V. USING A BALANCE
Balances are instruments used to determine weights of objects and substances. There are many types of balances, and two of those used commonly in bioresearch laboratories are available in the Tech Facility. These are the "beam balance" and the "electronic balance". Like all balances, these have been placed on a flat, level surface for proper use, located away from drafts that might affect the readings.
A. USE OF A BEAM BALANCE
The beam balance is a general purpose instrument, not intended for high precision weighing. Normally, it is not employed for weighing chemicals or other materials for use in preparing experimental media or solutions. Rather, it is frequently used for balancing unknown weights against standard weights, and for balancing centrifuge bottles or tubes against each other prior to centrifugation. The beam balance can also be used for "bulk" weighing jobs, such as determining the weight of a small animal (mouse, frog), or of a very large amount of a chemical.
The Tech Facility beam balance is a two platform balance, and can be used in two ways: (a) You can place objects or weights on both platforms and balance them against each other, or (b) you can weigh something on just one platform.
Before using the balance for weighing, it must be set to zero. This procedure is referred to as "zeroing the balance", or "taring".
EXERCISE #1: ZEROING THE BEAM BALANCE
1. Find the beam balance station, and refer to the picture of the balance (Figure 1; next page).
2. Make sure that the slide poise arrows are at the zero mark (hereafter called "slide weights") at the extreme left.
3. Also make sure any air conditioners are off and the balance is protected from drafts (see Figure 2).
4. Check the zero pointer; it should be aligned with the zero line on the zero indicator. If it is, the balance is zeroed and is ready for use.
Note: When reading the pointer, it helps to do so with one eye closed, in order to accurately determine its position.
5. If it is not aligned with the zero indicator, turn the zero adjust knob (located at the right end of beam, below platform) slowly. Allow the balance to settle, and check the zero indicator again.
6. Repeat step 5 if necessary, until a zero reading is obtained.
NOTE: Always make sure that a) the weighing platforms are free of any dust before zeroing the balance, and b) you have a true zero reading before using the balance.
EXERCISE #2: WEIGHING AN OBJECT
1. Zero the balance if not already zeroed.
2. Place a 100ml glass beaker (with Parafilm cover) on the left weighing platform. The pointer will move into the "-" position.
3. Slide the 200 gram beam weight to the right, so that it engages each notch one at a time, until the zero readout pointer passes into the "+" position.
4. Slide the 200 gm beam weight back to the left one notch. The pointer should now move back into the "-" position.
5. Slide the 10 gram beam weight to the right a little at a time until the pointer indicates zero.
6. The weight can be determined by adding the readings of the larger sliding weight to the reading of the smaller sliding weight (eg. if the arrow of the larger sliding weight points to 50 and the smaller sliding weight arrow points to 8 then the total weight is 58g). Since each line on the smaller sliding weight scale is 0.1 gram (= how many milligrams?), your measurement can be read to the nearest tenth of a gram.
7. Record the weight of the beaker in your Tech Facility notebook.
8. Remove beaker from weighing platform.
9. Slide beam weight back to the zero position.
10. Put the covered beaker back in the Labware cabinet.
EXERCISE #3: BALANCE CAPACITY WITHOUT ADDED WEIGHTS
Examine the balance to determine the weight that can be measured when no additional (external) weights are applied. Record this weight in your Tech Facility notebook.
EXERCISE #4: USE OF ADDITIONAL WEIGHTS
If your sample is greater than the capacity of the balance sliding weights, you may use the additional set of external weights provided (located on benchtop near balances). The maximum capacity of this beam balance, as specified by the manufacturer, is 210 gm without external weights & has a precision of 0.1 gm.
1. Zero the beam balance. (Refer to EXERCISE #1)
2. Place a 600 ml glass beaker (covered) on the left weighing platform.
3. Test whether you can determine its weight using the sliding weights.
Slide the 200 gm beam weight towards the right, engaging each notch one at a time, until the zero readout pointer passes into the pointer "+" position.
At what point does the 200 gram beam weight do this? Is the weight of the beaker within the parameters of the maximum capacity of the balance? In other words, is the maximum capacity of the beam balance adequate enough to determine the weight of the beaker? Or does it exceed it?
4. If not, leave the 200 gm sliding weight at the 200 gm setting.
5. Then gradually add the external weights, starting with the lowest weight and working upward, on to the right weighing platform until the zero pointer moves into the "+" position.
Note: Use the kimwipes, found in box along side the external weights, to pick up and place weights onto the weighing platform. Never use fingers to pick up weights. Handling with fingers can speed corrosion of weights and lead to inaccuracy.
6. Next, follow Steps 4-5, of EXERCISE #2.
Once a zero reading is obtained, add the sum of the external weights to the reading on the beams to obtain the total weight of the beaker.
7. Record the weight in Tech Facility Notebook.
8. Remove covered beaker from left weighing platform and return to Labware cabinet.
9. Return external weights back into storage box, again handling only with kimwipes. Once all weights have been replaced, return the box to its proper place.
10. Slide beam weights back to zero position.
EXERCISE #5: CHECKING THE BALANCE BEAM WEIGHTS AGAINST STANDARD EXTERNAL WEIGHTS
1. Zero the balance.
2. Place the 50 gram standard weight on the left weighing platform. Add external weights (using kimwipes) to the right platform. Try the combination totalling 50 grams. Do the same for 12,15, and 20 grams respectively.
3. When completed with the exercise, return external weights back into its storage box and return box to drawer.
EXERCISE #6: BALANCING CONTAINERS
In biological laboratories, one typical use of a beam balance is for balancing centrifuge tubes, bottles, or other components against each other prior to centrifugation. For balancing tubes, each platform should be equipped with a beaker or other container that will not tip easily, and the two containers should be of matching weight. In the instance where the weights don't match the balance should be zeroed with the containers on the pans.
1. Obtain two 250 ml glass beakers (covered) from Labware cabinet.
2. Place the beakers that are to be designated balance containers, onto the weighing platforms, one on each side.
3. Check to see that the zero pointer is negative to the zero line (i.e. zero pointer is in "-" position. If not, switch beaker positions.
NOTE: Usually when balancing containers, they are uncovered. But in this case, in order to acquaint you with balancing procedure they will remain covered since we will not be balancing sample tubes in this exercise.
4. Slide the 10 gm beam weight towards the right a little at a time until the pointer indicates zero. The beam balance is now zeroed to the containers, (i.e. the beam balance is reading zero with the containers on the weighing platforms).
5. Remove beakers from weighing platforms and return to Labware cabinet.
6. Slide the 10 gm weight back to the zero position.
7. Repeat this with two 250 ml plastic beakers.
EXERCISE #7: BALANCING TWO OBJECTS AGAINST EACH OTHER
When balancing one two objects against one another the beam weights are not used.
1. Obtain two 250ml plastic beakers next to the beam balance. These beakers are the designated balance containers.
2. Place the beakers onto the beam balance one on each weighing platform.
3. Zero the beam balance with the containers. (Refer to EXERCISE #6, steps 3-5.)
4. Obtain two 50ml conical plastic centrifuge tubes from the labeled drawer. Fill one to approximately 40ml with water, screw on its cap, and place it on the left platform container.
NOTE: When using squirt bottle take care not to touch tip of nozzle to any surface.
5. Place the other 50ml centrifuge tube, uncapped, in the container on the other platform. Also place its cap, inside surface laying face up, on the same platform; it must be included because the opposing tube is capped.
6. Add water (using a "squirt bottle") to the open tube until the pointer moves into the "+" position.
7. Using 3ml transfer pipette, slowly remove some of the water until a zero reading is obtained. The two tubes are now balanced.
8. When you are done remove the beakers from the weighing platforms.
9. Slide the 10gm beam weight back to zero position.
10. Discard 3ml transfer pipette into a biohazard waste container.
11. Take each of the 50ml centrifuge tubes and discard contents into sink.Place upside down in drying rack.
NOTE: In this case since deionized distilled water from a squirt bottle was used to fill up tubes, they can be reused. If, for instance, a sucrose solution were used the tubes would then be discarded in the biohazard receptacle.
12. Take the beakers and place into the bucket containing "micro" cleaning solution.
13. Clean up any spills on or around the beam balance.
A. USE OF AN ELECTRONIC BALANCE
An electronic balance is a high precision instrument designed to minimize weighing errors. In a bioresearch lab it is used primarily to weigh out ingredients during preparation of solutions. The chemicals are weighed out in "weighing dishes" made of disposable plastic, or on "weighing paper". The dishes are generally easier to use and permit greater accuracy.
The Tech Facility electronic balance has a "spirit" level located at the back of the unit, i.e., a small bubble that must be centered to level the instrument. The balance sits on "legs" whose length can be adjusted by turning them like screws. These aligning screws under bottom, front portion of the balance can be used to keep the balance level. For accuracy, this level should be checked periodically. An additional nice feature of this model is the draft cover over the weighing platform. Although this should block out drafts, it is still proper technique to eliminate any possible room drafts when weighing dry materials (see figure 2). There is a good chance that the electronic balance that you use in a research lab will not have such a cover.
When weighing dry materials, such as Sucrose and Sodium Chloride (sugar and salt), you should always use a weighing dish to hold the substance you are weighing. This leads us to the operation of Taring the balance.
1. Turn on the electronic balance by pressing the button marked On/Tare.
Nothing should be on the weighing platform when turning on the balance.
2. After a few moments, the digital display will read 0.000g.
(Remember your units of weight are in grams.)
The balance is now zeroed or Tared.
3. Raise the lid of the balance and place the object you would like to weigh in the Center of the weighing platform.
4. Lower the draft cover and wait for the reading to stabilize. (The balance has a stability indicator. This is the little circle in the upper left hand corner of the display.)
5. Once noted, the reading can be taken.
EXERCISE #1: WEIGHING DRY MATERIAL
Note: Before using the electronic balance, always check to see that weighing platform is clean and free of dust and "debris". If there is any residual substance on the weighing platform, DO NOT BLOW IT OFF. Use a dampened Kimwipe to gently wipe the surface clean, and then dry.
1. Obtain the following materials:
a. plastic (disposable) weighing dish
b. bottle of NaCl (Sodium Chloride; marked Tech. Fac.)
2. Before using the electronic balance, always check to see that nothing is on the weighing platform.
3. Turn on balance by pressing button marked ON/TARE.
4. After a few seconds, the balance will go on and the digital display will read 0.000 g. The balance is now tared.
5. Raise lid of balance and place weighing dish in center of weighing platform.
6. Lower lid and allow a few seconds for reading to stabilize.
Record this result in Tech. Fac. Notebook.
7. Raise lid and place two small scoops of NaCl onto weighing dish.
8. Lower lid and allow reading to stabilize. Record this result in Tech. Fac. Notebook.
9. To obtain the final weight of NaCl, subtract the result obtained in Step 6 from that obtained in Step 8. And record this in Tech. Fac. Notebook.
10. When completed, turn balance OFF.
11. Remove weighing dish and discard the NaCl by washing it down the sink.
12. Discard dish in biological waste container.
13. Place spatula in a tray filled with micro-cleaning solution.
14. Check to see that weighing platform is clean, and lid is lowered.
15. Clean area around balance.
1. Obtain the following materials:
a. weighing dish
b. bottle of NaCl (Sodium Chloride; marked Tech. Fac.)
2. Before using electronic balance, always check to see that nothing is on the weighing platform.
3. Raise lid of balance and place weighing dish on center of weighing platform.
4. Lower lid and turn on balance by pressing button marked ON/TARE.
5. After a few seconds, the balance will go on and the digital display will read 0.000 g. The balance is now tared to the weighing dish. (i.e. the balance will read zero with the weighing dish on it.)
6. Raise lid and place two small scoops of NaCl onto weighing dish.
7. Lower lid & allow reading to stabilize.
8. Record the result in Tech. Fac. Notebook.
9. When completed, turn balance OFF.
10. Follow clean-up procedures, Steps 11-15, in Part I of EXERCISE #1.
PART I and II of EXERCISE #1 illustrate two methods employed when measuring dry substances on the electronic balance. Though both procedures are good, the method outlined in PART II eliminates the step of calculating the final volume (as was done in PART I), and thus is the method of choice.