The ignition of flammable vapors via electrostatic discharge (ESD) is an attention-getting and also a highly instructive component of any electrostatics demonstration. An ignition chamber for use in lecture demonstrations can be constructed readily from a short, vertically mounted length of PlexiglasTM or LexanTM. The chamber, open at the top, is fitted with a pair of sparking electrodes mounted on metal rods that pass through holes drilled on opposite sides of the tube as shown. It is a good idea to place a watch glass in the bottom of the cylinder to hold the liquid as it evaporates. Interchangeable electrodes, including spherical balls (1/2" diameter) and pointed electrodes which can be attached to the rods that run into the chamber, are very convenient accessories.
The image above shows the ignition of acetone vapor in a 2 liter chamber. To obtain this photograph, ASA 400 film was used in a semi-darkened room. The aperture was f/16 and the shutter was held open for about 1 second. Because of the "time lapse"nature of the shot, the fireball appears to be more diffuse than it really is.
For a safe, convenient, and entirely adequate lecture demonstration, a smaller more conveniently sized unit of volume ~200 ml is quite adequate. Refer to the photograph below which shows such a chamber. Chambers significantly larger than this size are more difficult to transport and can be more dangerous to both the lecturer and his or her audience.
Reliably achieving ignitions in the lecture environment requires adding the
correct amount of liquid to the chamber each time so that the mixture is within the
flammable limits. The stoichiometric amount of liquid acetone
for a 200 ml chamber is ~0.03 ml. A comprehensive listing of flammable limits data for
liquid vapors in air has been collected by L. G. Britton of Union Carbide [Britton, 1997].
The chamber is closed up using a piece of copier paper secured by a tight-fitting
rubber band. After the liquid has had a minute or two to evaporate fully, one electrode
is grounded and a charged conductor such as the electrophorus is brought into contact with the contacter
ball connected to the other electrode. A spark jumps before true contact is made, and
simultaneously a spark jumps across the gap inside the chamber.
A number of attempts are sometimes
required to achieve an ignition. When difficulty is encountered, it is often due to poor
vapor/air mixing conditions in the chamber. A miniature, commutator-less muffin fan,
securely mounted inside the chamber near the bottom, will guarantee good mixing. The igniter tube in the
photograph above is equipped with such a mixing fan and the battery to drive it. These
little fans seem not to have sustained any damage even after hundreds of ignitions over the years. NOTE: ignition is unlikely
unless the mix is reasonably close to the correct stoichiometric limit. In addition, the
electrostatic discharge itself must have sufficient energy to reach the minimum ignition condition. Ignition of acetone vapor is usually
assured, as the capacitive discharge energy available from a fully charged electrophorus
is at least an order of magnitude higher than the minimum ignition energy of most HC
vapors.
To achieve the goal of an effective yet safe demonstration with this apparatus, the chamber should be well-constructed of PlexiglasTM or LexanTM, not glass, and its volume should not exceed approximately 200 cm3 (0.2 liters). Safety glasses must always be worn by the presenter and by anyone else involved in the demonstration because the rubber band and the paper cover are blown off the chamber with sufficient force to cause injury to unprotected eyes. Also, be sure to move any containers of flammable liquids well away from the chamber when using this demonstration. Never use gasoline in this demonstration.
The steadily increasing use of plastic components in automobiles combined with clothing made from synthetic fabrics may be combining to create greater risks of ignitions at gasoline service stations. The accidents reported, some quite serious, are usually spark-caused fires rather than explosions. To learn the views of one trade organization about this hazard, check out the Petroleum Equipment Institute link. This site contains useful safety suggestions plus very interesting data on the nature of accidents.
Substituting various types of sparking electrodes, such as balls of varied sizes, pointed electrodes, and wires will demonstrate that electrostatic discharges are influenced by electrode shape, and that the nature of an ESD strong influences the probability of ignition of flammable mixtures. For the lower voltage and higher capacitance of the piezoelectric sparker, wire electrodes seem more effective in producing good sparks for ignition. For the higher voltage and smaller capacitance of the electrophorus, smooth electrodes work better. This is so because higher voltages induce corona from sharp points which tends to dissipate any charge before an energetic spark can occur.
Interesting experiments have
been reported that show how the vapor ignition requirements depend on the nature of the
object being discharged. The apparent MIE of acetone is 2 to 3 times greater when
ignited by a capacitive discharge from the human body than from a standard capacitor
[Johnson, 1981]. This result is presumed to be due to the distributed series resistance
of the human body which slows the discharge transient. By comparison, the series
resistance of capacitors (and Leyden jars) is quite small.
Britton, L.G., unpublished collection of MIE and conductivity data for insulating materials, 1997.
Johnson, R.W., Loss Prevention (AIChE), vol. 14,
1981, pp. 29-34.