THIS INVENTION RELATES TO EQUIPMENT FOR INDICATING A PREDETERMINED PRESSURE DIFFERENTIAL BETWEEN GASEOUS FLUIDS AT LOW AND HIGHER PRESSURES. A HOLLOW SLEEVE OF CONICAL FORM HAS AN INNER TAPERED WALL. THE LARGE END OF THE HOLLOW SLEEVE IS IN COMMUNICATION WITH THE GASEOUS FLUID AT THE HIGHER PRESSURE AND ITS SMALL END IS IN COMMUNICATION WITH THE GASESOUS FLUID AT THE LOW PRESSURE. A MOVABLE MEMBER OF CONICAL FORM, WHICH HAS A TAPERED OUTER WALL, IS DISPOSED WITHIN THE HOLLOW SLEEVE WITH THE SMALL END OF THE MEMBER AT THE SMALL END OF THE SLEEVE. THE MEMBER, WHICH IS SPRING BIASED FROM THE SMALL END OF THE SLEEVE, HAS A SIGNALLING SYSTEM PART MOUNTED THEREON. THE PART AND MEMBER ARE IN ONE POSITION WITH RESPECT TO THE SLEEVE WHEN THE PRESSURE DIFFERENTIAL BETWEEN THE GASEOUS FLUIDS IS LESS THAN A PREDETERMINED VALUE AND IN A SECOND POSITION WITH RESPECT TO THE SLEEVE WHEN THE PRESSURE DIFFERENTIAL BETWEEN THE GASEOUS FLUIDS IS AT THE PREDETERMINED VALUE. THE RELATIVE POSITION OF THE PART AND MEMBER WITH RESPECT TO THE SLEEVE IS DEPENDENT UPON THE VELOCITY OF AIR FLOWING FROM THE LARGE TO THE SMALL END OF THE SLEEVE THROUGH A GAP BETWEEN THE SLEEVE AND MEMBER. THE PART IN ITS SECOND POSITION SIGNALS THAT THE PRESSURE DIFFERENTIAL BETWEEN THE GASEOUS FLUIDS IS AT THE PREDETERMINED VALUE.