from the balance design
The balances typically have 4 feet and a bubble to indicate that it is level. Sticky balance doors can cause the balance to move occasionally when the door is opened, and tape on the bench surfaces can both cause the balance to become off-balanced. When a balance becomes unlevelled, beside re-levelling it, what else should you do?
Hit the CAL button so it can do a quick internal calibration
How can you reduce errors from magnetic samples?
Weigh magnetic samples on top of a spacer. (Inverted weigh boat, beaker work well)
Weighing in drafty rooms can cause errors and unstable readings. Name some causes of drafts and how you can mitigate the associated weighing errors. (100 pts each pair)
-The AC unit can cause drafts, and it can be turned off during weighing
-Ventilation hoods/arms can cause a draft. Balances should be placed far from this if possible
-Doors opening/closing. Balances should be placed away from doors.
-High foot traffic. Balances should be placed away from high foot traffic areas.
-(others)
What should be checked prior to each weighing to ensure that the sample's mass is transmitting force vertically down on the load receptor, and therefore without error?
That the levelling bubble is centered
Rotating the balances feet __________________ (clockwise/counter-clockwise) will decrease the balance's height at that foot.
Counter-clockwise
The balance has an internal calibration weight that is periodically used to calibrate it automatically. You may see the balance calibrate often in a room that is ________________ , and should pause to allow the balance to adjust to current room conditions for the most accurate reading
Changing temperatures (ie. was hot and AC was turned on)
Statically charged samples produce weighing errors in the range of 1 - 100 mg, having the largest effect on small sample weighings (ie. ICP). What can cause charge separation (staticky samples)? (100 points each)
-tumbling samples for long periods
-poorly grounded floors / dragging your feet
-dry atmosphere / dry room
-friction from rubbing objects together
-others
Vibrations can cause unsteady readings and weighing errors. These are unavoidable in our lab, we have the train going by, equipment on shared benchtop, people moving around the lab, and other equipment causing vibrations. What is something that the balance can be placed on to minimize effects of vibrations on it?
A marble/concrete slab or a vibration-proof table
TRUE OR FALSE
After rolling the sample before weighing, it is sufficient to take scoops of sample from the top of the powder
False. You should always try to sample different locations and depths throughout the sample in case stratification of the particle sizes is occurring.
What type of force is used to balance the weight of the sample on the balance pan? This force is processed to readout in grams.
Electromagnetic force
A large difference in temperature between the sample and the metal balance pan can create ________________ within the enclosure, which are the rising of warm air and falling of cool air. These cause weighing errors! (hint: 3 words)
convective air currents
What is indicator that your sample is electrostatically charged and may produce errors? (200 points each)
-unidirectional drift in readout (discharging slowly)
-sample jumps off spatula/sample scatters in all directions
-sample sticking to walls of container
Humid rooms can cause moisture-sensitive samples to have falsely high mass readouts on the balance due to water absorption in the sample.
Dryrooms can cause samples to have falsely _____ (low/high) mass readouts due to ________________ (must have both)
falsely high mass readouts due to
electrostatically charged particles attraction to the metal balance pan registering as extra weight
If the operator notices that their sample is electrostatically charged, they can use a weigh boat made of what material to reduce the effects? (2 answers - 300 pts each)
- polystyrene or aluminum weigh boat
You have a weigh paper and a powder you want to weigh, and a lab balance with all the draft shield doors closed. Using the letters below representing each action, describe the order of steps you would take, trying to be as efficient and safe as possible, to weigh your sample (ex. ABCDEFG. can use as many letters as needed)
A. Open the door to the balance
B. Tare the balance
C. Roll the sample and allow powder inside to settle before opening container
D. Place the weigh paper/weigh boat onto balance
E. Record mass
F. Close the balance door
G. Allow balance to stabilize
H. Weigh the sample onto weigh paper
CAD FGB AHF GE
Will also accept:
ADF CGB AHF GE (you roll sample while balance stabilizes)