Set a micropipette inside its volume range
The same digits mean three different volumes on three different instruments. Learn to read the dial for the pipette in your hand and to stop before you force it.
- The name of an instrument can be a specification: P-20, P-200 and P-1000 are not model codes chosen at random. The number is the largest volume in microliters the instrument sets, and reading it that way answers most range questions on the spot.
- A decimal place depends on the scale it sits on: Students read a dial the way they read a clock, as if the digits alone carry the meaning. Here the digits are meaningless until you know which instrument they are printed on.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now handle the two decisions that come before a single drop moves: which instrument, and how far the dial may turn.
A protocol calls for 8 microliters. The bench holds a P-20, a P-200 and a P-1000. Which one is the right choice, and on what grounds?
Reviewed- A.The P-1000, since a bigger instrument is more powerful
- B.The P-200, since 8 is comfortably below its top
- C.Any of them, since all three can be dialed to 8
- D.The P-20, since it is the smallest one that reaches 8
Show the worked solution ▾
Answer: D. The P-20, since it is the smallest one that reaches 8
- Step 1: Cross off what cannot reach it: All three tops are above 8, so nothing is ruled out on that basis alone.
- Step 2: Ask where 8 sits on each: On the largest instrument it is a scrap of the range. On the smallest it is a good portion of it.
Why it's right: All three tops exceed 8, so the deciding question is where 8 falls within each range. On the smallest instrument it sits well up the scale, which is where a pipette is at its most trustworthy.
- A: Capacity is not power. A large instrument asked for a tiny volume is being used in the worst part of its range.
- B: This is the right idea and the wrong instrument. Being below the top is necessary and not sufficient, since something smaller also clears it.
- C: Whether a dial physically reaches a number is a different question from whether the volume it then delivers can be relied on.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A student holding a P-200 needs 260 microliters and keeps turning the dial, which has become stiff. What should happen next?
Reviewed- A.Stop, and take an instrument whose range covers 260
- B.Set it to 130 and dispense twice into the same tube
- C.Keep turning, since stiffness means it is nearly there
- D.Stop, and record the volume as 200 in the notebook
Show the worked solution ▾
Answer: A. Stop, and take an instrument whose range covers 260
- Step 1: Compare the volume to the top: 260 is above 200, so the volume is outside what this instrument can set.
- Step 2: Decide what stiffness is telling you: The mechanism has reached a limit that was built in on purpose, and forcing past a built-in limit damages it.
Why it's right: 260 is more than the P-200 can set, so no amount of turning will get there. The stiffness is the built-in limit doing its job, and the fix is a different instrument.
- B: Two half deliveries would reach the volume, but they double every error and this student has not established that the instrument is undamaged.
- C: Stiffness here is the limit, not the last stretch before the target. Continuing pushes the mechanism against a hard stop.
- D: Writing down a number you did not deliver puts a false value into the record, and the reaction still received the wrong amount.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student meets a protocol volume that no instrument on the bench covers and asks for the right one instead of forcing the dial.
Fill these in as you work through the lesson.
- Volume range (printed on the instrument, both ends):
- Full scale (the number in the name, in microliters):
- Overturning (forcing the dial past the top of the range):
- Mid range (where an instrument is most trustworthy):
The number in the name is the volume in microliters. Convert your dial reading to before you trust it, and choose the instrument whose range reaches the volume you need.
- Explain how the same four digits can mean two different volumes.
- Describe what you check before turning a dial upward.
- Give the reason two instruments can both reach a volume and one is still the better choice.
A protocol asks for 155 microliters. A P-1000 can reach it, since 155 is under 1000, and a P-200 can reach it too, since 155 is under 200. On the P-1000 that volume sits near the very bottom of the range. On the P-200 it sits high in the range. Take the P-200.