TM 1-1500-204-23-4 Figure 4-2.  Manometer Reading Level WARNING A    scratched    tube    may    shatter under relatively low internal pressure. (2) Cleaning.  When visually determined as necessary,    periodically    clean    manometer    using    the information provided in the following paragraphs. (a)     Mercury     cleaning.          Use     the following procedures tor mercury cleaning: WARNING Be extremely careful when handling  mercury.    A  very  small amount  of  mercury  may  produce hazardous  vapors.    For  methods that are used to dean up mercury spillage, see paragraph 4-2a (6). 1 Clean  mercury  by  washing  in  a solution of nitric acid, Federal Specification O-N-350 (10 percent acid and 90 percent distilled water), or a solution of    sulfuric    acid,    Federal    Specification    O-S-09    (80 percent acid and 20 percent distilled water). 2 Using apparatus shown in figure 4-3,  pour  mercury  through  funnel  and  filter  paper  and allow to pass through acid solution. 3 After mercury collects at bottom of  glass  tube,  open  pinch  cock  and  let  dean  mercury pass   into   beaker.      Control   pinch   cock   carefully   to prevent acid solution from entering beaker. 4 Next, pass clean mercury through  distilled  water.    Place  beaker  of  dean  mercury on a sheet of asbestos and heat until dry over an open flame not to exceed 230°F (110°C). (b)   Manometer   tube   cleaning.      Use the following procedures for manometer tube cleaning: CAUTION When   manipulating   swab   inside the  glass  tubing,  take  care  not  to scratch the tube bore. A scratched  glass  tube  may  shatter under relatively low pressure. 1   Clean   manometer   tubes   using   a swab attached to a medium stiff piano wire.  First swab out Figure 4-1.  Manometer Change 3  4-3