The calibration sheet on the wall said 42 grams per second. The operator had checked it in March with a stopwatch and a bucket. In August the same setting was delivering 39 grams, and nobody knew, because nobody had weighed anything since March. Across a shift that is a 7 percent shortfall, which is precisely the kind of error that does not produce a complaint until a customer opens a drum and finds free liquid at the bottom.
Bulk density is the least glamorous number in polymer handling and the one that quietly decides whether a volumetric feeder tells the truth. It drifts with grade, with moisture, with how the material was poured, and with how long it sat in the hopper. A five-minute check once a week catches all of it.
Two densities, not one
Every powder has a loose bulk density and a tapped bulk density, and the gap between them is the number that predicts how your feeder will behave. Loose density is what you get by pouring into a measuring cup from a height. Tapped is what you get after the same cup has been dropped on a bench 50 times, which is roughly what a hopper does to the material over an hour of vibration.
A coarse granule might measure 0.72 kg/L loose and 0.78 tapped — a 8 percent difference. A fine powder with a broad particle size distribution can go from 0.55 to 0.71, which is a 29 percent difference. That second material will dose accurately for ten minutes and then progressively over-deliver as the hopper settles and the screw fills more completely.
The method, in full, because half a method is worse than none
Use a 1 litre graduated cylinder or a 1,000 ml stainless cup with a known tare. Pour from about 150 mm through a funnel so the material lands in the centre, fill to the brim, then strike off level with a straight edge without pressing. Weigh. Divide the net weight in grams by 1,000 to get kg/L. Repeat three times from three different bags and use the average.
For the tapped figure, use the same cup on a bench and lift one edge 20 mm, drop it, and repeat 50 times at about one per second. Top up after settling, strike off, weigh again. The ratio of tapped to loose is your Hausner number, and anything above 1.25 tells you the feeder needs an agitator or a different grade.
| Material | Loose kg/L | Tapped kg/L | Ratio | Volumetric error over 8 h |
|---|---|---|---|---|
| Coarse 1–3 mm | 0.72 | 0.76 | 1.06 | Under 2% |
| Mid 0.5–1 mm | 0.66 | 0.71 | 1.08 | 3–5% |
| Fine, broad PSD | 0.55 | 0.71 | 1.29 | 10–14% |
The last column is the one that matters commercially. A 12 percent over-dose on a 500 kg a week line is 60 kg a week, and at industrial prices that is a line item nobody has put in the budget.
Moisture is the hidden input
Density and moisture move together. A granule that has taken up 1 percent water by weight swells slightly, flows worse, and gains density as the particles pack differently. In a humid month, the same grade can read 3 to 5 percent heavier in the cup than it did in winter, which means your volumetric feeder is now over-dosing by the same amount if it is set by volume.
This is why the check belongs next to the hygrometer, not in a folder. If the store has been above 65 percent RH for a week, weigh the sample and note the date. Two months of those notes will show you the seasonal swing on your own site, which is far more useful than a generic figure from a supplier.
When the check finds something
If density has moved more than 5 percent from the last reading, do three things in order. Check the lot number — a different lot can legitimately differ. Check the hopper for segregation, which shows up as fines at the bottom and coarse at the top. Then check the feeder screw for wear, because a worn screw delivers more per revolution and looks exactly like a density change.
One plant I worked with replaced a feeder motor before anyone weighed a cup. The density was fine; the auger flighting had worn 2 mm over four years, which was enough to double the slip and raise the feed rate by 9 percent.
Record the check on the same sheet as the calibration, with the date, the lot, the room humidity and the operator’s initials. It takes five minutes and it turns an argument about “the powder changed” into a conversation about numbers. The density and particle size data for super absorbent powder gives the expected range for each grade so you know what normal looks like.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
