A valve comprises an assembly of elastomeric elements confined in a cylindrical space, with one end of the assembly engageable with a valve seat at one end of the cylindrical space. The output compensation signal is used to set an output compensation voltage that sets the duty cycle of the waveform. The tips define a pair of outwardly facing indentations that adapt the tips to engaging the locking ring in order to spread the locking ring by operation of the person squeezing the handles toward each other.