Scientific Analytical Calorimeter Solutions

Oxygen Filling Pressure and Why Change It

The following information applies to the: CAL3K-AP

The oxygen filling pressure can only be changed from the PC. For details on PC operations:


And enter a suitable value from 20 to 30 bar.

The filling pressure can also be changed from the Keyboard after the password is entered. For details see: Keyboard password

WARNING: The filling pressure must be less than the regulated oxygen cylinder pressure.

The most common used pressure is 30 bar (calorimeter standard). This is because some ancient documents recommended it. But, this is not always required.

The main reason for the high pressure oxygen is that the sample should burn completely. For this an oversupply of oxygen is required, otherwise oxygen starvation sets in.

The final oxygen pressure depends on:

Sample Size: 0.5g 1.0g
Energy Content: 15MJ/Kg 45MJ/Kg
Sample Consistency: Solid Powder
Vessel Volume: Small Big

All the laboratory requirements differ. For this reason, and if there is an incentive to reduce the oxygen pressure, I recommend running a typical sample at ever decreasing pressure to measure the effect the lower pressure has on the result. A typical test would use: ~0.5gr Sample

Filling pressure: 30bar, 25bar, 20bar, 15bar

The thermal transfer of the vessel changes slightly with the reduced pressure, because the gas is less dense at lower pressure. For this reason I recommend calibrating the vessel at a lower pressure once it is chosen as being sufficient.

By reducing the oxygen pressure you reducing the cost of operation significantly.

One other reason for reducing the oxygen pressure is the sample consistency. If it is a VERY FINE POWDER then it may explode on ignition and the fine powder ‘scatters’ and settles on the walls of the vessel. This is evident after the determination when one wipes the wall with a tissue.

This ‘scattering’ of the sample can be overcome by two methods:


The Filling pressure parameter is checked to be in the range of 0 to 35bar.

During the main period the vessel pressure is monitored, and if it falls significant below the pressure before firing then an ‘OXYGEN LEAK’ is declared and the main period is terminated.


The vessel is filled from a bottle of industrial oxygen. An adjustable pressure regulator reduces the pressure to about 20-33 bar. The exact regulated pressure depends on the CAL3K settings.

There are various reasons why the vessel firing pressure should be lowered:

  1. Less pressure is more economical because less oxygen is used per firing
  2. Less pressure prevents explosion of powdery substances (sample scattering)
  3. Less pressure slows the vessel equalization.

It is unknown why the vessel equalization should be slowed down, but: The less dense atmosphere inside the vessel transmits heat slower.

The historical standard filling pressure is 30bar (3000kPa). This is a very conservative pressure. The point is that the sample must burn completely without any sign of oxygen starvation. Oxygen starvation sets in when there is:

  • Too little oxygen


The oxygen filling pressure is pre-regulated to 30 bar. This is the standard filling pressure for bomb calorimetry. However, special samples such as powders or wood samples require less filling pressure to obtain good results. It is also possible to save oxygen for the determination of routine samples. In all these instances, the vessel filling pressure can be set lower from the keyboard.

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The CAL3K_AP is connected to the oxygen supply and will FILL the vessel automatically. It will also DEFLATE the vessel. The process is monitored by the microprocessor. The mechanical components must work together with the electrical (parameters). To this end all valves and their performance are checked (User test) and the oxygen filling progress is monitored.

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The broad term "Not Filling" can be dissected into the following scenarios: Filling too slow : The vessel pressure does not reach target within 40 to 60 seconds. Replace the restrictor, The filling pressure stays at zero during the filling process. The CAL3K-AP has a "DEFLATE" keyboard command, which opens the oxygen inlet valve. It is used to deflate the oxygen line when the oxygen bottle is closed.

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