Parr Instrument 1108 - Manual (Page 7)

1108 Oxygen Combustion Vessel
w w w . p a r r i n s t . c o m 7
Fire the charge by pressing the firing button on the
ignition unit, keeping the circuit closed for about 5
seconds. The indicator light will come on when the
button is depressed and will remain on while cur-
rent flows through the fuse. When the fuse burns off
and breaks the circuit, the light will go out. Normally
this takes about ½ second, but it is good practice
to keep the push switch closed for about 5 seconds
regardless of the light. If the light continues to glow
while the button is depressed, there is either a short
circuit in the firing system or the fuse was not prop-
erly arranged. If a 26 gauge platinum wire is used to
fire the charge, hold the firing button down for only
one or two seconds which should be sufficient to ig-
nite the auxiliary fuse. A longer period may melt the
wire. If the wire melts, use the 7 cm terminals on the
ignition unit to obtain a lower firing voltage; or add
a heavy, one-ohm resistor to the 10 cm firing circuit
to lower the voltage.
If the indicator light does not come on when the
firing button is pressed there is either a fault in the
2901 or an open circuit in the system. Check for
voltage between the 10 cm and common termi-
nals of the 2901. Approximately 23 VAC should be
measured. If there is no voltage present, check the
fuse inside the 2901. An open circuit can usually be
located with an ohmmeter. Flex the lead wires dur-
ing any continuity check as the wires may be broken
and making only intermittent contact. If the red indi-
cator light glows during ignition but the bomb fuse
does not burn, check the system for a voltage leak
to ground, most likely in the insulated electrode on
the bomb head. Check the electrode using the high
impedance scale on an ohmmeter and replace the
electrode insulator and seal if leakage is indicated.
Recovering the Combustion Products
Let the bomb stand in the calorimeter or water bath
for at least 3 minutes, then lift it out of the water
and wipe with a clean towel. Open the valve knob
slightly to release all residual gas pressure before
attempting to remove the screw cap. Gas release
should proceed slowly over a period of not less than
one minute to avoid entrainment losses. After all
pressure has been released, unscrew the cap; lift the
head out of the cylinder and place it on the support
stand. Do not twist the head during removal. Pull it
straight out to avoid sticking. Examine the interior of
the bomb for soot or other evidence of incomplete
combustion. If such is found the test will have to be
discarded. Wash all interior surfaces of the bomb
and the combustion capsule with a jet of distilled
water and collect the washings. If any precipitate
or residue is present, remove it with a rubber po-
liceman. Do not filter the washings as this might
remove valuable constituents. Titrate the washings
and measure the unburned fuse wire as required for
calorific tests, then analyze the washings for sulfur
and other elements, if required.
Caution!
Do NOT have any part of the body in the
exhaust path of the bomb.
An Optional Recovery Procedure
If desired, a luer fitting, 518A, can be attached to the
bomb to provide a means for washing the bomb
and recovering the combustion products with a
syringe without opening the bomb and removing
the bomb head. To use this procedure, remove the
standard A420A valve needle and replace it with an
A420A2 needle to which a syringe, 244C, can be at-
tached.
To recover the combustion products via a luer fit-
ting, let the bomb stand in a cooling bath for at
least 3 minutes after firing to allow for complete
condensation of all residual vapor. Then remove the
bomb from the water and attach only the barrel of a
244C syringe to the luer fitting. Open the valve and
release the pressure at a slow rate, using at least
a full minute to bring the bomb pressure back to
atmospheric. The attached syringe barrel will help to
retain any condensate spray that might be carried
out of the valve during the exhaust period.
Add 30 mL of distilled water to the attached syringe
barrel and use the syringe plunge to force the water
into the bomb, then close the valve while holding
the plunger down. This will develop sufficient pres-
sure within the bomb to seat the inlet check valve
and provide enough positive pressure to help re-
move the washings. Agitate and rotate the bomb in
a horizontal position to wet all inner surfaces, then
turn the bomb upside down over a 600 mL beaker
and open the valve to discharge the washings into
the beaker. Tilt the bomb slightly toward the valve
to get as much of the water out as possible. Repeat
this back-flushing procedure two times, collecting
a total of 90 to 100 mL of washings, then open the
bomb and recover any liquid that may remain in the
cylinder. The three complete back flush and rinse
cycles should recover better than 99 percent of the
combustion products.