Free Air or Blast Chamber? Answering My MNGN 333 Students with the PETS Edgar Mine Calculator
Petr Explosives Group · Practical Explosives Training School ·
By Dr.-Ing. Vilem Petr
This semester, I am teaching shock wave physics in air to students of the Mining Engineering Department in MNGN 333 at The Colorado School of Mines. After one of our classes, several students asked me a very good question: What would happen if we fired the same charge in the blast chamber at the Edgar Mine instead of in free air?
This article is my answer. I dedicate it to my new friends in MNGN 333, who keep asking such good and important questions. I hope it helps you understand the difference between a blast in open air and a blast in a chamber.
To answer the question with numbers, we built the PETS Edgar Mine Blast Chamber Calculator. You enter the charge, the room, and the gauge position, and the calculator predicts the shock wave parameters before the shot. Next semester, I hope we can fire real charges in the chamber and compare these predictions with real gauge records. That is the true test of any calculation.
When a charge is fired in the open, the shock wave travels outward, and the hot gases expand away. When the same charge is fired in a closed room, nothing gets away. The shock bounces off every wall, the floor, and the ceiling, and the gases stay trapped until they can escape through the opening. The room is loaded twice.
What the calculator works out
- Shock at a point. The direct wave reaches the gauge first. Then come the reflections from every surface of the room, each one weaker and later than the one before.
- Gas pressure. Hot gas fills the room and vents through the opening. The calculator gives the peak gas pressure, the vent time, and the gas impulse.
- Adjacent room. The shock weakens as it turns the bend (angle θ) into the next room and strikes the far wall at angle α. The gas, however, fills both rooms.
- Sound level. Peak pressure is expressed in decibels. In the default case, the chamber wall sees 232 dB and the adjacent room wall 183 dB. Both are far above the common 140 dB peak hearing limit.
- Ground vibration. Peak particle velocity (PPV) at a receptor at distance R, compared with the 12.7 and 50 mm/s (0.5 and 2.0 in/s) guideline levels.
- Ground or air. A charge on the floor acts like 1.8 × W because the floor reflects the shock back. The same charge in the air acts like W.
How to use the calculator
The calculator runs in a web browser and switches between SI (kg, m, kPa) and Imperial (lb, ft, psi) units with one click. Students:
- Choose the explosive and charge mass. TNT, ANFO, bulk emulsion, C-4, PETN, RDX, Composition B, Pentolite or a custom product. The TNT equivalence factor fills in automatically and can be edited. If the chamber’s rated limit is entered and the charge exceeds it, a red warning appears.
- Set up the chamber. Length, width, height, the opening area and the bend angle of the passage. The default room is 20 × 10 × 10 ft. Ambient pressure matters: sea level is 101.3 kPa, but the Edgar Mine sits at roughly 77 kPa.
- Place the charge and the gauge anywhere in the room. A plan view and a side elevation, drawn to scale, show where they are. A gauge placed on a wall, floor or ceiling reads reflected pressure.
- Read the results. Peak pressure, impulse, arrival time, scaled distance Z, sound level, gas pressure, and vibration, with graphs and tables for every quantity.
A worked example: 8 kg of TNT on the floor
This is the pressure at the gauge over the first 50 milliseconds for the default case: 8 kg of TNT on the floor in the centre of the room, with the gauge on the side wall 2.14 m (7.0 ft) away.
- The tall spike at about 1 ms is the direct shock: 7,849 kPa (1,138 psi), 232 dB. It is very high but lasts only about 0.2 ms.
- The smaller sawtooth peaks from about 4.5 ms are shocks that bounced off the other walls, the floor, and the ceiling. The first is about 1,400 kPa because two reflections arrive almost together. With reflections up to the third order, 43 separate arrivals reach the gauge.
- The red dashed line is the gas pressure, 237 kPa (34 psi). It is 33 times lower than the shock peak, but it lasts about 72 ms while the room vents.
Here is the lesson students usually do not expect. The direct shock contributes only 584 kPa·ms of impulse. The reflected shocks bring the total shock impulse to 10,318 kPa·ms, and the gas adds another 6,046 kPa·ms. The highest peak is not what delivers most of the push.
Five differences between free air and the chamber
- A chamber loads the room twice. In the open, only the shock matters. In a chamber, the gases are trapped, so there is a shock load and a gas load.
- Structures respond to impulse as well as peak. A light, stiff part, such as a gauge, a window, or a bolt, responds to the shock peak. A heavy part, such as a door, a wall, or a rock, responds to the total push, where the gas and the reflected shocks dominate.
- The controls are different. Moving the charge away from a wall reduces the shock on that wall, but does nothing to the gas pressure. Doubling the distance cuts the shock pressure by far more than half.
- Bends stop shock, not gas. In the adjacent room, the shock at the wall falls to about 29 kPa, but the gas pressure next door still reaches 127 kPa because the two rooms equalize.
- Vent size sets the gas impulse. Halving the opening doubles the vent time and the gas impulse, with no change in the peak gas pressure.
What is behind the numbers
Incident pressure and impulse use the Kinney-Graham fits for TNT in free air, with Sachs scaling for the lower air pressure at altitude. Reflections are calculated with image sources, one for each bounce, and added together. Gas pressure comes from the explosive’s energy and the room volume, with optional afterburning, and venting is modelled as a blowdown through the opening. Ground vibration uses the standard scaled-distance formula, and the site constants K and β can be replaced with values measured at your own site.
An important note: this is an educational estimate only. It has not been validated against BlastX, and results can differ by 30% or more from ConWep, BlastX, or UFC 3-340-02. It is a teaching tool, not a design tool. Every charge fired in the chamber must follow the approved procedures and the chamber’s rated limit.
Predict it, fire it, measure it
The best way to understand a shock wave is to predict it, watch it happen, and then explain the difference. This article is the first step: the prediction. My hope is that next semester we can take the next steps together at the Edgar Mine, fire real charges in the chamber, and validate these calculations with real experiments.
Thank you to the students of MNGN 333 for the questions that started this article. Keep asking them.
Open the PETS Edgar Mine Blast Chamber Calculator
Want to go further? The Fundamentals of Explosives short course returns to Golden, Colorado, February 2–5, 2027.
