Oxyhydrogen is a mixture of diatomic hydrogen and oxygen gases, normally assumed to be in a 2:1 atomic ratio, the same proportion as water. When ignited, this mixture combusts to water, making 142.35 kJ (34,116 gram calories) of heat for each gram of hydrogen burned: that is 286.97 kJ/mol of enthalpy.
The flame is hottest in the burning of a stoichiometric mixture. It is less hot when there is an excess of oxygen or hydrogen, or when an inert gas such as nitrogen is mixed in, because there the same amount of heat is added to a larger amount of matter.
Oxyhydrogen is usually made from water electrolysis, which also ensures a stoichiometric proportion.
At normal temperature and pressure, oxyhydrogen can burn when it is between about 4% and 94% hydrogen by volume. Although, if the gas velocity out of a torch tip is too great, there will be ignition difficulty as the flame will tend to blow itself out.
A water torch is an oxyhydrogen torch which is fed by oxygen and hydrogen generated on demand by water electrolysis, avoiding the need for supplied oxygen and hydrogen. . It is used in small-scale applications such as making jewelry and electronics, and other oxyhydrogen torch applications.
An oxyhydrogen flame is used in the glass industry for "fire polishing", i.e. slightly melting the surface of glass to remove scratches and dullness. Another application is in forming preform blanks via chemical vapor deposition in making fiber optics. Oxyhydrogen torches are also used in the ceramics and sensor industry, where the temperature and velocity of the flame is beneficial.
Oxyhydrogen torches must be designed to mitigate flashback by making the electrolytic chamber strong enough; an intermediary water bubbler makes potential electrolyzer damage from flashback negligible, and also captures any remaining electrolyte (sodium or potassium hydroxide) in the output gas. A flashback arrestor is useless due to the flame velocity.
Some water torch models mix the two gases immediately after production (instead of at the torch tip) making the gas mixture more accurate; this electrolyzer design is called common ducting. Common ducted electrolyzers are typically series cell parallel plate design, but can also be built using cylindrical cells. The main criteria for common ducting is a single gas output hose. Oxyhydrogen gas produced this way is sometimes called Brown's gas (see below). Oxyhydrogen gas produced in an independently ducted electrolyzer is not considered Brown's Gas. Independently ducted electrolyzers have substantially separated anodes and cathodes, are typically rod type design, and have separate hydrogen and oxygen gas output hoses.
Many forms of oxyhydrogen lamps have been invented, but the explosiveness of the gas mixture made them all more or less dangerous at that time.
It was much used in platinum works, as platinum could be melted only in an oxyhydrogen flame, or an electric furnace (which is now used instead).
For making limelight, as in optical ("magic") lanterns; nowadays electric light is used instead.
The factual accuracy of this section is disputed. Please see the relevant discussion on the talk page
Brown's gas, as presented by Yull Brown and subsequent investigators is a mixture of science and pseudoscience. Hoaxes are also claimed to be associated with Brown's gas due to a sourceless distinction from the water fuel cell; although, the lack of a defined relationship establishes a reasonable distinction in itself. Overall, considering the original claims of Yull Brown and those of subsequent investigators, Brown's gas is a sometimes overly hyped technology conforming with standard electrolysis parameters. Many of the claims about the gas are well-understood properties of oxyhydrogen, including Atomic welding: "An electric arc is passed through the mixture of gas before burning, so that the gas molecules break into atomic oxygen and hydrogen, using the electrical energy to produce a hotter flame when the atoms recombine".
A Brown's Gas electrolyzer comprises "the cells as a single unit in which a number of electrodes, effectively in series, are arranged adjacent each other in a common electrolytic chamber, the chamber being provided with a gas collection space and an outlet for connection to, for example, gas burner means. Furthermore, only the end electrodes need be connected to an external source of electrical energy and the arrangement as a whole can be made extremely efficient and compact. Additionally the need for a transformer for most applications can be eliminated by such an arrangement so that the apparatus can be designed to be electrically connected directly to a main electrical supply, through a bridge rectifier if desired. By eliminating the need for a transformer, the gas generating equipment as a whole can be made surprisingly compact, to be well suited for small domestic requirements as well as heavy industrial requirements".
Electrolytic cells arranged in series experience voltage division. The successive addition of subsequent cells will reduce the voltage across each cell accordingly. Conversely the removal of subsequent cells increases the voltage across each individual cell. The current passing through the entire cell arrangement can be limited with the properties of a capacitor. The current through a capacitor is proportional to the change in voltage with respect to time, multiplied by the specific capacitance value. Therefore the current passing through the capacitor can be addressed by modifying the frequency of the voltage source. Having less total power delivered, to each individual cell, reduces the reaction temperature. Too many cells and the voltage across each cell will not be enough to sustain reasonable current flow. Too few cells and the voltage across each individual cell will be too high, and the water can reach boiling temperature.
In a parallel cell electrolyzer current division occurs. Because electrolysis is a current driven reaction the addition of subsequent cells reduces the quantity of current delivered to each individual cell, thus proportionally reducing the degree of the electrolytic reaction.
George Wiseman believes "that anything above 2.2 volts won't make much Brown's Gas"  This is referring to the voltage across each individual cell, and not the entire cell apparatus. Keeping the voltage at such a relatively low value will mitigate waste heat associated with an unnecessarily high electrolytic reaction temperature. Minimizing waste heat will increase overall efficiency. Overall "as a good rule of thumb. If your electrolyzer stays fairly cool during heavy power, you are likely making Brown's Gas". Since Brown's Gas is produced by minimizing the voltage across each individual cell, it production is more efficient than other oxyhydrogen electrolyzer designs.
Torch use. Yull Brown claimed "a method of oxy/hydrogen welding, brazing or the like".
Fuel enhancement; hydrogen based fuels effect the lean combustion capabilities of internal combustion engines. Fuel Enhancement systems are designed "to feed the hydrogen and oxygen gases directly to an internal combustion engine without intermediate storage". For Diesel applications; "When the hydrogen enriched air is compressed, the diesel fuel is introduced with a resulting improvement in fuel efficiency and maximized combustion of the fuel". For internal combustion engines in general; a fuel enhancement system, with proper air/fuel ratio modification, can produce "hydrogen and oxygen gases that provides up to 1,000% percent increase in fuel mileage". Overall George Wiseman believes that "we can alread run normal internal combustion engines on Brown's Gas assist, reducing the actual cost of operation while making the engine last longer. (See also: Water Injection)
Heating. Sang Nam Kim claims "an energy generating apparatus using the cyclic combustion of Brown gas wherein a heat generating unit is heated to a temperature of 1,000.degree. C".
"A Brown gas incinerator can reduce radioactive rays to 1/3–1/120 when it burns the trash from an atomic power generator".
Brown's gas is "a mixture of hydrogen and oxygen generated in substantially stoichiometric proportions in an electrolytic cell by electrolytic dissociation of water". Because Brown's Gas is produced via electrolysis it's production conforms to the 1st and 2nd laws of electrolysis.
A Brown's gas electrolyzer is designed with "an outlet" emitting hydrogen and oxygen "in substantially stoichiometric proportions". A single gas output is typically referred to as common ducted, and a stoichiometric proportion of hydrogen and oxygen is typically referred to as oxyhydrogen.
Yull Brown claimed that Brown's Gas eliminates "many of the disadvantages associated with conventional gas welding practice", "particularly for users working remote from a supply depot and for whom there might be an appreciable delay between the placing of an order for a delivery of gas", and "the actual delivery". This is specifically with regard to "cylinders (or "bottles") of gas, usually oxygen and acetylene". 
"Hydrogen-oxygen welding has the advantage that it does not pollute the atmosphere as does oxy-acetylene welding". 
Varying flame temperature: this effect is explained by inaccurate infrared thermometry, and measurement of the target material rather than the flame itself.
"The implosion characteristics of Brown gas cannot be explained by modern physics, whilst the crystallizing π-bonding of atoms can interpret it clearly".
George Wiseman has "experimentally proven many of Yull Brown's statements to be wrong!".
George Wiseman believes "that Brown's Gas is a viable option to apply to a self-sufficient home, but not in the ways that are normally outlined". This claim is most likely referring to fuel substitution rather than fuel enhancement. Fuel substitution using Brown's Gas will always be less efficient than consuming the electricity directly. Therefore fuel substitution using Brown's Gas is conclusively not economically viable.
Noah Seidman claims "the components necessary for separation of hydrogen and oxygen gasses can be eliminated from the design of an electrolyzer for certain applications". Applications that require pure hydrogen must still contain the components necessary to separate the gases.
HHO gas or Klein gas is an oxyhydrogen mixture made by water electrolysis and has been trademarked Aquygen by the firm Hydrogen Technology Applications.
Dennis Klein claims his patented electrolyzer "differs from" "U.S. Pat. No. 4,081,656, titled Brown Mar. 28, 1978 Arc-assisted Oxy/hydrogen Welding, invented by Yull Brown". The main difference being that his output gas "is directed to a torch which has a pair of tungsten electrodes in the out put path of the gas". Asserted is a difference in the choice of torch, rather than an explicit difference in the gas itself. The gas coming from a common ducted electrolyzer will be the same independent of secondary processing. Secondary processing is not in question, therefore HHO is conclusive a trademark given to Brown's Gas being passed through an electric arc. Substantiating this conclusion is Yull Brown's reference to an electric arc in his patent literature ("An electric arc is passed through the mixture of gas before burning, so that the gas molecules break into atomic oxygen and hydrogen, using the electrical energy to produce a hotter flame when the atoms recombine".)
Ruggero Santilli holds a trademark for the term "PlasmaArcFLow". "The arc decomposes the liquid molecules into their atomic constituents, and forms a plasma in the immediate vicinity of the electrodes at about 10,000°F".
Unverified claims  have been made about the properties of "HHO gas", claiming as basis an unproven new state of matter called magnegases and an unproven theory about magnecules.
^ O'Connor, Ken. "Hydrogen", NASA Glenn Research Center Glenn Safety Manual.
^ ab William Augustus Tilden. Chemical Discovery and Invention in the Twentieth Century. Adamant Media Corporation, 80. ISBN 0543916464.