In  order  to  determine  the  accuracy  of  its  indications^
however,  it  should  be  tested  with  a  column  of  mercury.
PRACTICAL  ENGINEERING


53


Ques.  What  precaution  is  taken  to  prevent
the  steam  taking  the  temper  out  of  the  discs
or  tubes  of  steam  gauges?

Ans.  They  are  put  on  with  a  turn  or  two  of  pipe
between  the  boiler  and  the  disc ;  the  bend  of  the  pipe
gradually  filling  with  condensed  steam,  which  pre-
vents the  live  steam  touching  the  elastic  discs
or  tubes.
Figs.  36  to  38. — Various  forms  of  connection  for  steam  gauge.  The  pocket
formed  by  the  connection  becomes  filled  with  water  of  condensation  which
protects  the  spring  from  the  heat  of  the  steam.
Ques.  What  kind  of  pressure  does  a  steam
gauge  indicate?

Ans.  It  shows,  what  is  called  the  gauge  pressure,
as  distinguished  from  absolute  pressure;  that  is,
pressure  measured  above  the  atmospheric  pressure,
instead  of  above  a  perfect  vacuum,  as  in  the  case  of
absolute  pressure.
54  AUDELS  ANSWERS  ON

Ques.  To  what  should  the  hand  of  the
steam  gauge  point  when  there  is  no  pressure
in  the  boiler?

Ans.     To  0,  or  zero.
Ques.  Does  this  indicate  that  the  gauge  is
correct?

Ans.  No;  if  it  point  to  a  figure  above  0,  it  is  cer-
tainly out  of  order;  but  the  fact  of  its  pointing  to  0
when  there  is  no  pressure  does  not  prove  its  correct-
ness even  at  low  pressures ;  it  might  continue  pointing
to  0  when  there  was  pressure  on  it ;  or  it  might  point
to  90  when  there  was  100  pounds  pressure  on  it,  and
to  160  when  there  was  170  pounds.  Gauges  may  be
"  fast  "  at  some  steam  pressures,  and  "  slow  "  at
others.
