Transmitter 10-11 Vacuum Pressure Pressure Switches Hydraulic Pressure Figure 10-17.  A typical analog fuel pressure gauge.  Figure 10-18.  A hydraulic pressure transmitter senses and converts  pressure into an electrical output for indication by the cockpit gauge  or for use by a computer that analyzes and displays the pressure in  the cockpit when requested or required.  which the gyros spin needs to be within a certain range for  correct operation. This speed is directly related to the suction  pressure that is developed in the system. The suction gauge  is extremely important in aircraft relying solely on vacuum- operated gyroscopic flight instruments.  due to the potential for fire should a leak develop. Therefore,  the preferred arrangement is to have whichever sensing  mechanism that is used be part of a transmitter device that  uses electricity to send a signal to the indicator in the cockpit.  Sometimes, indications monitoring the fuel flow rate are  used instead of fuel pressure gauges. Fuel flow indications  are discussed in the fuel system chapter of this handbook. Vacuum is a differential pressure indication, meaning the  pressure to be measured is compared to atmospheric pressure  through the use of a sealed diaphragm or capsule. The gauge  is calibrated in inches of mercury. It shows how much  less pressure exists in the system than in the atmosphere.  Figure 10-19  shows a suction gauge calibrated in inches  of mercury. Numerous other pressure monitoring gauges are used on  complex aircraft to indicate the condition of various support  systems not found on simple light aircraft. Hydraulic systems  are commonly used to raise and lower landing gear, operate  flight controls, apply brakes, and more. Sufficient pressure in  the hydraulic system developed by the hydraulic pump(s) is  required for normal operation of hydraulic devices. Hydraulic  pressure gauges are often located in the cockpit and at or  near the hydraulic system servicing point on the airframe.  Remotely located indicators used by maintenance personnel  are almost always direct reading Bourdon tube type gauges.  Cockpit gauges usually have system pressure transmitted  from sensors or computers electrically for indication.  Figure 10-18  shows a hydraulic pressure transmitter in place  in a high-pressure aircraft hydraulic system.  In aviation, it is often sufficient to simply monitor whether  the pressure developed by a certain operating system is  too high or too low, so that an action can take place should  one of these conditions occur. This is often accomplished  through the use of a pressure switch. A pressure switch is  a simple device usually made to open or close an electric  circuit when a certain pressure is reached in a system. It can  be manufactured so that the electric circuit is normally open  and can then close when a certain pressure is sensed, or the  circuit can be closed and then opened when the activation  pressure is reached. [ Figure 10-20] Gyro pressure gauge, vacuum gauge, or suction gauge are  all terms for the same gauge used to monitor the vacuum  developed in the system that actuates the air driven  gyroscopic flight instruments. Air is pulled through the  instruments, causing the gyroscopes to spin. The speed at  Pressure switches contain a diaphragm to which the pressure  being sensed is applied on one side. The opposite side of  the diaphragm is connected to a mechanical switching  mechanism for an electric circuit. Small fluctuations or  a buildup of pressure against the diaphragm move the  diaphragm, but not enough to throw the switch. Only when 