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Journal of Electrostatics 70 (2012) 97e104
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics
journal homepage: www.elsevier.com/locate/elstat
MIE experiments and simultaneous measurement of the transferred
charge e A verification of the ignition threshold limits
T. Langer*, G. Gramse, D. Möckel, U. von Pidoll, M. Beyer
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 January 2011
Received in revised form
26 October 2011
Accepted 6 November 2011
Available online 17 November 2011
Using the apparatus for the determination of the MIE a wide series of experiments have been carried out
in hydrogen/air, ethene/air, propane/air and acetone/air mixtures. The transferred charge as a criterion to
judge the ignition potential is determined to verify the thresholds of transferred charge given in the
standards. The stored charge in the capacitance before the discharge is compared to the transferred
charge in the spark. The correlation of ignition energy and transferred charge is examined and the
thresholds of the transferred charge are discussed. The MIE of the above-mentioned mixtures are
reviewed taking into account the measurement uncertainty.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Minimum ignition energy (MIE)
Spark discharge
Transferred charge
Hand-held coulombmeter
Ignition threshold
Measurement uncertainty
1. Introduction
The determination of the minimum ignition energy (MIE)
using a capacitive spark discharge is well established in the
literature as well as in standards [1e6]. Thereby, a capacitor with
a defined capacitance is supplied by a defined voltage. Then the
capacitor is discharged in a test vessel filled with a certain flammable gas/air mixture via an electrode configuration with
a defined electrode diameter, electrode material and electrode
gap width. The energy stored in the capacitor is calculated using
the equation
W ¼ 0:5$C$U 2 ;
(1)
where W is the energy, C is the capacitance and U is the voltage
supplied to the capacitance.
The energy of the spark discharge is assumed to have the
amount of energy stored in the capacitance before the discharge.
The MIE is determined by varying the capacitance, the charging
voltage, the electrode diameter, the electrode gap width and the
mixture composition in order to find the lowest energy necessary
to ignite the given mixture.
* Corresponding author. Tel.: þ49 531 592 3438; fax: þ49 531 592 3705.
E-mail address: Tim.Langer@ptb.de (T. Langer).
0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.elstat.2011.11.001
Based on MIE measurements, von Pidoll et al. [6] have calculated
the charge stored on the capacitance in order to obtain an estimation of the transferred charge using the equation.
Q ¼ C$U;
(2)
where Q is the charge. By correlating the MIE with the lowest
transferred charge leading to ignition of the different mixtures, von
Pidoll et al. [6] have suggested transferred charge thresholds in
order to judge if electrostatic discharges are potentially incendive
or not. These threshold values are now defined in the standards IEC
60079-0 and EN 13463-1 [3,4]. There, different gases, vapours and
dusts are classified in explosion groups according to their hazards
(I, IIA, IIB, IIC, III) [3,4]. For each explosion group, a maximum
transferred charge has been defined which should ensure that
a discharge with a transferred charge less than the defined
maximum does not ignite the representatives of the specific group.
In a recent study Langer et al. have found that these thresholds are
sufficient concerning brush discharges regarding explosion groups
IIA and IIC. But in case of explosion group IIB the achieved results
were close to the defined threshold. It was concluded that the
thresholds represent different levels of safety [7].
However, in [6], the transferred charge was not used for the
definition of the transferred charge thresholds. In fact the charge
stored in the capacitance before the discharge happened was taken.
Since Chubb [8] and Walmsley [9] additionally stated that
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
depending on the used electrode radius, gap distance and charging
voltage a transferred charge detected with unshielded probes
might give wrong results e or more precisely an underestimation
due to measured values being to low e it seems appropriate to first
validate the thresholds in the standards by comparing both the
charge stored before the discharge and the charge transferred in
the discharge. Secondly, at the same time, the suitability of the
method “transferred charge” using unshielded probes can be
reviewed since the charging of the capacitance before the discharge
occurs is well known using equation (2). This is in contrast to the
latest verification of the thresholds using brush discharges [7]
where the charging of an insulating surface cannot be determined that easily.
Four different gases were examined e hydrogen as a representative of explosion group IIC, ethene for IIB and propane for IIA.
Additionally, acetone was used since it has a higher minimum
ignition energy compared with the other gases and, therefore,
offers the opportunity to measure higher values of transferred
charge. Performing the tests near the MIE of the mixtures is
necessary to validate the transferred thresholds in the standards
[3,4].
The aim of this work is to examine the correlation between the
charge stored in the capacitance before the discharge Qc and the
charge transferred in the discharge Qt. Furthermore, the dependence of the ignition energy and the transferred charge is studied.
Based on the results, the thresholds of transferred charge and the
suitability of the test method “transferred charge” using unshielded
probes are discussed.
Since the validation of the transferred charge threshold can only
be reliably performed near the MIE of the certain mixtures, another
objective of this work is to recheck the MIE of hydrogen, ethene,
propane and acetone. Especially concerning propane and acetone,
there have been some discussions in the recent past [10,11].
Furthermore, using the “Guide to the expression of uncertainty in
measurement” [12], the received values are examined for the first
time concerning the measurement uncertainty which has not been
discussed in detail in the past. The considerations concerning the
uncertainty should lead to an estimation as to how accurate the
results given in the literature can be expected to be.
This paper is structured as follows: section 2 contains the
description of the test set-up and the experimental procedure. In
section 3 the measurement uncertainty is discussed. The results
and their discussion are presented in section 4. Finally, the resulting
conclusions are given in section 5.
2. Test set-up and experimental procedure
A schematic top view of the test set-up is given in Fig. 1. The test
set-up consists of a metal vessel with a round base. Its inner
diameter is 100 mm. The height is 85 mm. Therefore, the volume
consists of approximately 670 cm3. An electrode configuration with
an adjustable gap width and an electrode diameter is integrated in
the vessel. Ball bearings were used as electrodes. The high voltage
electrode is led insulated through the grounded vessel. A voltage in
the range of several kV is given to a capacitor which is an air
capacitor or just the capacitance of the electrode configuration
together with the test set-up depending on the demanded size of
the capacitance. The capacitance is measured using a calibrated
capacitance bridge (Wayne-Kerr 4210). The voltage is delivered by
a high voltage supply unit (F.u.G., HCN 140M-35000). The voltage is
measured using a calibrated voltage divider (Spellman HVD 100,
divider ratio 1:10008) and a voltage measuring device (Agilent
34410A). The voltage supply and the capacitance are connected in
series via a resistor. Its resistance is e for all experiments e
between 1 GU and 50 GU. The other electrode is grounded via
a hand-held coulombmeter (Schnier Elektrostatik GmbH, Type
HMG 11/02). Using this hand-held coulombmeter (HC) the transferred charge during a spark discharge can be measured.
The measurement principle of the HC (see Fig. 2) is based on the
discharge current which charges a capacitor with defined capacitance C. This leads to an increase in the voltage U at the capacitor.
The voltage rise is measured internally by a processor which scans
the voltage to find increasing or decreasing edges. The sampling
rate is 100 kHz. The difference between the final and the initial
value represents the voltage rise at the capacitor due to the
transferred charge Qt during the discharge. The measurement
result is calculated and given on a display.
Fig. 1. Experimental set-up.
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
99
The results presented here are based on several thousand
discharge events. In fact, more than 3.500 spark discharges have
been used for hydrogen/air, more than 5.000 for ethene/air, more
than 12.000 for propane/air and more than 2.500 for acetone/air.
3. Measurement uncertainty
Fig. 2. Schematic view of the hand-held microprocessor-operated coulombmeter.
A UV radiation source is installed at the vessel in order to make
spark discharges more reliable. The mixture is prepared using the
partial pressure method [13]. Therefore, the vessel is evacuated.
Thereby, pressure values lower than 2 mbar are reached with the
test set-up. Afterwards the required amount of flammable gas is
inserted in the vessel before the volume is filled with dried air
(relative humidity 20%) until the ambient pressure is reached.
The temperature during the experiments was 22 C 2 K.
For the tests, the electrode configuration and the gap width are
chosen. A defined charging voltage is given to the capacitance.
Thereby, the voltage is close to the onset voltage for the formation
of spark discharges for the given electrode configuration which has
been observed in pretests. The transferred charge of each spark
discharge is recorded with the HC. The time constant (s ¼ RC) of the
capacitance and the series resistor was chosen to be more than
30 ms in order to prevent additional energy supply from the power
supply during the spark discharge which is considerably shorter in
time. In fact, the additional energy supply can be calculated using
this equation
W ¼ U$I$t ¼ U 2 $t=R
(3)
In order to obtain a worst case estimation, a voltage of 10,000 V,
a resistance of 1 GU and a discharge duration of 100 ns are assumed.
The average voltage used is lower than 10,000 V, the resistance
higher and the discharge duration can be expected to be shorter
than 100 ns. However, using these values, an additional energy
supply of 10 nJ results. Therefore, the decoupling of the capacitance
from the power supply using a high resistance is justified.
The experiment runs until an ignition of the mixture is
observed. The corresponding voltage is noted and the ignition
energy is calculated using equation (1). Additionally, the transferred charge of the incendive spark is measured. Furthermore, the
originally stored charge in the capacitor Qc can be calculated using
equation (2). Reducing the ignition energy was attempted by
varying the capacitance and the charging voltage, respectively.
For the tests the flammable gas/air mixtures prepared were in
the most ignitable range for hydrogen, ethene and propane
according to [14], since this is the most hazardous situation one can
examine in matters of explosion protection. Hence, a concentration
of 22.0% hydrogen in air, 8.0% ethene in air and 5.2% propane in air
was used for the experiments. For acetone, a concentration of 6.5%
was used which is most ignitable according to [14].
Using hydrogen/air mixtures, an electrode configuration of
a 15 mm diameter electrode and of a 2 mm diameter electrode was
used for one part of the experiments. For the determination of the
MIE of hydrogen/air mixtures a small capacitance was needed.
Therefore, the capacitance of the electrode 15 mm/2 mm configuration together with the cable capacitance was sufficient. Thereby,
the 15 mm electrode was at high voltage and the 2 mm electrode
grounded. Later the configuration 2 mm/2 mm was tried too, which
then led to the lowest results concerning the ignition energy. For
the examination of the propane/air mixtures, a configuration of
1.5 mm (high voltage) and 2 mm electrodes was used. For all other
mixtures, a configuration of two 2 mm diameter electrodes was
used.
Before presenting the results, the consideration of the
measurement uncertainty is explained. The expanded measurement uncertainty is determined according to the internationally
accepted “Guide to the expression of uncertainty in measurement”
[12]. The important parameters which influence the uncertainty of
the ignition energy and the calculated charge are the voltage U and
the capacitance C. Considering the voltage measurement, an
accuracy of 6 104 results when taking into account the uncertainties of the calibration of the voltage divider and the voltage
measuring device. The uncertainty in the determination of the
capacitance was estimated to be 0.5 pF which results mainly from
the zero point adjustment before the actual measurement is performed. This uncertainty is much more than what the capacitance
bridge contributes and can be ascribed to the problem of the
determination of small capacitances (in the range of a few pF) e.g.
due to the influence of stray capacitances. For all uncertainties
given in this work, the coverage factor k is 2.0. By multiplying the
measurement uncertainty with the coverage factor, it is assured
that all measurement results have a probability of 95% of being
within the resulting expanded measurement interval. Considering
the ignition energy according to equation (1) and the calculated
charge stored on the capacitance according to equation (2), it is
found that the influence of the uncertainty in C is much higher than
the uncertainty in U. The uncertainty consists in all cases to more
than 99.8% of the contribution in the uncertainty in C.
The measuring range, in which the HC can be used adequately, is
between approximately 10 nC and 200 nC. In [15] Langer et al.
discussed the relevant influences on the measurement uncertainty
of the HC. This knowledge was then used to develop a calibration
method for the HC. According to this calibration method, the HC has
a deviation DQ between the measured transferred charge Qt and
the reference value Qr. Moreover, an expanded measurement
uncertainty was determined according to [12] with a coverage
factor k ¼ 2.0. Its dependence on the amount of transferred charge
is as follows: Table 1.
The measuring results can be corrected by considering the
deviation in dependence of the measuring interval of the HC. The
expanded uncertainty remains. For all results presented here, the
deviation is already taken into account.
The electrode gap width can be adjusted with an accuracy of
0.1 mm. The mixture compositions have an accuracy of 0.1%.
4. Results and discussion
The HC did not measure the transferred charge Qt of every spark.
The probability of measuring the transferred charge during the
Table 1
Results of the hand-held coulombmeter calibration.
Qr/nC
Qt/nC
DQ/nC
Expanded measurement uncertainty/nC
k
12.4
30.6
60.9
91.2
121.5
151.9
182.2
9.8
28.7
59.4
89.9
119.9
150.3
181.4
2.6
1.9
1.5
1.3
1.6
1.6
0.8
0.7
1.1
1.8
2.4
3.4
4.2
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
spark occurrence is lower when getting near the minimum trigger
level of approximately 10 nC as is necessary in case of hydrogen/air
mixtures. In case of hydrogen/air mixtures even a discrepancy
between the two different electrode configurations used can be
seen. By using the 2 mm/2 mm configuration, the capacitance was
reduced again compared to the 15 mm/2 mm configuration.
Therefore, the amount of charge stored on the capacitance before
the spark occurred decreased too, resulting in an even lower
measurement probability of Qt. For the other mixtures with
transferred charges higher than the trigger level, the frequency of
measuring Qt was more than 80% as can be seen in Table 2. Only the
experiments with an ignition of the mixture are considered here.
Non-triggering of the HC has a minor part at this result. Mainly the
manual resetting between two measurements with the HC caused
a probability lower than 95% due to occurring sparks before the HC
was reset.
In Figs. 3e6 the correlation between the transferred charge Qt
and the calculated charge Qc is presented for the four different
examined mixtures.
The “ignition” points represent the experiments in which
a spark ignited the mixture and the HC measured the transferred
charge Qt. For the clearness of the figures the expanded measurement uncertainty in the calculated charge Qc is only shown for the
lowest and highest Qc. However, for all other Qc, it is in the same
range. In all figures a fit of Qt ¼ Qc is given. In reality the transferred
charge cannot be expected to match the calculated charge as there
is always some residual charge on the capacitance after a spark
discharge [16]. In [6] a 5% residual charge is given as an estimation.
In [17] a residual charge of up to 50% was found. Furthermore,
losses through corona cannot be excluded even though the
capacitance is continuously charged by the supplied voltage since
the time constant is quite high. Therefore, all transferred charges Qt
should be lower than the calculated charge Qc and, thus, should lie
under the fitted line. Taking the expanded measurement uncertainty into account, almost every measurement not depending on
the mixture showed the expected Qt Qc. Furthermore, considering propane/air (Fig. 5) and acetone/air mixtures (Fig. 6) it can be
seen that the ratio Qt/Qc decreases for higher Qc compared to
hydrogen/air (Fig. 3) and ethene/air mixtures (Fig. 4). The average
charging voltage was about 3 kV for hydrogen/air, 6 kV for ethene/
air and 9 kV for propane/air and acetone/air mixtures. Therefore,
higher charging voltages could lead to more corona losses, for
instance, resulting in a lower transferred charge Qt. Furthermore,
higher charging voltages correspond to higher gap distances.
Considering the results of Bane [17], there seems to be a dependence of the electrode gap width leading to higher residual charges
at higher gap distances. This is not understood so far and should be
studied in the future. In summary, it can be stated that the HC
measured reasonable values. The theoretical work of Walmsley [9]
investigating the effect of induced charge on the measurement of
the transferred charge during brush and spark discharges when
using unshielded probes showed that for the electrode radii, gap
Fig. 3. Correlation between transferred charge Qt and calculated charge Qc for ignition
in a 22.0% hydrogen/air mixture.
distances and added capacitances used in the tests of this work
induced charge would be of negligible amount and therefore,
would not significantly influence the measurement of the transferred charge in this test set-up. This is in accordance to the results
presented in Figs. 3e6.
In the following, the criterion of the transferred charge Qt in
order to judge the safety of products and items is reviewed based
on the values presented in Figs. 3e6. Therefore, the transferred
charge Qt in dependence of the ignition energy (calculated using
equation (1)) for the observed ignitions is presented in Figs. 7e10
for the different mixtures.
Thereby, the results are from different electrode gap widths. The
criterion of transferred charge is based on the MIE values received
in former studies [6] which are strongly connected with finding the
optimum in all important parameters (capacitance, charging
voltage, electrode diameter, electrode gap width, mixture composition). However, the criterion of transferred charge can only be
suitable for judging the safety of test items when for all ignition
energies e depending on the mixture e a transferred charge higher
than the threshold value given in [3,4] is found.
Table 2
Frequency of measuring the transferred charge in case of an ignition.
Mixture
Measurement
of Qt
No recorded
measurement of Qt
Probability of
measurement/%
Hydrogen/aira
Hydrogen/airb
Ethene/air
Propane/air
Acetone/air
3
12
27
69
34
15
12
3
8
8
17
50
90
90
81
a
b
Electrode configuration 2 mm/2 mm.
Electrode configuration 15 mm/2 mm.
Fig. 4. Correlation between transferred charge Qt and calculated charge Qc for ignition
in an 8.0% ethene/air mixture.
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
101
Fig. 5. Correlation between transferred charge Qt and calculated charge Qc for ignition
in a 5.2% propane/air mixture.
Fig. 7. Transferred charge Qt in dependence of the ignition energy for a 22.0%
hydrogen/air mixture.
In Figs. 8e10, the threshold value according to the explosion
group given in the standard is drawn. Additionally, the hatched box
represents the interval of ignition energy which is under the MIE
given in [18]. For acetone in Fig. 11, the value of minimum transferred charge of 127 nC given in [6] is used. The MIE value of 0.55 mJ
is also taken from [18].
In case of hydrogen/air mixtures which represent explosion
group IIC (Fig. 7), all observed transferred charge values are over
the threshold of 10 nC given in [3,4]. The lowest transferred charge
igniting the mixture was 10.8 nC 0.7 nC corresponding to an
ignition energy of 25.1 mJ 4.0 mJ. The transferred charge was not
measured for the lowest ignition energy. However, taking the
measurement uncertainty into account and the general difficulty in
measuring transferred charges near the trigger level of the HC, the
received results fulfil the requirements concerning the criterion of
transferred charge since all received measurement results are over
the threshold of 10 nC.
In case of ethene/air mixtures which represent explosion group
IIB (Fig. 8), sparks with a transferred charge less than the threshold
of 30 nC ignited the mixture. Furthermore, ignition energies lower
than the MIE are observed. The lowest transferred charge igniting
the mixture was 24.1 nC 1.1 nC corresponding to an ignition
energy of 91.1 mJ 9.9 mJ. A recalculation of the values which
formed the base of the given threshold of IIB [6] in the standards
showed that even the minimum transferred charge Qmin based on
the voltage and capacitance given in [6] does not lead to a Qmin of
32.0 nC but to 30.2 nC.
For propane/air mixtures as a representative of explosion group
IIA (Fig. 9), the lowest transferred charge igniting the mixture was
66.1 nC 1.8 nC corresponding to an ignition energy of
0.382 mJ 0.025 mJ. However, the existing threshold seems
adequate since all received transferred charges are over the
threshold of 60 nC.
In case of acetone/air mixtures (Fig. 10), transferred charges
lower than the value given in [6] were observed. The lowest
transferred charge igniting the mixture corresponds to the MIE
measured in out tests. However, ignition energies lower than the
MIE given in [18] were found. Therefore, for acetone, a revision of
the MIE might be advisable.
Since the results presented here are for incendive sparks only,
these tests form an excellent base for the revision of the thresholds
given in the standard [3,4] and the literature [6]. As could been
Fig. 6. Correlation between transferred charge Qt and calculated charge Qc for ignition
in a 6.5% acetone/air mixture.
Fig. 8. Transferred charge Qt in dependence of the ignition energy for an 8.0% ethene/
air mixture.
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
Fig. 9. Transferred charge Qt in dependence of the ignition energy for a 5.2% propane/
air mixture.
Fig. 11. Ignition energy in dependence of the electrode gap width using a 22.0%
hydrogen/air mixture, determination of the MIE.
seen, the criterion of transferred charge is suitable but the
thresholds represent different levels of safety. In case of explosion
group IIB, the achieved values of transferred charge leading to
ignition are close to the threshold whereas in case of explosion
group IIC, the gap between the achieved results and the threshold is
somewhat bigger.
In the last part of this paper, the MIE values of hydrogen, ethene,
propane and acetone/air mixtures given in the literature e.g
[1,2,6,10,11,14,18e20]. are reviewed. Thereby, especially the
measurement uncertainty is the focus of this study. In contrast to
the data presented above, all observed ignitions are taken into
account. Thus also the experiments in which the HC non-triggered
the discharges are used for this study, since the transferred charge
is not necessary to obtain the ignition energy. The ignition energy is
once more calculated using equation (1).
The MIE of hydrogen was recently verified by Ono et al. [20].
They observed a MIE of 17 mJ for a hydrogen/air mixture with
a 22%e26% hydrogen fraction. This is in accordance with the value
of 17 mJ given in [18]. In [2] 19 mJ and in [14] 16 mJ are given.
As mentioned before two different electrode configurations
were used for the 22.0% hydrogen/air mixture (Fig. 11). This is
indicated with different symbols in the figure. The MIE determined
in our test was observed at d ¼ 0.5 mm with two 2 mm electrodes at
a voltage of 2.80 kV and a capacitance of 4.41 pF. Its value is
17.3 mJ 2.3 mJ and agrees well with the values between 16 mJ and
19 mJ given in [2,14,18,20]. For this value no transferred charge was
measured. The corresponding calculated charge is 12.4 nC 1.6 nC.
Concerning 8.0% ethene/air mixtures (Fig. 12), an MIE of
80.8 mJ 9.7 mJ was found at a gap width of 1.2 mm using two 2 mm
electrodes at a voltage of 5.80 kV and a capacitance of 4.80 pF. As
described above, the biggest influence with respect to the
expanded measurement uncertainty originates from the determination of the capacitance. The transferred charge was
26.9 nC 1.1 nC, the calculated charge 27.9 nC 3.3 nC. Regarding
the measurement uncertainty, the MIE is the same as the value of
82 mJ given in [18].
The MIE of propane/air mixtures (Fig. 13) is in discussion [10,19]
since the value of 0.25 mJ given by Lewis and von Elbe [2] and in
[14] seems to be quite low. In [18] 0.24 mJ is given. Its origin could
not be verified. For 5.2% propane in air, a 1.5 mm/2.0 mm electrode
configuration and a gap width of 1.7 mm was used. An MIE of
0.330 mJ 0.020 mJ was found using a voltage of 8.32 kV and
Fig. 10. Transferred charge Qt in dependence of the ignition energy for a 6.5% acetone/
air mixture.
Fig. 12. Ignition energy in dependence of the electrode gap width using an 8.0%
ethene/air mixture, determination of the MIE.
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
103
Table 3
Overview of achieved results concerning the MIE determination.
Fig. 13. Ignition energy in dependence of the electrode gap width using a 5.2%
propane/air mixture, determination of the MIE.
a capacitance of 9.53 pF. The corresponding value of transferred
charge was 66.9 nC 1.8 nC, the calculated charge 79.3 nC 4.8 nC.
It should be pointed out that the results for propane are based on
more than 12.000 spark discharges. The MIE is over 30% higher
than the values given by Lewis and von Elbe [2] and Brandes and
Möller [18]. On the other hand, it is significantly lower than the
recently published value of 0.46 mJ [10] and the value of 0.48 mJ
given by Moorhouse et al. [19]. The value given by Moorhouse was
measured in a slightly different mixture compared to the mixture
used for this work (stoichiometric ratio 1.30 instead of 1.25 in this
work). The gap width is not given in detail but is between 2.3 mm
and 2.8 mm in the study of Moorhouse. Moreover, the used pressure was only 0.75 atm. Eckhoff et al. [10] used a gap width of
2.0 mm and two 1.6 mm flanged electrodes. The mixture was 5.2%
propane in air as used for this work as well.
Concerning 6.5% acetone/air mixtures (Fig. 14) the MIE observed
in our tests was 0.499 mJ 0.018 mJ with an electrode gap of
1.9 mm and two 2.0 mm electrodes at a voltage of 7.90 kV and
a capacitance of 16.00 pF. This value is about 10% lower than the
value of 0.550 mJ given in [18]. In [11] it was criticised that for the
Fig. 14. Ignition energy in dependence of the electrode gap width using a 6.5%
acetone/air mixture, determination of the MIE.
Mixture
Qt at
MIE/nC
Expanded
measurement
uncertainty/nC
MIE
Expanded
measurement
uncertainty
22.0% hydrogen/air
8.0% ethene/air
5.2% propane/air
6.5% acetone/air
e
26.9
66.9
112.8
e
1.1
1.8
3.4
17.3 mJ
80.8 mJ
0.330 mJ
0.499 mJ
2.3 mJ
9.7 mJ
0.020 mJ
0.018 mJ
determination of the MIE of acetone/air mixtures often a stochiometric and not the most ignitable mixture is used leading to too
high MIE values. The corresponding transferred charge is
112.8 nC 3.4 nC, the calculated charge 126.4 nC 4.6 nC. The
transferred charge measured is more than 10% lower than the value
given in [6].
In summary especially concerning the determination of MIE and
their corresponding transferred charge, it seems essential to give
the corresponding measurement uncertainties in order to compare
the achieved results. The measurement of the capacitance seems to
be the main challenge in this context. In Table 3 all achieved results
are given for an overview.
5. Conclusions
Using the apparatus commonly utilized for the determination of
the minimum ignition energy (MIE), a wide series of experiments
based on several thousand discharge events have been carried out
in hydrogen/air, ethene/air, propane/air and acetone/air mixtures.
Thereby, the transferred charge as a criterion to judge the ignition
potential is determined to verify the thresholds of transferred
charge given in the standards IEC 60079-0 and EN 13463-1. The
stored charge in the capacitance before the spark discharge is
compared to the transferred charge in the spark. The measurement
of the transferred charge performed with a hand-held coulombmeter using an unshielded probe gave reasonable values for all
examined mixtures. It was found that the transferred charge is less
than the stored charge. However, this difference can be explained
by a residual charge on the capacitance or corona, respectively. For
higher gap distances or higher charging voltages, respectively,
a decreasing ratio of the transferred charge to the stored charge was
observed.
Furthermore, the correlation of the ignition energy and the
transferred charge is examined. Based on these results the
thresholds of the transferred charge and the suitability of the
method “transferred charge” are discussed. In case of ethene/air
mixtures (explosion group IIB) a minimum transferred charge of
24.1 nC 1.1 nC led to ignition which is lower than the current
threshold of 30 nC. Moreover, a lower transferred charge igniting
acetone/air mixtures than given in the literature was found
(112.8 nC 3.4 nC compared to 127 nC). For hydrogen/air and
propane/air mixtures the lowest transferred charge leading to
ignition was higher than the given threshold. In general, the lowest
transferred charge igniting the mixture does not necessarily
correspond to the MIE of the mixture. All in all, the criterion of the
transferred charge seems suitable but the thresholds of the
different explosion groups represent different levels of safety.
The MIE values of the above-mentioned mixtures are reviewed
taking into account the measurement uncertainty according to the
“Guide to the expression of uncertainty in measurement”. For
hydrogen/air mixtures, a value of 17.3 mJ 2.3 mJ was achieved
which agrees well with the literature. The MIE of ethene/air
mixtures was determined to be 80.8 mJ 9.7 mJ which matches well
with the given value of 82 nC in the literature. The MIE of propane/
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T. Langer et al. / Journal of Electrostatics 70 (2012) 97e104
air mixtures has been in discussion during the past few years. An
MIE of 0.330 mJ 0.020 mJ was achieved which is less than the
recently published values by Eckhoff et al. and Moorhouse et al. of
0.46 mJ and 0.48 mJ, respectively, but significantly more than the
0.24 mJ and 0.25 mJ given by Brandes and Möller and Lewis and von
Elbe, respectively. The MIE of acetone/air mixtures was
0.499 mJ 0.018 mJ which is lower than the value of 0.550 mJ given
in the literature. The main part of the measurement uncertainty
results from the determination of the capacitance (setting the zero
point). More work is planned in the future concerning the determination of MIE values especially regarding statistics along with
the uncertainty.
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