Detail A-B thermal lockout Hot condition Cold condition A B Indicator button Thermal lockout Magnet Figure 12-18.  Filter bypass valve. Pumps Hand Pumps  12-15 Differential pressure indicators are a component part of the  filter assembly in which they are installed and are normally  tested and overhauled as part of the complete assembly.  With some model filter assemblies, however, it is possible  to replace the indicator itself without removal of the filter  assembly if it is suspected of being inoperative or out of  Some button indicators have a thermal lockout device  incorporated in their design that prevents operation of the  indicator below a certain temperature. The lockout prevents  the higher differential pressure generated at cold temperatures  by high fluid viscosity from causing a false indication of a  loaded filter element. Double-action hand pumps produce fluid flow and pressure  on each stroke of the handle.  [Figure 12-19]  The double- action hand pump consists essentially of a housing that has  a cylinder bore and two ports, a piston, two spring-loaded  check valves, and an operating handle. An O-ring on the  piston seals against leakage between the two chambers of  the piston cylinder bore. An O-ring in a groove in the end of  the pump housing seals against leakage between the piston  rod and housing. Several types of hand pumps are used: single action, double  action, and rotary. A single action hand pump draws fluid  into the pump on one stroke and pumps that fluid out on the  next stroke. It is rarely used in aircraft due to this inefficiency. Differential pressure indicating devices have many  configurations, including electrical switches, continuous- reading visual indicators (gauges), and visual indicators  with memory. Visual indicators with memory usually take  the form of magnetic or mechanically latched buttons or  pins that extend when the differential pressure exceeds that  allowed for a serviceable element.  [Figure 12-18, top]  When  this increased pressure reaches a specific value, inlet pressure  forces the spring-loaded magnetic piston downward, breaking  the magnetic attachment between the indicator button and  the magnetic piston. This allows the red indicator to pop out,  signifying that the element must be cleaned. The button or  pin, once extended, remains in that position until manually  reset and provides a permanent (until reset) warning of a  loaded element. This feature is particularly useful where it  is impossible for an operator to continuously monitor the  visual indicator, such as in a remote location on the aircraft.  The hydraulic hand pump is used in some older aircraft for the  operation of hydraulic subsystems and in a few newer aircraft  systems as a backup unit. Hand pumps are generally installed  for testing purposes, as well as for use in emergencies. Hand  pumps are also installed to service the reservoirs from a  single refilling station. The single refilling station reduces  the chances for the introduction of fluid contamination. All aircraft hydraulic systems have one or more power-driven  pumps and may have a hand pump as an additional unit when  the engine-driven pump is inoperative. Power-driven pumps  are the primary source of energy and may be either engine  driven, electric motor driven, or air driven. As a general rule,  electrical motor pumps are installed for use in emergencies  or during ground operations. Some aircraft can deploy a ram  air turbine (RAT) to generate hydraulic power. calibration. It is important that the external surfaces of button- type indicators be kept free of dirt or paint to ensure free  movement of the button. Indications of excessive differential  pressure, regardless of the type of indicator employed,  should never be disregarded. All such indications must be  verified and action taken, as required, to replace the loaded  filter element. Failure to replace a loaded element can result  in system starvation, filter element collapse, or the loss of  filtration where bypass assemblies are used. Verification of  loaded filter indications is particularly important with button- type indicators as they may have been falsely triggered by  mechanical shock, vibration, or cold start of the system.  Verification is usually obtained by manually resetting the  indicator and operating the system to create a maximum flow  demand ensuring that the fluid is at near normal operating  temperatures.