Stop relying on operator attention and a rod in the existing tank
Presses fed from a big bag through a 50*50*40 tank stop because nothing warns anyone before the bag runs dry. A reminder sign or a walk-by check fails on night shift. A vibrating rod dropped into the existing small tank fails too: the tank is so small that by the time the rod uncovers, the press is seconds from starving.
Two fixes work. Pick one after the check below.
- Time one bag-empty event: note when the bag visibly empties, when the tank level drops below the press pickup, and when the press stops.
- Read the press powder consumption rate from the press data or a timed test. You need it to size any warning time.
Check: you have a number (minutes) between "bag empty" and "press stops". That number decides which option below is acceptable.
Choose between a microwave barrier and a deeper buffer with a vibrating rod
| Option | Where it senses | Output | What you gain | What it costs |
|---|---|---|---|---|
| Microwave barrier (emitter + receiver) | Through the bag wall, emitter and receiver facing each other outside the bag | Digital (on/off): beam blocked by product, beam passes when product is gone | Warns while product is still in the tank, and needs no modification of the tank | Two units to mount and wire; usually the more expensive option |
| Vibrating rod on a deeper tank | Inside the buffer tank, at the high-level point | Digital (on/off) | Cheaper sensor; the extra tank volume buys running time | Requires modifying or replacing the tank |
Both are viable. The rod route is the low-cost one when a deeper tank is acceptable. Powder density of 850 g/l is not an issue: vibrating detectors are available with sensitivity down to 8 g/l on some models, so 850 g/l is far above the limit. Confirm the minimum density setting on the datasheet of the model you buy.
Check: you have picked one option and know whether you can modify the tank.
Mount the microwave barrier across the bag
- Place the emitter and receiver on opposite sides of the bag, outside it, on the same axis.
- Mount them low enough to leave working time. Height sets the warning: when the powder falls below the beam, the beam passes and the output switches.
- Fix both units to a rigid support, not to the bag frame straps, so alignment holds when the bag is changed.
- Align the pair per the manufacturer instructions and set the sensitivity with a full bag in place.
Two field conditions to test: a bag that collapses as it empties can leave fabric folds in the beam path, and a metallic or foil liner attenuates microwaves. Woven plastic bags are normally transparent to the beam; verify with your actual bag and powder.
Check: with a full bag the output shows "product present". Lower the powder below the beam (or use an empty, partly collapsed bag) and the output flips to "no product". Repeat with a new bag, not only the one used for setup.
Deepen the buffer tank and mount the vibrating rod
Make the tank deeper so it holds enough powder to run the press through a bag change. Mount the rod at the high-level point. While the bag feeds the tank, the rod stays covered. When the bag empties, the level drops, the rod uncovers, and the output signals "bag empty" while the tank still holds a reserve.
Size the reserve. Volume below the rod times 850 g/l gives the powder mass. Example under a stated assumption: if the current 50*50*40 tank is in cm, it holds 100 L, about 85 kg at 850 g/l. Divide the mass below the rod by the press consumption rate you measured to get the warning time. If the result is shorter than a bag change plus operator response, deepen the tank further or mount the rod higher.
- Install the rod on the tank wall away from the falling powder stream, or shield it with a deflector, so falling material does not trigger it.
- Set the sensitivity or density range on the rod for 850 g/l according to its datasheet.
- Add an uncover delay in the controller so splashing at the surface does not flicker the alarm.
Check: fill the tank until the rod is covered, then run the press until it uncovers. The output must change state, and the time from uncover to starvation must match your calculation.
Wire it fail-safe and send it to the operators
Wire the output so a wire break, lost supply, or dead sensor reads as "bag empty", not "bag full". Set the switching mode on the sensor (min/max or normally open/normally closed, per its datasheet) so the healthy, covered state is the energized state. Feed it to a beacon or horn at the operator station, and to the PLC input if the press line has one.
Temporary restore: a beacon wired directly to the sensor output gets production running tonight. Permanent repair: land the signal on a PLC input, add the uncover delay in logic, and log the event so you can see how often bags run dry.
Check: disconnect the sensor cable. The alarm must activate.
Prove the alarm before the press does
- Run a full production cycle from a full bag to an empty one with the press running.
- Confirm the alarm activates before the tank falls below the press pickup level.
- Confirm the elapsed time from alarm to starvation is longer than the time an operator needs to change the bag.
- Change to a new bag and confirm the alarm clears without manual reset or adjustment.
- Repeat with a partially collapsed bag (barrier option) or a fast fill (rod option) to check for false clears.
Check: two consecutive bag changes complete with no press stop.
FAQ
Why does a vibrating rod in the existing 50*50*40 tank not give enough warning?
The tank is too small: the rod uncovers only a short time before the press pickup runs dry. Deepen the tank so the volume below the rod, times 850 g/l, divided by the press consumption rate, exceeds your bag-change time.
Why does a microwave barrier work through a big bag?
The beam passes through the bag wall and is blocked by the powder between the emitter and receiver. When the powder drops below the beam, the beam becomes passable again and the digital output changes state.
Why does the microwave barrier stay in "product present" when the bag looks empty?
Collapsed fabric folds or residual powder can sit in the beam path, and a metallic or foil liner attenuates the beam. Move the pair to a different height or axis and retest with the actual bag and powder.
Why does the vibrating rod alarm flicker while the tank fills?
Falling powder or surface splashing repeatedly covers and uncovers the rod. Move the rod out of the fill stream, add a deflector, and set an uncover delay in the controller.
Why stop and call the manufacturer when the sensor will not switch reliably?
If the output still does not follow the powder after realignment, sensitivity adjustment, and a check of the datasheet density and mounting limits, stop adjusting. Contact the sensor manufacturer's official support with the powder density (850 g/l), bag material, and mounting geometry.