Pump versus gravity: mL per hour against drops per minute
A cutaway drip chamber on the left with spike, falling drops, fluid meniscus and roller clamp, paired on the right with the same tubing run ending in a blank pump cassette, a vertical rule between them carrying gtt/min on one side and mL/hr on the other.
Column 1 — Electronic pump
- Device wants: mL/hr
- Formula: total volume in mL ÷ total time in hours
- Worked: 1000 mL ÷ 8 hr = 125 mL/hr
- Rounding: whole number
- The tell: the stem names a pump, or gives no drip factor at all.
Column 2 — Gravity set
- Device wants: gtt/min
- Formula: (volume in mL × drip factor in gtt/mL) ÷ time in minutes
- Worked, same order at 15 gtt/mL: (1000 mL × 15 gtt/mL) ÷ 480 min = 15000 ÷ 480 = 31.25 = 31 gtt/min
- Rounding: whole drop, always, because a fractional drop does not exist.
- The tell: the stem gives a drip factor. A drip factor is only ever printed when it is needed.
Second pass
Order: 500 mL over 4 hours.
- Pump: 500 mL ÷ 4 hr = 125 mL/hr
- Gravity at 10 gtt/mL: (500 mL × 10 gtt/mL) ÷ 240 min = 5000 ÷ 240 = 20.83 = 21 gtt/min
Third pass
Order: 250 mL over 2 hours.
- Pump: 250 mL ÷ 2 hr = 125 mL/hr
- Gravity at 20 gtt/mL: (250 mL × 20 gtt/mL) ÷ 120 min = 5000 ÷ 120 = 41.67 = 42 gtt/min
Notice that all three orders run at the same pump rate and three different gravity rates. The drip factor, not the order, sets the drop count.
The microdrip shortcut
At 60 gtt/mL the two answers are numerically identical, because dividing by 60 minutes and multiplying by 60 drops cancel each other out. An order running at 125 mL/hr on a microdrip set runs at 125 gtt/min. That is a checkable coincidence, not a separate formula, and it is worth one second of verification rather than assumption.
Watch outWRONG UNIT. Candidates lose these items by computing a correct number in the unit the question did not ask for.
Margin notea printed drip factor means the answer is in drops per minute. → BOOK 15 · P11
Takeawayread the equipment first; it names the unit before the math begins.