1906

When cold, vitreous enamel has excellent insulation resistance.

In common with all insulants the specific resistance of the enamel decreases with increased temperature; therefore, if operated at any temperature approaching the maximum, the resistor cannot be classed as an insulated type and should not be used in contact with any conducting materials.

The recommended dissipations for each of the resistor hot spot temperatures applies to resistors mounted horizontally. If the bore is completely blocked a 15% derating is recommended. However, wherever possible, resistors should be mounted vertically with unobstructed bore.

This makes best use of the chimney effect of the heated tube and will encourage a cooling stream of air through the bore.

Allowances must be made, when tubular resistors are mounted in banks, for the effects produced by radiation between tubes.

Appreciable reduction of hot spot temperature can be achieved by arranging that resistors are subjected to some measure of forced draught. In general, it is most efficient to extract air from the resistor enclosure and arrange that an air inlet is adjacent to the bottom of the tubes.

If soft soldered connections are used the resistors should be derated where applicable to limit the hot spot temperature to 300°C.

Vitreous enamelled wirewound resistors are capable of withstanding a higher dissipation size for size, than any other protected type; the 1906  is attributable to the higher operating temperature which the wire and enamel can withstand. Vitreous enamel provides exceptionally good protection to the wire element and is essentially impervious to moisture.

The resistors 1906 can safely be used in harsh environmental conditions.

The Welwyn range is based on thirteen sizes of tube, each of which has a recommended maximum dissipation which limits operating surface temperature to a maximum of 375°C. The stability and high reliability of Welwyn tubular vitreous resistors is a direct result of the best quality materials being used in their construction.

High purity ceramic tubes 1906 have been matched with nickel chromium resistance wires and specially formulated enamel to ensure that the resistors can withstand repeated heat cycling without damage.

Connections between the resistance element and end terminations are welded; other connections are brazed.
Mounting devices are available which permit resistors to be mounted by both ends or by one end. Single ended mounting is recommended for applications which do not subject the resistor to shock or vibration.

Ferrules and the F type mounting plug are anchored into the tubes with high temperature adhesive.