SPARK-GAP VOLTAGES
Based on results of the American Institute of Electric Engineers
Air at 760 mmHg, 25 C
Peak Voltage	Needle points	Diameter of spherical electrodes, [cm]
		2,5	5	10	25
[kV]	[cm]	[cm]	[cm]	[cm]	[cm]
5	0,42	0,13	0,15	0,15	0,16
10	0,85	0,27	0,29	0,30	0,32
15	1,30	0,42	0,44	0,46	0,48
20	1,75	0,58	0,60	0,62	0,64
25	2,20	0,76	0,77	0,78	0,81
30	2,69	0,95	0,94	0,95	0,98
35	3,20	1,17	1,12	1,12	1,15
40	3,81	1,41	1,30	1,29	1,32
45	4,49	1,68	1,50	1,47	1,49
50	5,20	2,00	1,71	1,65	1,66
60	6,81	2,82	2,17	2,02	2,01
70	8,81	4,05	2,68	2,42	2,37
80	11,10		3,26	2,84	2,74
90	13,30		3,94	3,28	3,11
100	15,50		4,77	3,75	3,49
110	17,70		5,79	4,25	3,88
120	19,80		7,07	4,78	4,28
130	22,00			5,35	4,69
140	24,10			5,97	5,10
150	26,10			6,64	5,52
160	28,10			7,37	5,95
170	30,10			8,16	6,39
180	32,00			9,03	6,84
190	33,90			10,00	7,30
200	35,70			11,10	7,76
210	37,60			12,30	8,24
220	39,50			13,70	8,73
230	41,40			15,30	9,24
240	43,30				9,76
250	45,20				10,30
300	54,70				13,30
CORRECTIONS FOR TEMPERATURE AND PRESSURE
Values found in the above table may be corrected for temperature and pressure
by multiplying the values given by the appropriate correction factor found below:
Pressure [mmHg]
Temp. [C]	720	740	760	780
0	1,04	1,06	1,09	1,12		kV	cm
10	1	1,02	1,05	1,08		20	1,75
20	0,96	0,99	1,02	1,04		22,78	2	Interpolation
30	0,93	0,96	0,98	1,01		25	2,2
Rod Gap Breakdown Voltage Tables
More details on Rod Gap procedures and construction
The following table values valid for:
1.27 cm square rods
27 deg C
760 torr (1013 mBar)
15.5 torr water vapor pressure
(11 g/cc water content for impulses).
The voltages in the table reflect the breakdown of the rod gap +/- 8%
Rod Gap	Voltage	1/5 uSec	1/5 uSec	1/50 uSec	1/50 uSec
Spacing		Pos impulse	Neg impulse	Pos impulse	Neg impulse
cm	kV	kV	kV	kV	kV
2	26
4	47
6	62
8	72
10	81	101	111	90	97
15	102
20	124	179	208	160	178
25	147
30	172	256	301	226	262
40	225	348	392	279	339
50	278	431	475	334	407
60	332	513	557	397	470
70	382
80	435	657	701	511	585
90	488
100	537	820	855	629	703
Humidity Correction
H20 Vapor Press. (torr)	2,5	5	10	15	20	25	30
Correction Factor %	-16,5	-13,1	-6,5	-0,5	4,4	7,9	10,1
Data taken from tables and graphs in Naidu & Kamaraju, High Voltage Engineering.
Copyright 1998, Jim Lux / rodv.htm / 5 May 1998 /
Sphere Gap Breakdown Voltage Tables
The following two tables give the breakdown voltages for a standard
sphere gap of the specified sphere size and spacing. The standard
accuracy is ±3% for gaps less than half the sphere diameter and 5% for
gaps larger than that. The first table is for a gap with one terminal
grounded, typically a vertical gap. There are two columns for each sphere
size depending on whether it is a positive or negative impulse. The second
table is for a gap with both terminals at HV. The values from these tables
will need to be corrected for current atmospheric conditions, particularly
humidity.
Sphere gap with one terminal grounded
A -> AC, DC, either polarity and full negative standard impulse voltages (one sphere grounded)
B -> positive polarity standard impulse voltages and impulse voltages with long tails (either polarity)
	Sphere Diameter (cm)
Gap Spacing	5		10		15		25		50		100		150		200
cm	A	B	A	B	A	B	A	B	A	B	A	B	A	B	A	B
0,5	17,4	17,4	16,9	16,8	16,9	16,9
1	32	32	31,7	31,7	31,4	31,4	31,2	31,4
1,5	44,7	45,5	44,7	45,1	44,7	45,1	44,7	44,7
2	57,5	58	58	58	58	58	58	58
2,5			71,5	71,5	71,5	71,5	71,5	71,5	71,5	71,5
3			85	85	85	85	85	85	85	85
3,5			95,5	96	97	97	97	97	97	97
4			106	108	108	110	110	110	110	110
5			123	127	127	132	135	136	136	136
7,5					181	187	195	196	199	199
10							257	268	259	259	262	262	262	262	262	262
12,5							277	294	315	317
15							309	331	367	974	383	384	384	384	384	384
17,5							336	362	413	425
20									452	472	500	500	500	500	500	500
25									520	545	605	610
30									575	610	700	715	730	735	735	740
35									725	755	785	800
40											862	885	940	950	960	965
45											925	965
50											1000	1020	1110	1130	1160	1170
75											1210	1260	1420	1460	1510	1590
100															1870	1900
symmetrical sphere gaps
20deg C, 760 torr, either polarity impulse, AC, DC
spacings <0.5D Accuracy +/- 3% ; spacings >=0.5D accuracy +/- 5%
	Sphere Diameter (cm)
gap (cm)	5	10		15	25	50	100	150	200
0,5	17,5	16,9		16,5
1	32,2	31,6		31,3	31
1,5	46,1	45,8		45,5	45
2	58,3	59,3		59,2	59
2,5	69,4	72,4		72,9	73
3	79,3	84,9		85,8	86
4		107		111	113	112
5		128		134	138	138	137	137	137
8		177		194	207	214
10					248	263	266	267	267
12					286	309
14					320	353
16					352	394
18						452
20						495	504	511	511
25						558	613	628	632
30							744	741	746
35							812	848	860
40							902	950	972
50							1070	1140	1180
60							1210	1320	1380
70								1490	1560
80								1640	1730
90									1900
100									2050
Copyright 1998, Jim Lux / spherev.htm / 8 March 1998
Spark Gap Correction Factors
Effects of atmospheric conditions
V = k* V0
where k is a function of the air density d
d = p/760 * (293 / (273+T)) where p in Torr and T in degrees C
d	0,7	0,75	0,8	0,85	0,9	0,95	1	1,05	1,1	1,15
k	0,72	0,77	0,82	0,86	0,91	0,95	1	1,05	1,09	1,12
The voltage increases with humidity varying some 2-3% over a normal atmospheric range of 8-15 g/m3.
Irradiation
If the gap is illuminated with UV or X-rays, it is more easily ionized. The effect is more pronounced with smaller gaps, a 20% reduction
for short gaps of a few cm when irradiated. For this reason, UV irradiation is specified for accurate breakdown voltage
Copyright 1998, Jim Lux / spherev.htm / 8 March 1998