The negative readings given in the pressure gauge in the SCBA is to show that if the gauge is operational. Normally there will be a support near the zero reading for the needle to rest upon, however since its an important safety equipment, showing negative scale readings is an additional safety feature so that we made aware that the gauge is operational and the spring inside is not broken. If there was no negative readings scale then it can be mislead that the cylinder is empty. Hence this is to reduce all possible human errors during an emergency situation.