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Bulletin 4700

A Built-in Pressure Indicator

Sample Preparation Vessels - 28323 A

s pressure in the sample cup increases and the relief disc is compressed, a retaining screw, which is normally flush with the top of the bomb cap, will protrude abovethe top surface. The head of this screw will rise approximately 1/32 inch for each 500 psi of pressure in the bomb. Thus, by monitoring the extension of the screw head, the user will have a visual estimation of the pressure within the bomb.This release mechanism has been designed to protect againstthe relatively gradual pressure build-up which can result from overheating the contents of the bomb. But no relief mechanism can protect against the destructive forces produced by materials which detonate or explode within a vessel of this type.

A Microwave-Transparent Body

he body and screw cap for these bombs are made of anew, microwave-transparent polymer which has good mechanical strength at temperatures up to 150 °C, andwhich serves also as an excellent heat insulator for the Teflon sample cup. Since heating is developed internally within the cup, temperatures in the outer, high strength body will seldom exceed 50 °C.

4782

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45 mL - 250 °C1200 psig

Pressure and TemperatureLimits

orking pressures up to 1200 psi (8.27 Mpa) andreactant temperatures up to 250 °C can be developed safely in these bombs, but these limits must not beexceeded. Obviously, it is much easier to specify these limits than it is to describe how to ensure that they are carefully observed, since there is not pressure gage on these vessels and no internal temperature probe. Actually, the 1200 psi pressure rating assumes that the containment parts of the vessel will always remain at temperatures below 50 °C. This will normally be the case, since microwave energy is directed into the sample itself and not into the containment vessel. Although high temperatures are developed within the sample, this is accomplished without the introduction of large amounts of heat. And the energy that is directed into the sample is rapidly dissipated into the Teflon cup and the outer vessel. The bomb itself will become warm to the touch as the contents cool, but it should not reach temperatures above 50 °C.Users can get a preliminary estimate of the effective heatingrate of their microwave oven by heating a small amount of the sample and its digestion medium in an open Teflon cup, and observing the time required to bring the medium to boiling. Most domestic ovens will generate internal temperatures in the range of 200 °C within one minute when using the 4781 or 4782 bomb. More powerful units will be significantly faster. After removing the bomb from the oven it normally will cool down to ambient temperature within 20 or 30 minutes with forced air circulation. Excessive deformation of the cup, it’s sealing ring, or significant acid leakage are clear indications that excessive temperatures and/or pressures are being generated.
4782 Microwave Digestion Bomb

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Unique Safety Features

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everal unique sealing and safety features have been incor-porated into the design of these vessels. The Teflon sample cup is closed with a self-sealing, Teflon O-ringwhich eliminates the need to pre-load the cup in order to secure a tight seal. This also eliminates the effects of differential thermal expansion during heating and cooling cycles while providing a chemically inert, all-Teflon system.For overpressure protection, the closure in these bombsincludes a compressible relief disc which operates in conjunction with the O-ring cup seal to release any excess pressure if the internal pressure should exceed a point at which it might destroy the bomb and oven. When pressure in the bomb reaches approxi- mately 1500 psi, the relief disc will be compressed sufficiently to release the support for the O-ring. At this point, the unsupported O-ring will blow out, releasing pressure from within the cup. In most cases all parts of the bomb except the O-ring will be reusable after this event if they are promptly and carefully cleaned and inspected following the pressure release.
Parr Instrument Microwave Digestion Bombs 9