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Spectral character as a function of spark source parameters
Spectrochimica Acta, 1958, Vol. 13, pp. 99 to 106. Pergamon Press Ltd., London
Spectral character as a function of spark source parameters T. P SCHREIBER and D. L. FRY Research Staff, General Motors Corp., Warren, Michigan (Received 12 December 1957) Abstract-The possibility of specifying high voltage spark source conditions on the basis of only two parameters, the average (rectified) current and the intensity ratio of an Fe1 to an Fe111 line, has been investigated and found feasible. Because of difficulties encountered in measuring the average current directly, a method was developed whereby a good approximation of it can be computed from the root-mean-square current, as measured by a radio-frequency meter, and the circuit parameters, using the equation:
The spectral character as measured by the FeI/FeIII following relationship to source parameters: &l/Z
Fe1 3016.2 Fe111 3013.1
intensity ratio was found to have the
Oc-c v*/r
Introduction OF basic importance to spectrochemical analysis is the specification of spark source parameters and their correlation with spectral character. The specification of spark source conditions by means of the circuit constants is not completely satisfactory because the circuit constants are not uniquely related to the spectrochemical character of the discharge. There are also circuit variables which are difficult or impossible to specify. The work of KAISER [l], ENNS [Z] and others has led to a better understanding of the mechanisms of spark excitation but quantitative application of this information has not been realized. The work of LAQUA [3] in which spectral character is related to the average (rectified) current and the duration of the discharge indicates that source specification by means of a few critical parameters may be possible. This paper covers an investigation of LAQUA’S ideas as applied to the point-toplane spark excitation of low alloy steels with a high voltage spark source. Average current-theory The usefulness of average current in calculating the power dissipated in the analytical gap is based upon KAISER’S [l] finding that after the initial breakdown the spark gap assumes the character of an arc. The voltage drop across the gap is independent of current and is determined by gap length and electrode material. Thus the average power dissipated can be computed with the equation: Power where T is the period,
= 1T
s
OpWv,(t)
i(t) is the instantaneous 99
dt current
and v,(t)
is the voltage
T. P. SCHREIBER and D. L. FRY
across the gap. In an oscillatory discharge v,(t) is constant except that its sign Equation (1) varies as i(t) varies so that the product, i(t)v,, is always positive. can thus be rewritten: T
Power
= 5 i(t) at = v& T so
Attempts to measure the average current with a rectifying ammeter as that described by LAQUA failed because of the frequency sensitivity resistor. (This work was performed at higher frequencies than those by LAQUA.) Several shunt designs were tried but none was found Because of these difficulties, three methods of calculating Iaav from parameter were developed. Figure 1 is a condenser voltage oscilloscope trace obtained with
(2) device such of the shunt encountered satisfactory. measurable an air inter-
Fig. 1. Voltage oscillograph trace of transient discharge. Source conditions-C = 0.0080 pF; I; = 29 pH; R (added) = 0; V = 20 kV; B = 3 discharges/helf cycle.
rupted source under standard conditions for analysing steel. The current trace, which is much more difficult to obtain, appears much the same since it is the first derivative of the voltage curve. The envelope of the curve lies between an exponential decay, which would be the case if all the resistance in the circuit were ohmic, and a linear decay, which would hold if only the voltage drops across the gaps were deducted on each half cycle. With this curve shape in mind three different approaches in calculating average current were tried. (1) In the first method linear damping was assumed and 0 was measured with an oscilloscope. The calculation takes the form: (3) where J, is the first current peak (J, = V2/(C/L) if R is small) and 2/n is the form factor for obtaining the average of a sine wave. N is the number of discharges per see and 6 is the transient duration in seconds. For a 60 c/s supply and with B as the number of discharges per half cycle the equation becomes: Ia,, = %+lJ,BO (2) The second method of computing I,, involved the determination terms of the normally measured or known circuit parameters. The general solution for an oscillatory discharge is given by i(t) = J, exp 100
(4)
of 0 in
Spectral character as a function of spark source parameters
If a linear decay is assumed, than R must be a function of t if equation (5) is applicable. The function R(t) can be obtained by equating the decay portion of equation (5), exp ( - R/2L t), to a linear function with a slope of -l/O and an interaept of 1
Solving for R(t): R(t) = The average resistance,
1 -
(
8 s
R, can then be obtained
R = f This can be integrated
5: In
s
f
1
from the following:
- f In (1 &R(t) & = ‘$
t/0)
dt
0
to give: R_%
Experimentally, it has been established resistance of the circuit. R,, = s
(7)
(f% that R is very nearly equal to the effective = “$V’
(9)
rf
where I,, is the radio frequency or root mean square current as measured on a thermocouple-type meter. Equating R and R,, the following relationship for 0 is obtained: ?PL I,,” !!I= (10) 180BCV2
Substituting
0 of equation
(10) into equation
2.09 I,f” Iav
=
7
(4):
L
2.09 I,,”
J( c J =-
Jll
(11)
With this equation I,, can be computed from the easily measured source parameters C, L, and V and the almost universally measured variable I,,. (3) The third method used in computing Iav was based on the assumption of an exponential decay [see equation (5)]. Integrating the transient for each half cycle by substituting equation (5) into equation (12) leads to the expression for the average current given by equation ( 13). I,”
I,,
R in this equation
= f
s
or~i(t)l at
(12)
= 12OBCV
(13)
was assumed to be R,, [see equation 101
(g)].
T. P. SCHREIBER and I>. L.
FRY
Experimental Because average current could not be measured directly, the most reliable values were obtained by method (1) above. Since this procedure requires equipment not normally found in a spect’rographic laboratory, experiments were undertaken to check the usefulness of methods (2) and (3). Average current values were obtained by the three methods for forty-five different source conditions in which parameters Methods (1) and (2) gave values which agreed within were varied individually. &lo per cent. Method (3), however, averaged 10 per cent higher than (1) or (2).
I
1
6
1
2 Breaks
Fig. 2. Variations of average current with capacitance. Average current computed by ( x) Method 1; (0) Method 2; (0) Method 3. Other source settings L = 56 ,uH, V = 13.5 kV, B = 5 breaks/half cycle, and R (added) = 0.
10
5 per
half
cycle
20
-
Fig. 3. Average current obtained by method (2) as a function of breaks per half cycle for three different inductance settings; 29 PH (O), 56 PH (x), and 115 ,uH (0). Other parameters C = 0.0080 pF, V = 12 kV, and It (added) = 0.
To obtain accurate values for the above data it was necessary to calibrate the radio-frequency ammeter and the oscilloscope used to measure breakdown voltage. The radio-frequency current reading is especially critical when method (2) is used since it appears as the square in equation (11) and its per cent error is thus doubled. Errors in breakdown voltage cause difficulty in comparing methods (1) and (2) since the resultant errors in the average current values are in opposite directions. Average current as a function of capacitance, breaks per half cycle, voltage and resistance is shown in Figs. 2, 3, 4 and 5. Average current was found to be independent of inductance. The fact that equations (4), (11) and (13) show a relationship between Iav and J, might lead one to believe that Iav is not independent of inductance (J, = Vv1/C/L). This can be explained by the fact that other variables, e.g. 8 in (4), I,, in (ll), and R,, in (13), are dependent upon inductance. From the slopes of these curves the following relat,ionship can be obt’ained: I
a.v
cc BG V3’2
R
(14)
LAQUA [3] found a correlation between average current and the total intensity of the spark as measured with a phototube. These measurements were not made, but an attempt was made to correlate the intensity of the 3016.2 A Fe1 line with average current. Good correlation was obtained for conditions in which capacitance, voltage and breaks per half cycle were varied but inductance variations
102
Spectral character as a function of spark source parameters
caused some peculiar effects. When samples were sparked in air, the intensity decreased as inductance was added, but when a nitrogen atmosphere was used the intensity was much more constant and showed no specific trend except that it increased for the first few indu~tanee steps. These variations indicate that spectral
o 4.0 E 0 2.0
I > 9
1.0
05’
5
““I
10
1 20
v-
I
02
kV
50
10 R-
Fig. 8. Average current obtained by method (2) as a function of breakdown voltage. Other parameters, G = O@I80 ,uF, L = 56 ,IJH, 3 = 5 breaks~ha,lfcycle and R (added) = 0.
10.0
200 ohm
Fig. 5. Average current obtained by method (2) aa a function of R for three different conditions. The R value used here is R (ttdded) + R (residual). R (residual) was obtained by plotting R (added) vs. l/IPr8 and estrapolating to obtain the R at l/lrtz = 0. The other parameters were (0)C = 0.0080 ,uF, L = 56 ,uH; (x) C = O*OOSO,aF, L = 115 ,uH; (0) C = 0.0107 pF, L = 56 pH. For all conditions V = 12 kV and B = 5 breaks/half cycle.
intensity is a function of more than just electrical source parameters. Other work performed in this laboratory [4] has shown considerable dependence of spectral intensity upon oxidation of the metal in the spark area.
Spectral character was measured by the ratio of an iron arc line (FeI 3016.18 A) to a doubly-ionized iron line (Fe111 3013.12 A). Lines from singly-ionized atoms were investigated but the iron-III line was found to be more sensitive to source parameter changes.’ Figures 6 thro~lgh 9 show how the ratio of the Fe1 3016.18 A ~ex~itation potential 5.10 eV) to the FeIII 3013.12 A (excitation potential 38.8 eV) varies with different parameters. These curves indicate that the intensity ratio of: _
Fe1 3016.2 ,!%
FeIII Figure 9 indicates. that the inverse of the peak current independent of breaks per LAQUA [3] claimed that is a function of transient
pz
3016.1 A K ___ CP
05)
FeI/FeIII intensity ratio is roughly proportional to the squared. The FeI/l?eIII intensity ratio is substantially half cycle and resistance. spectral quality, e.g. the ratio of an arc to a spark line, duration time. Figures 6, 7 and 8 indicate that this is 103
T. P. SCHREIBER and D. L. FRY
not true. The transient duration time increases as capacitance, voltage and inductance increase, while the FeI/FeIII intensity ratio increases with inductance but decreases with capacitance and voltage. 5.0
~
c9 $2 WEI l?[ I? 2.0 3 x
1.0
5 E
0 002
0005 c-
z 0010
0020
0.5
5
20
10
Fig. 6. FeI/FeIII intensity ratio as a function of capacitance. Other parameters L = 56 ,uH, V = 13.5 kV, B = 5 breaks/half cycle and R (added) = 0.
50
100
L-
Pu’
300
Fh
Fig. 7. FeI/FeIII intensity ratio as a function of inductance. Other parameters C = 0.0080 pF, V = 13.0 kV, B = 5 breaks$alf cycle and R (added) = .
80
50
v-
Fig. 8. FeI/FeIII intensity ratio as a function of breakdown voltage. Other parameters C = 0.0080 pF, L = 56 ,uH, B = 5 breaks/half cycle, R (added) = 0.
Specification
100 Peak
kV
current
200 -
400
800 amp
Fig. 9. FeI/FeIII ratio as a function of peak current (Jo = Vd(C/L)) for conditions with C varying (0 ), L varying ( x ) and V varying ( 0). Other parameters as indicated in Figs. 6, 7 and 8.
of source conditions
Since average current gives a measure of a very basic quantity, the energy dissipated in the analytical gap, it should be useful in the specification of source parameters. This study shows that spectral character is best specified by means of the FeI/FeIII intensity ratio. Application of this intensity ratio has the disadvantage that the Fe111 line disappears with very arc-like conditions. Arc-like conditions, however, are not as critical to parameter settings as the spark-like conditions. To test the validity of this approach, three source conditions with widely different source settings but giving the same FeI/FeIII intensity ratio and average current (see conditions 1, 2 and 3 in Table 1) were examined. Equations (14) and 104
Spectral character as a function of spark source parameters
(15) were very helpful in selecting suitable source parameters. Analytical curves for low alloy steel obtained with these conditions were compared with conditions which gave varying FeI/FeIII intensity ratios (conditions 4 and 5) and average current (conditions 6 and 7). Both air and nitrogen atmospheres were employed.
Condition
C (@)
Table
-
T L (PH)
1
I
I
Intensity ratio
Breaks per half cycle
V (kv)
I rf
I av
Fe1 3016.2 Fe111 3013.1 (A)
-
-L
I 1 2 3 4 5 6 7
0.0027 0.0078 0.0155 O-0078 0.0078 0.0078 0.0078
-
10 4 1 10 10 2 8
11.5 10.0 19.0 10.0 10.0 10.0 10.0
9 (22)
22 (22) 89 (38) 9 38 22 22
10.0 (7.9) 9.6 (9.6) 11.8 (14.4) 11.8 8.2 6.6 14.4
1.0 1.0 1.0 1.0 1.0 0.5 2.3
1.1 1.0 1.1 0.8 1.9 1.1 1-l
(1.65) (1.65) (1.65) (1.0) (3.0) (2.1) (1.3)
Preburn : 60 see Exposure : 30 set Analytical gap :3mm Counter electrode : 120” pointed graphite
(The values in parentheses refer to the tests run with a nitrogen atmosphere.)
I
4
-
l/IN 3215 I
I I
II
o-2 Intensity
ratio Cr -
2988
6 8
Fe29904!i
Fig. 10. Analytical curves obtained for different souroe osnditions using a nitrogen atmosphere. The numbers indicate the source conditions as specified in Table 1.
Since the nitrogen caused a mismatch of the FeI/FeIII intensity ratio it was found The analytical lines used for these necessary to vary the conditions slightly. experiments were carefully matched in excitation potential and wavelength [5]. Figure 10 is an example of the type of curves obtained. The curve shifts shown are somewhat larger than the average for all elements. In general, the greatest
T. P. SCHREIBER
and D. b.
FRY:
Spectral character
as a function of spark sowce pamnet~ers
curve shifts were observed when the FeI/FeIXI intensity ratio varied rather than when the average current changed. The interesting feature of the curves is that conditions 1, 2 and 3 gave similar analytical curves and produced the same spectral intensity while 4 and 5 showed larger curve shifts. Conditions 6 and ‘i showed curve shifts comparable to 1 and 3 and as expected the inte~lsities varied a.s the average current. These results indicate that average current and the FeI/FeIII intensity ratio are more unique parameters for specifying a spark source condition than are capacitance, inductance, voltage, radio frequency (r.m.s.) current, etc. The average current determines the power dissipated in the analytica gap and the general spectral intensity. The FeIFeIII intensity ratio (or similar lines of other elements) Repeatability is not directly determines the type of analytical curves obtained. related to these two parameters so that the best combination of the other parameters must be determined experiment,alIy. Equations (14) and (15) can be used to maintain the average current and FeI/PeIII intensity ratio constant while doing this.
References [I] KAISER H. and WALLRAFF A., Arm. Phgs. 1939 34, 297. [Z] Emm J. H. and WOLFE R. A., Special Tech. E%blicalion No. 76. Amer. Materials 1948. [3] IAQUA K., Spectmchim. Acta 1952 4, 446. [4] NAJICOWSKI Et. F. and SCHREIBER T. P. To be published. [a] FRY D. L. and ~~HREIBER T. P., i. Opt. Sot. Amer. 1954 44, 159.
106
Sot.
for Testing
Spectral character as a function of spark source parameters T. P SCHREIBER and D. L. FRY Research Staff, General Motors Corp., Warren, Michigan (Received 12 December 1957) Abstract-The possibility of specifying high voltage spark source conditions on the basis of only two parameters, the average (rectified) current and the intensity ratio of an Fe1 to an Fe111 line, has been investigated and found feasible. Because of difficulties encountered in measuring the average current directly, a method was developed whereby a good approximation of it can be computed from the root-mean-square current, as measured by a radio-frequency meter, and the circuit parameters, using the equation:
The spectral character as measured by the FeI/FeIII following relationship to source parameters: &l/Z
Fe1 3016.2 Fe111 3013.1
intensity ratio was found to have the
Oc-c v*/r
Introduction OF basic importance to spectrochemical analysis is the specification of spark source parameters and their correlation with spectral character. The specification of spark source conditions by means of the circuit constants is not completely satisfactory because the circuit constants are not uniquely related to the spectrochemical character of the discharge. There are also circuit variables which are difficult or impossible to specify. The work of KAISER [l], ENNS [Z] and others has led to a better understanding of the mechanisms of spark excitation but quantitative application of this information has not been realized. The work of LAQUA [3] in which spectral character is related to the average (rectified) current and the duration of the discharge indicates that source specification by means of a few critical parameters may be possible. This paper covers an investigation of LAQUA’S ideas as applied to the point-toplane spark excitation of low alloy steels with a high voltage spark source. Average current-theory The usefulness of average current in calculating the power dissipated in the analytical gap is based upon KAISER’S [l] finding that after the initial breakdown the spark gap assumes the character of an arc. The voltage drop across the gap is independent of current and is determined by gap length and electrode material. Thus the average power dissipated can be computed with the equation: Power where T is the period,
= 1T
s
OpWv,(t)
i(t) is the instantaneous 99
dt current
and v,(t)
is the voltage
T. P. SCHREIBER and D. L. FRY
across the gap. In an oscillatory discharge v,(t) is constant except that its sign Equation (1) varies as i(t) varies so that the product, i(t)v,, is always positive. can thus be rewritten: T
Power
= 5 i(t) at = v& T so
Attempts to measure the average current with a rectifying ammeter as that described by LAQUA failed because of the frequency sensitivity resistor. (This work was performed at higher frequencies than those by LAQUA.) Several shunt designs were tried but none was found Because of these difficulties, three methods of calculating Iaav from parameter were developed. Figure 1 is a condenser voltage oscilloscope trace obtained with
(2) device such of the shunt encountered satisfactory. measurable an air inter-
Fig. 1. Voltage oscillograph trace of transient discharge. Source conditions-C = 0.0080 pF; I; = 29 pH; R (added) = 0; V = 20 kV; B = 3 discharges/helf cycle.
rupted source under standard conditions for analysing steel. The current trace, which is much more difficult to obtain, appears much the same since it is the first derivative of the voltage curve. The envelope of the curve lies between an exponential decay, which would be the case if all the resistance in the circuit were ohmic, and a linear decay, which would hold if only the voltage drops across the gaps were deducted on each half cycle. With this curve shape in mind three different approaches in calculating average current were tried. (1) In the first method linear damping was assumed and 0 was measured with an oscilloscope. The calculation takes the form: (3) where J, is the first current peak (J, = V2/(C/L) if R is small) and 2/n is the form factor for obtaining the average of a sine wave. N is the number of discharges per see and 6 is the transient duration in seconds. For a 60 c/s supply and with B as the number of discharges per half cycle the equation becomes: Ia,, = %+lJ,BO (2) The second method of computing I,, involved the determination terms of the normally measured or known circuit parameters. The general solution for an oscillatory discharge is given by i(t) = J, exp 100
(4)
of 0 in
Spectral character as a function of spark source parameters
If a linear decay is assumed, than R must be a function of t if equation (5) is applicable. The function R(t) can be obtained by equating the decay portion of equation (5), exp ( - R/2L t), to a linear function with a slope of -l/O and an interaept of 1
Solving for R(t): R(t) = The average resistance,
1 -
(
8 s
R, can then be obtained
R = f This can be integrated
5: In
s
f
1
from the following:
- f In (1 &R(t) & = ‘$
t/0)
dt
0
to give: R_%
Experimentally, it has been established resistance of the circuit. R,, = s
(7)
(f% that R is very nearly equal to the effective = “$V’
(9)
rf
where I,, is the radio frequency or root mean square current as measured on a thermocouple-type meter. Equating R and R,, the following relationship for 0 is obtained: ?PL I,,” !!I= (10) 180BCV2
Substituting
0 of equation
(10) into equation
2.09 I,f” Iav
=
7
(4):
L
2.09 I,,”
J( c J =-
Jll
(11)
With this equation I,, can be computed from the easily measured source parameters C, L, and V and the almost universally measured variable I,,. (3) The third method used in computing Iav was based on the assumption of an exponential decay [see equation (5)]. Integrating the transient for each half cycle by substituting equation (5) into equation (12) leads to the expression for the average current given by equation ( 13). I,”
I,,
R in this equation
= f
s
or~i(t)l at
(12)
= 12OBCV
(13)
was assumed to be R,, [see equation 101
(g)].
T. P. SCHREIBER and I>. L.
FRY
Experimental Because average current could not be measured directly, the most reliable values were obtained by method (1) above. Since this procedure requires equipment not normally found in a spect’rographic laboratory, experiments were undertaken to check the usefulness of methods (2) and (3). Average current values were obtained by the three methods for forty-five different source conditions in which parameters Methods (1) and (2) gave values which agreed within were varied individually. &lo per cent. Method (3), however, averaged 10 per cent higher than (1) or (2).
I
1
6
1
2 Breaks
Fig. 2. Variations of average current with capacitance. Average current computed by ( x) Method 1; (0) Method 2; (0) Method 3. Other source settings L = 56 ,uH, V = 13.5 kV, B = 5 breaks/half cycle, and R (added) = 0.
10
5 per
half
cycle
20
-
Fig. 3. Average current obtained by method (2) as a function of breaks per half cycle for three different inductance settings; 29 PH (O), 56 PH (x), and 115 ,uH (0). Other parameters C = 0.0080 pF, V = 12 kV, and It (added) = 0.
To obtain accurate values for the above data it was necessary to calibrate the radio-frequency ammeter and the oscilloscope used to measure breakdown voltage. The radio-frequency current reading is especially critical when method (2) is used since it appears as the square in equation (11) and its per cent error is thus doubled. Errors in breakdown voltage cause difficulty in comparing methods (1) and (2) since the resultant errors in the average current values are in opposite directions. Average current as a function of capacitance, breaks per half cycle, voltage and resistance is shown in Figs. 2, 3, 4 and 5. Average current was found to be independent of inductance. The fact that equations (4), (11) and (13) show a relationship between Iav and J, might lead one to believe that Iav is not independent of inductance (J, = Vv1/C/L). This can be explained by the fact that other variables, e.g. 8 in (4), I,, in (ll), and R,, in (13), are dependent upon inductance. From the slopes of these curves the following relat,ionship can be obt’ained: I
a.v
cc BG V3’2
R
(14)
LAQUA [3] found a correlation between average current and the total intensity of the spark as measured with a phototube. These measurements were not made, but an attempt was made to correlate the intensity of the 3016.2 A Fe1 line with average current. Good correlation was obtained for conditions in which capacitance, voltage and breaks per half cycle were varied but inductance variations
102
Spectral character as a function of spark source parameters
caused some peculiar effects. When samples were sparked in air, the intensity decreased as inductance was added, but when a nitrogen atmosphere was used the intensity was much more constant and showed no specific trend except that it increased for the first few indu~tanee steps. These variations indicate that spectral
o 4.0 E 0 2.0
I > 9
1.0
05’
5
““I
10
1 20
v-
I
02
kV
50
10 R-
Fig. 8. Average current obtained by method (2) as a function of breakdown voltage. Other parameters, G = O@I80 ,uF, L = 56 ,IJH, 3 = 5 breaks~ha,lfcycle and R (added) = 0.
10.0
200 ohm
Fig. 5. Average current obtained by method (2) aa a function of R for three different conditions. The R value used here is R (ttdded) + R (residual). R (residual) was obtained by plotting R (added) vs. l/IPr8 and estrapolating to obtain the R at l/lrtz = 0. The other parameters were (0)C = 0.0080 ,uF, L = 56 ,uH; (x) C = O*OOSO,aF, L = 115 ,uH; (0) C = 0.0107 pF, L = 56 pH. For all conditions V = 12 kV and B = 5 breaks/half cycle.
intensity is a function of more than just electrical source parameters. Other work performed in this laboratory [4] has shown considerable dependence of spectral intensity upon oxidation of the metal in the spark area.
Spectral character was measured by the ratio of an iron arc line (FeI 3016.18 A) to a doubly-ionized iron line (Fe111 3013.12 A). Lines from singly-ionized atoms were investigated but the iron-III line was found to be more sensitive to source parameter changes.’ Figures 6 thro~lgh 9 show how the ratio of the Fe1 3016.18 A ~ex~itation potential 5.10 eV) to the FeIII 3013.12 A (excitation potential 38.8 eV) varies with different parameters. These curves indicate that the intensity ratio of: _
Fe1 3016.2 ,!%
FeIII Figure 9 indicates. that the inverse of the peak current independent of breaks per LAQUA [3] claimed that is a function of transient
pz
3016.1 A K ___ CP
05)
FeI/FeIII intensity ratio is roughly proportional to the squared. The FeI/l?eIII intensity ratio is substantially half cycle and resistance. spectral quality, e.g. the ratio of an arc to a spark line, duration time. Figures 6, 7 and 8 indicate that this is 103
T. P. SCHREIBER and D. L. FRY
not true. The transient duration time increases as capacitance, voltage and inductance increase, while the FeI/FeIII intensity ratio increases with inductance but decreases with capacitance and voltage. 5.0
~
c9 $2 WEI l?[ I? 2.0 3 x
1.0
5 E
0 002
0005 c-
z 0010
0020
0.5
5
20
10
Fig. 6. FeI/FeIII intensity ratio as a function of capacitance. Other parameters L = 56 ,uH, V = 13.5 kV, B = 5 breaks/half cycle and R (added) = 0.
50
100
L-
Pu’
300
Fh
Fig. 7. FeI/FeIII intensity ratio as a function of inductance. Other parameters C = 0.0080 pF, V = 13.0 kV, B = 5 breaks$alf cycle and R (added) = .
80
50
v-
Fig. 8. FeI/FeIII intensity ratio as a function of breakdown voltage. Other parameters C = 0.0080 pF, L = 56 ,uH, B = 5 breaks/half cycle, R (added) = 0.
Specification
100 Peak
kV
current
200 -
400
800 amp
Fig. 9. FeI/FeIII ratio as a function of peak current (Jo = Vd(C/L)) for conditions with C varying (0 ), L varying ( x ) and V varying ( 0). Other parameters as indicated in Figs. 6, 7 and 8.
of source conditions
Since average current gives a measure of a very basic quantity, the energy dissipated in the analytical gap, it should be useful in the specification of source parameters. This study shows that spectral character is best specified by means of the FeI/FeIII intensity ratio. Application of this intensity ratio has the disadvantage that the Fe111 line disappears with very arc-like conditions. Arc-like conditions, however, are not as critical to parameter settings as the spark-like conditions. To test the validity of this approach, three source conditions with widely different source settings but giving the same FeI/FeIII intensity ratio and average current (see conditions 1, 2 and 3 in Table 1) were examined. Equations (14) and 104
Spectral character as a function of spark source parameters
(15) were very helpful in selecting suitable source parameters. Analytical curves for low alloy steel obtained with these conditions were compared with conditions which gave varying FeI/FeIII intensity ratios (conditions 4 and 5) and average current (conditions 6 and 7). Both air and nitrogen atmospheres were employed.
Condition
C (@)
Table
-
T L (PH)
1
I
I
Intensity ratio
Breaks per half cycle
V (kv)
I rf
I av
Fe1 3016.2 Fe111 3013.1 (A)
-
-L
I 1 2 3 4 5 6 7
0.0027 0.0078 0.0155 O-0078 0.0078 0.0078 0.0078
-
10 4 1 10 10 2 8
11.5 10.0 19.0 10.0 10.0 10.0 10.0
9 (22)
22 (22) 89 (38) 9 38 22 22
10.0 (7.9) 9.6 (9.6) 11.8 (14.4) 11.8 8.2 6.6 14.4
1.0 1.0 1.0 1.0 1.0 0.5 2.3
1.1 1.0 1.1 0.8 1.9 1.1 1-l
(1.65) (1.65) (1.65) (1.0) (3.0) (2.1) (1.3)
Preburn : 60 see Exposure : 30 set Analytical gap :3mm Counter electrode : 120” pointed graphite
(The values in parentheses refer to the tests run with a nitrogen atmosphere.)
I
4
-
l/IN 3215 I
I I
II
o-2 Intensity
ratio Cr -
2988
6 8
Fe29904!i
Fig. 10. Analytical curves obtained for different souroe osnditions using a nitrogen atmosphere. The numbers indicate the source conditions as specified in Table 1.
Since the nitrogen caused a mismatch of the FeI/FeIII intensity ratio it was found The analytical lines used for these necessary to vary the conditions slightly. experiments were carefully matched in excitation potential and wavelength [5]. Figure 10 is an example of the type of curves obtained. The curve shifts shown are somewhat larger than the average for all elements. In general, the greatest
T. P. SCHREIBER
and D. b.
FRY:
Spectral character
as a function of spark sowce pamnet~ers
curve shifts were observed when the FeI/FeIXI intensity ratio varied rather than when the average current changed. The interesting feature of the curves is that conditions 1, 2 and 3 gave similar analytical curves and produced the same spectral intensity while 4 and 5 showed larger curve shifts. Conditions 6 and ‘i showed curve shifts comparable to 1 and 3 and as expected the inte~lsities varied a.s the average current. These results indicate that average current and the FeI/FeIII intensity ratio are more unique parameters for specifying a spark source condition than are capacitance, inductance, voltage, radio frequency (r.m.s.) current, etc. The average current determines the power dissipated in the analytica gap and the general spectral intensity. The FeIFeIII intensity ratio (or similar lines of other elements) Repeatability is not directly determines the type of analytical curves obtained. related to these two parameters so that the best combination of the other parameters must be determined experiment,alIy. Equations (14) and (15) can be used to maintain the average current and FeI/PeIII intensity ratio constant while doing this.
References [I] KAISER H. and WALLRAFF A., Arm. Phgs. 1939 34, 297. [Z] Emm J. H. and WOLFE R. A., Special Tech. E%blicalion No. 76. Amer. Materials 1948. [3] IAQUA K., Spectmchim. Acta 1952 4, 446. [4] NAJICOWSKI Et. F. and SCHREIBER T. P. To be published. [a] FRY D. L. and ~~HREIBER T. P., i. Opt. Sot. Amer. 1954 44, 159.
106
Sot.
for Testing