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Gas pressure measurements on cartridges with or without silicone grease


Effect of different protection mechanisms for the pressure transducer diaphragms of piezoelectric sensors on the point pressure during the gas pressure measurement on ammunition for short firearms.

In the case of type tests and officially prescribed fabrication controls of ammunition, the maximum gas pressure which develops inside the cartridge during the shot must be measured. The cartridge is triggered in a measuring barrel which - for pressure measurements - is nowadays equipped with a piezoelectric pressure transducer in the chamber. To let the pressure act on the transducer, the cartridge is drilled and further prepared. According to the decisions of the "Permanent International Commission for Firearms Testing" (C.I.P.), the test laboratories must, for this purpose, always select the most exact and most practical procedure. On the basis of this requirement, the drill hole is, first of all, covered with a circular kapton foil. During the measurements, the free space between the drilled cartridge casing and the diaphragm of the piezoelectric pressure transducer had, up to now, to be additionally filled completely with silicone grease. In particular the ammunition manufacturers and some users of piezoelectric pressure transducers describe this point of the measurement regulation as not practicable and as an application which is no longer up to date. On behalf of the C.I.P., PTB has now analyzed the suitability of different alternative preparation procedures and concentrated its work on the - particularly critical - shortarm calibre 9 mm  Luger.

The silicone grease is to protect the sensor diaphragm against mechanical damage and thermal stress. In addition, the grease filling attenuates a possible occurrence of resonances (air hole oscillations) and prevents an increase in the firing capacity volume around the free space mentioned above. According to information from manufacturers of ammunition and pressure sensors, new pressure transducers as well as accessories for the protection of the sensor diaphragm (see Figure 2) shall make the use of grease unnecessary and be, thus, more suitable for practice. The time required for the tests would be considerably reduced if a disassembly of the transducer for the checking of the measurement hole, a possibly required grease addition, and the subsequent assembly became unnecessary. It is, however, questionable whether the measurements with alternative sensor protection could in this way be compared with the reference procedure.

Prior to the gas pressure measurements with the prepared cartridges, the bullet velocity was first of all determined with a series of 10 shots with the measuring barrel in the normal state, i.e. without drilling of the casing which is otherwise required for gas pressure tapping. These measurements furnished a mean value of 383 m/s. The brass casings fired off showed, however, strong powder residues at their bottom edge zone, which indicated a serious powder flow over the casing mouth towards the breech. Therefore, to reduce the gap between the casing and the chamber wall, the circumference of the cartridge casing was completely sealed with kapton tape (see Figure 1, left). With this kapton tape, a higher bullet velocity with a mean value of 389 m/s was achieved and is, therefore, regarded as a reference. The preparation for the pressure measurement should affect the burning behaviour and the acceleration of the bullet as little as possible. The decisive measure for it is the deviation of the velocity. For the gas pressure measurements described in the following, the above-mentioned kapton tape was used in the same way as a sealing measure (see Figure 1, right).

Links: Patronenhülse 9 mm Luger, nicht angebohrt ohne Verschmauchung, mit Ausbeulung im Bereich Druckaufnehmerbohrung, Rechts: Hülse angebohrt, mit nicht verschmauchtem Bodenrand

Figure 1: Left: Cartridge casing 9 mm Luger, not spot-drilled and without powder residues, with distortion in the area of the pressure transducer hole, Right: Casing spot-drilled, without powder residues in the bottom edge

In the procedure with pressure measurement, i.e. with spot-drilled casing, three alternative preparation and protection procedures with unfilled free space were investigated in addition to the prescribed procedure "Measurement of the gas pressure of central firing cartridges for weapons with rifled barrel [XX-9]": "Teflon disk as diaphragm protection", "Diaphragm protection" and "Thermal protection shield with thermal protection plate", see Figure 2. In addition, the effects which present themselves when only the diaphragm is lubricated were investigated.

Aufnehmer mit Silikonfett vor der Membran, Membranschutz (Kistler 6567) und Thermoschutzschild mit Thermoschutzplatte (Kistler 6565A mit 1181A)

Figure 2: Transducer with silicone grease in front of the diaphragm, diaphragm protection (Kistler 6567) and thermal protection shield with thermal protection plate (Kistler 6565A with 1181A)

The procedure without the use of grease was started to rule out - already from the outset - the influence of the lubrication on the results obtained for the bullet velocity and the gas pressure. After that, the procedures were carried out with lubrication. Each time, 10 shots were evaluated. After each shot, the silicone grease filling of the measurement hole in the chamber wall of the measuring barrel was checked with an endoscope. Although a grease loss could not be detected during the measurement series, the measurement hole of the barrel was - after the fifth shot - filled from the sealing side with silicone grease in the chamber (using a suitable spatula), without removing the pressure transducer.

Figure 3 shows the results of the measurements of the gas pressure and of the bullet velocity for the different combinations.

Figure 3: Deviations of the mean bullet velocities and of the gas pressure, related to the measurement series without spot-drilled casings and measures for gas leak reduction in the case of alternative preparation and/or protection procedures

It can be seen that the procedure prescribed by the C.I.P. deviates with regard to the velocity clearly less from the mean reference velocity (without hole). The values of the pressure measurements and of the velocity measurements in relation to each other are to be seen in the context of a relative procedure comparison, without analyzing the occurring systematic error. In the case of the alternative procedures, the maximum deviations of the velocity amount on average to up to 21.4 m/s (which corresponds to 5.5 %), related to the measurement without spot drilling and those of the gas pressure to up to 153 bar (which corresponds to 6.4 %), related to the free space filled with grease. The available measurement results clearly show that the procedure described in the decisions comes closest to the specifications of the C.I.P.

Before the gas pressure measurements are carried out, the transducers have to be calibrated. These calibrations have so far always been performed without attached casings. Corresponding calibrations carried out by PTB with the drop ball device show a decrease in the sensitivity by 1.5% when a thermal protection shield is used.

According to the provisions of the C.I.P. decisions, calibrations of mechanical-electrical pressure transducers must be performed quasi-statically - with a pressure balance - or dynamically, in the reference procedure, on a drop ball device. The different free-space arrangements used for the previous gas pressure measurements also served as a measurement condition arrangement for the measuring series with the drop ball device of PTB.

The application of the different measuring arrangements showed - related to the current C.I.P. procedure for determining the sensitivity in comparison with the reference transducer - a relative sensitivity change in the pressure transmission hole of the test piece.

The deviations from the current standard procedure of the C.I.P. for measurements of the velocity and of the gas pressure have already been discussed in the "Working Group Proof Houses and PTB" (in 2010).

Contact person:

Ernst Franke, Dept. 1.3, WG 1.33, E-Mail: dynamische_druckmessung@ptb.de