Aquadag
Encyclopedia
Aquadag is a trade name for a graphite
based coating commonly found in cathode ray tube
s. It is manufactured by Acheson Industries, a subsidiary of ICI
. The name is a shortened form of "Aqueous Deflocculated Acheson Graphite". Other related products include Oildag, Electrodag and Molydag. The product names are often printed with DAG in upper case (e.g. AquaDAG).
The coating can be on the inside of the tube, where it helps to maintain a constant electrical field near the screen. Tubes intended for television
use normally have an aluminised interior which acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag is also painted on part of the outside of many tubes. The glass, tube, internal aluminium coating and the outer Aquadag then form a capacitor
used to filter the high voltage EHT supply.
Normally there are sprung contacts that touch the Aquadag. If this contact is missing or loose, arcing is often observed at times.
When removing a CRT from a TV set, it is wise to discharge this built-in capacitance to avoid a shock hazard when handling the tube.
Aquadag is a water based colloidal graphite suspension, and may be painted on to items to avoid build up of static, or provide electrical screening.
Aside from its use in the production of CRTs, Aquadag is used in many types of high voltage lab apparatus where the creation of a highly predictable electrostatic field is required. The surfaces of some metals (most notably aluminum) can develop nonconductive oxide layers which tend to disrupt the electrostatic field produced around the surface of the metal when used as an electrode. Aquadag is not subject to such effects, and provides a completely uniform equipotential surface for electrostatics.
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
based coating commonly found in cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s. It is manufactured by Acheson Industries, a subsidiary of ICI
Imperial Chemical Industries
Imperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...
. The name is a shortened form of "Aqueous Deflocculated Acheson Graphite". Other related products include Oildag, Electrodag and Molydag. The product names are often printed with DAG in upper case (e.g. AquaDAG).
The coating can be on the inside of the tube, where it helps to maintain a constant electrical field near the screen. Tubes intended for television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
use normally have an aluminised interior which acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag is also painted on part of the outside of many tubes. The glass, tube, internal aluminium coating and the outer Aquadag then form a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
used to filter the high voltage EHT supply.
Normally there are sprung contacts that touch the Aquadag. If this contact is missing or loose, arcing is often observed at times.
When removing a CRT from a TV set, it is wise to discharge this built-in capacitance to avoid a shock hazard when handling the tube.
Aquadag is a water based colloidal graphite suspension, and may be painted on to items to avoid build up of static, or provide electrical screening.
Aside from its use in the production of CRTs, Aquadag is used in many types of high voltage lab apparatus where the creation of a highly predictable electrostatic field is required. The surfaces of some metals (most notably aluminum) can develop nonconductive oxide layers which tend to disrupt the electrostatic field produced around the surface of the metal when used as an electrode. Aquadag is not subject to such effects, and provides a completely uniform equipotential surface for electrostatics.