a vehicle emissions test shed is able to accommodate internal pressure changes due to temperature changes or other causes during a test cycle , which would otherwise cause a measurement threatening air exchange . the pressure is kept constant , without changing the shed volume and without withdrawing air and fuel vapor from within the shed . airtight compensation bladders within the shed are inflated or exhausted in response to a sensed incipient pressure drop or rise . the pressure is automatically kept substantially constant , preventing leaks in or out of the shed .