if a spillage occurs sufficient to cover the dome 39 the refractive index of the dome changes allowing light from the led to escape this reduces the light received from the phototransistor and produces a signal indicative of the presence of liquid a liquid level sensor operating on the principal described above is an lle series liquid level sensor available from honeywell international inc motherwell u k www honeywell com the sensor housing 40 is preferably formed of material resistant to attack by the chemicals being dispensed and as shown in fig1 is mounted on or immediately adjacent the floor of the deck the dome 39 faces downwardly towards the floor 14 of the containment deck so as to generate an alarm signal before the spill becomes a major catastrophe the signal indicative of the presence of liquid may be communicated by conductors 42 to a separate display on the weight indicator 36 or may be an audible or visual alarm system or both and may also include a display of the amount of the spill alternative placements of the sensor 38 considered to be within the scope of the invention include placement on the inner surface of wall 16 of the deck 10 where use takes place in locations where the humidity might cause moisture to condense on the dome sufficient to cause a signal indicative of a spill the sensor could be placed with the dome 39 facing away from the support surface in summary the invention comprises a low profile.