece under test.

In conclusion

These are in summary the most evident peculiarities of the new technology described here. In addition to these aspects, the measurement certification is always and only inherent to a relative measure and in practice is respected all the simplicity and reliability of in a system with absolute decline.
In practice, even if you lose a few decimals of Pascal resolution and with operating pressures over 6 bar test, you get an incredible simplification of the most famous differential systems.
With this technology, nothing more necessarily revolves around the differential transducer, but the hardware is reduced to a minimum while the software is constantly evolving.

We therefore urge both industry technicians and equipment manufacturers to contact ForTest for testing and more details and not to hesitate to test this promising new technology.

 

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