IMP+ INSTRUCTION MANUAL 
 
42 
P860 Sound Velocity 
This option allows for the velocity of sound to be changed according to the 
atmosphere the transducer is operating in. By default, the velocity is set for 
342.72m/sec which is the speed of sound travelling in air at a temperature 
of 20
o
C. 
The table below gives details of the velocity of sound in various gaseous 
atmospheres.  In all cases the velocity indicated is that in a 100% gaseous 
atmosphere at 0
o
C. In atmospheres less than 100% it may be necessary to 
check the level indicated at near empty and near full and compare with the 
actual level, several times, then adjust the Sound Velocity accordingly to 
obtain an accurately displayed reading. 
P645 Vapour Temperature Compensation 
The sound velocity in air increases or decreases at a uniform rate of 
60cm/sec. per 
o
C, however in atmospheres other than air it will change at a 
different rate. 
This option allows the rate of change in cm/sec. per 
o
C to be set according 
to the atmosphere and temperature present. The level indicated, should be 
compared with the actual level, several times, then Vapour Temperature 
Compensation adjusted accordingly, to obtain an accurately displayed 
reading. Default = 60 cm/sec. per 
o
C