Overview
The 6725 Semimicro Calorimeter is a compact and easily operated, static jacket, combustion calorimeter designed specifically for measuring the heat produced by the combustion of small samples when only limited amounts of test material are available. It is equally useful for procedures in which it would be difficult to obtain a measurable temperature rise in a standard calorimeter, or where the nature of the sample makes semimicro operation desirable.
<>Features
• A 22ml Oxygen Bomb
The 1109 oxygen bomb used in the 6725 Calorimeter is the smallest of all Parr oxygen bombs. This 22 ml bomb will handle samples ranging from 25 to 200 milligrams, liberating up to 1200 calories when burned in oxygen, using initial oxygen pressures up to 35 atmospheres. Outputs up to 2400 calories can be accommodated if the sample is self-oxidizing, provided it is burned in an inert atmosphere and does not produce gas. Combustion in oxygen must be limited to a maximum output of 1200 calories.
The 1109 Semimicro Oxygen Bomb is a self sealing design with an O-ring main seal. The head is equipped with an inlet/outlet valve for oxygen filling and release, and insulated and grounded electrodes for the firing circuit.
Samples are held in a shallow nickel alloy dish which is supported on an adjustable Inconel loop attached to the oxygen inlet tube. Electrical connections for the firing circuit are made through a connector which slides into the central terminal stem.
• The Oxygen Connection
Oxygen is supplied to the 1109 bomb with a Parr 1824 Filling Connection which provides all of the valves and fittings needed to fill the bomb from a commercial oxygen tank, including an automatic relief valve to prevent overcharging.
• An Effective Thermal Jacket
Effective static insulation is provided in these calorimeters by using a fully silvered glass Dewar as the calorimeter vessel. This vacuum flask holds the 1109 semimicro bomb in 460 grams of water. It is supported within a stainless steel air can which is easily removed for filling or cleaning. The air can is surrounded by a plastic foam block, and the entire assembly is mounted in a sturdy aluminum case.
A thermistor probe and stirrer are mounted on the calorimeter cover and remain attached to the cover when the calorimeter is opened. A small motor attached to the side of the case drives the stirring shaft at a constant speed (approximately 450 rpm) through a flat, ribbed drive, belt which is easily removed when opening the calorimeter.
• Precise Temperature Measurements
Temperature measurements in the 6725 Semimicro Calorimeter are taken with a Parr 6772 Calorimetric Thermometer. In this application, the thermometer module is included with the calorimeter.
• Good Precision
The precision obtainable with the 6725 Calorimeter will depend primarily upon the amount of energy released by the sample. If the release is less than 100 calories, the deviation from the mean will not exceed 1.0 percent of the measured value. With a 400 calorie release, the deviation will be less than 0.4 percent of the mean value.
Applications
The ability of the 6725 Calorimeter to produce complete combustion and a measurable temperature rise with small samples in the 25 to 200 milligram range makes this an excellent instrument for use in marine biology and related ecological studies where only limited amounts of sample are available. It also can be used for testing a variety of heat powders and pyrotechnic mixtures, particularly slow burning thermite types which are used to produce heat. Samples which contain their own oxidisers can be burned in an inert atmosphere, while others can be burned in oxygen. There is, however, an important limitation to be observed: the 6725 Calorimeter must not be used for testing self-oxidising samples if the reaction liberates a considerable amount of gas, due to the dangerous high pressures which might develop within the bomb.
