You are simply using more compressed air than the compressor can produce. The compressor is working at full capacity but the application demands more. Pressure drops because you are emptying the system faster than it can fill. The compressor is too small for the job.

What you'll see

The engine runs at full RPM, the compressor is fully loaded, everything is working correctly -- but pressure keeps dropping when all tools are running simultaneously. When you disconnect one tool, pressure comes back. When you reconnect it, pressure drops again. The compressor is genuinely too small for the total air demand of the connected tools or equipment.
Before you assume this is the problem

Before concluding the compressor is too small, check for leaks (they reduce effective capacity significantly) and make sure the engine is reaching full RPM (a derating factor). Also check if the compressor inlet filter is dirty -- a dirty filter reduces actual capacity below rated capacity. See: Air Leaks, Engine Not Reaching Full RPM.

See all causes of low pressure / low air output →

How to diagnose

  1. Compare actual demand to compressor capacity

    Add up the air consumption of all connected tools. Every pneumatic tool has a rated air consumption (in CFM or l/s). If the total exceeds the rated capacity of the compressor, you have a sizing problem. Remember that rated capacity is the maximum output -- in practice, account for 10-15% loss from altitude, temperature, and normal system losses.
    Result: Total tool demand exceeds compressor rated output = undersized.

  2. Consider altitude derating

    If the compressor is operating at significant altitude (above 1,000m), both the engine power and the compressor capacity are reduced. The air density is lower, so the compressor takes in less air per revolution. As a rough rule, capacity drops approximately 1% per 100m of altitude. At 2,000m, you have lost about 20% of capacity compared to sea level.
    Result: Significant altitude = derate the compressor capacity accordingly.

How to fix it

  1. Reduce air demand

    Use tools sequentially rather than simultaneously. Fix all air leaks. Lower the working pressure to the minimum the tools need (every 1 bar reduction gives about 7% more available capacity). Use the most efficient tools available for the job.

  2. Bring in a larger or additional compressor

    If the demand genuinely exceeds the compressor capacity, you need more air production. Either bring in a larger compressor, or add a second compressor in parallel. When running two compressors in parallel, install check valves on both outlets to prevent one from pressurizing the other. Adjust pressure settings so one machine leads.

Common mistakes

Do not increase the pressure setpoint to compensate for low pressure at the tools. Higher pressure does not mean more air -- it means more power consumed, more heat, and more fuel. If the pressure drops because demand exceeds capacity, raising the setpoint just makes the compressor work harder without actually delivering more usable air. Also: do not confuse pressure with flow. The tools need a certain FLOW of air at a certain PRESSURE. If flow is insufficient, pressure drops. Increasing pressure without increasing flow does not solve the problem.

Parts & tools

Air consumption data for connected tools. Compressor capacity specification. Check valves if running machines in parallel.

Review safety precautions before starting →

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