6.4 Type B Sequence: Water, Petcocks, Pressure, and Reading
Key Takeaways
- The Type B sequence requires open petcocks, water introduced until air is expelled, pressure pumped to the initial line, and controlled release into the bowl.
- Petcocks must not be closed too early because trapped air above the concrete can create a false reading.
- The gauge is read after pressure equalization, tapping as required, and stabilization, not while the needle is drifting.
- Pressure must be released safely before disassembly; removing the cover under pressure is a serious procedural error.
The Type B Sequence Is An Order-Control Test
After the bowl is filled, consolidated, struck off, and cleaned, the Type B pressure sequence begins. This is where many performance-exam errors happen because the steps must occur in the right order. The basic purpose is to seal a known volume of concrete under a water-filled cover system, apply a calibrated pressure, and read the air content response.
First, the cover is clamped onto the clean bowl with the main air valve between the air chamber and bowl closed. The petcocks are opened. Water is introduced through one petcock until it flows from the other, and the meter is handled as required to remove air bubbles from the space above the concrete. The point is to eliminate free air in the cover cavity that is not part of the concrete sample.
If the petcocks are closed before water has displaced the air, the meter may measure trapped air above the concrete along with the air in the concrete. That creates an invalid apparent air reading. Water flow through both petcocks is a visible control that the cover space has been filled and vented.
Type B sequence map:
| Order | Action | Why it matters |
|---|---|---|
| 1 | Clamp cover on clean rim | Creates sealed test system |
| 2 | Open petcocks with main valve closed | Allows water filling and venting |
| 3 | Add water until it exits the opposite petcock | Removes free air above concrete |
| 4 | Pump to the initial pressure line | Sets calibrated starting pressure |
| 5 | Close petcocks and open main air valve | Transfers pressure to bowl system |
| 6 | Tap meter or gauge as required and read | Stabilizes apparent air reading |
| 7 | Release pressure through petcocks before opening | Prevents unsafe disassembly |
The air chamber is pumped until the gauge reaches the initial pressure line. The pressure should stabilize, and any fine adjustment should be made according to the method and meter design. Then the petcocks are closed, and the main air valve is opened to let pressure into the bowl. This pressure compresses air in the concrete, producing the gauge response.
The reading should not be taken from a bouncing or drifting needle. The technician may need to tap the meter sides and lightly tap the gauge to help stabilize the indication. Read at eye level and record the apparent air content to the required precision. Do not invent extra decimal places from a gauge that cannot support them.
After reading, pressure must be released before the cover is removed. Opening petcocks relieves pressure safely. Removing clamps or lifting the cover while the meter is still pressurized is a serious safety and performance error. The release step also demonstrates control of the equipment to the examiner.
Common sequence traps:
- Closing petcocks before the cover space is full of water.
- Forgetting to pump to the initial pressure line.
- Opening the main air valve before the chamber is properly pressurized.
- Reading the gauge while it is still moving.
- Reporting the apparent reading without subtracting correction.
- Removing the cover before releasing pressure.
The sequence is learnable as a story: seal the bowl, vent the cover with water, set the pressure, release pressure to the concrete, read the gauge, then release the system safely. Memorize the reason for each step, not just the hand movement.
Why is water introduced through the petcocks before pressurizing a Type B meter?
When should the Type B gauge be read?
What must happen before removing the cover from a Type B pressure meter?