CHAPTER  11 INSTRUMENTS As   a   watch   stander,   you   observe   or   monitor operating equipment and take the necessary steps to detect   malfunctions   and   prevent   damage   to   the equipment.  The  word  monitor  means  to  observe,  record, or  detect  an  operation  or  condition  using  instruments. Measurement, in a very real sense, is the language of engineers.  The  shipboard  engineering  plant  has  many instruments  that  indicate  existing  conditions  within  a piece  of  machinery  or  a  system.  By  reading  and interpreting the instruments, you can determine whether the machinery or the system is operating within the prescribed  range. Recorded instrument readings are used to make sure the plant is operating properly. They are also used to determine the operating efficiency of the plant. The instruments  provide  information  for  hourly,  daily,  and weekly entries for station operating records and reports. The data entered in the records and reports must be accurate since they are used to determine the condition of  the  plant  over  a  period  of  time.  Remember,  for accurate data to be entered on the records and reports of an engineering plant, you must read the instruments carefully. In  this  chapter,  we  describe  various  types  of indicating instruments that you, as a Fireman, come in contact  with  while  working  and  standing  watch  on  an engineering  plant.  Upon  completion  of  this  chapter,  you should  be  able  to  describe  the  various  types  of temperature  and  pressure  measuring  instruments, indicators, alarms, and the functions for which they are used. Engineering  measuring  instruments  are  typically classified  into  the  following  groups: l l l l l l Pressure  gauges Temperature  detectors Temperature  measuring  devices Electrical  indicating  instruments Liquid-level   indicators Revolution counters and indicators . Salinity indicators . Torque wrenches We  will  discuss  each  of  these  categories  in  the following   sections. PRESSURE GAUGES The types of pressure gauges used in an engineering plant  include  Bourdon-tube  gauges,  bellows,  diaphragm gauges,  and  manometers.  Bourdon-tube  gauges  are generally used for measuring pressures above and below atmospheric  pressure.  Bellows  and  diaphragm  gauges and   manometers   are   generally   used   to   measure pressures  below  15  pounds-per-square-inch  gauge (psig). They are also used for  low vacuum  pressure.  Low vacuum   pressure   is   slightly   less   than   14.7 pounds-per-square-inch  absolute  (psia).  Often,  pressure measuring  instruments  have  scales  calibrated  in  inches of water (in. H2O) to allow greater accuracy. NOTE: On  dial  pressure  gauges,  set  the  adjustable red hand (if installed) at or slightly above the maximum normal  operating  pressure,  or  at  or  slightly  below  the minimum  normal  operating  pressure,  (Refer  to  Naval Ships’  Technical  Manual,  chapter  504,  for  specific instructions.) BOURDON-TUBE  GAUGES The device usually used to indicate temperature changes by its response to volume changes or to pressure changes is called a Bourdon tube. A Bourdon tube is a C-shaped, curved or twisted tube that is open at one end and sealed at the other (fig. 11-1). The open end of the tube is fixed in position, and the scaled end is free to move. The tube is more or less elliptical in cross section; it does not form a true circle. The tube becomes more circular when there is an increase in the volume or the internal pressure of the contained fluid. The spring action of the tube metal opposes this action and tends to coil the tube. Since the open end of the Bourdon tube is rigidly fastened, the sealed end moves as the pressure of the  contained  fluid  changes. 11-1