SO/H Detatchable Low Pressure Sodium

Updated
16-VI-2013
Developed by Philips in early 1932, the SO/H design was the first commercial sodium vapour lamp to become popular and enter mass production. The discharge tube is U-shaped and fabricated from 2-ply sodium-resistant glass, and to allow this to attain a wall temperature of 260C under which conditions the discharge is most efficient, it is operated inside a dewar vacuum jacket to provide thermal insulation. At end of life it was customary to retain the dewar and only the inner tube need be replaced.

Illustrated here is a selection of SO/H lamps from a variety of different manufacturers, exhibiting some of the different construction techniques employed. It is interesting to note the extra efforts made by Mazda - for instance the small nickel cap to protect the delicate vacuum tip-off, a barium getter to preserve the vacuum in the dewar, and a variety of methods to keep moisture away from the discharge tube. Not so noticeable is that the earlier lamps also have hard borosilicate glass discharge tubes which are much more resistant to cracking.

Another point worth noting here is that the argon clean-up rate of old glasses can be very high if the lamp has not been lit for more than twenty years or so. Thus when an old lamp is first re-lit, the glass can absorb the argon content very rapidly and if the lamp is then turned off, it is virtually impossible to ever strike that lamp up again. However if it is left burning for some five hours or so after its first start, this will be sufficient to re-release much of the argon and it will then be safe to turn off the lamp and know that it should strike again.

Osram

140W

SO/H lamp of first production style
1949

Philips

45W

SO/H lamp of first production style
1954

Mazda

140W

SO/H lamp with hard glass discharge tube
1953

Mazda

60W

SO/H lamp with hard glass discharge tube
1950

Philips

85W

SO/H lamp with bamboo style discharge tube
1960

Mazda

85W

SO/H discharge tube of hard glass design
1957

Mazda

60W

SO/H discharge tube of soft glass design
1965

Osram

45W

SO/H discharge tube of last production style
1965

MELZ

140W

DNAO-140 Soviet Style low pressure sodium
1966