Regents Prep: Living Environment: Laboratory: Measurement

Volume Measurement
A commonly used instrument to measure liquid volume is the graduated cylinder.
This instrument usually measures liquid volume in milliliters (ml).

 Using a Graduated Cylinder It is important to remember to read to the bottom of the curved line or meniscus when measuring solutions involving water or most liquids.   The graduated cylinder at the left is divided into increments of 2 ml, so the volume in it is 12 ml.    The graduated cylinder on the right is divided into increments of 1 ml, so the volume in it is 16 ml.

Mass Measurement
The triple beam balance is commonly used to measure mass in the biology lab. This device is named for its three long beams on which sliding bars called riders (or tares) are used to determine the mass of an object placed on its platform.    It is very important that the riders on the rear beams are in the notch for the whole number of grams and not in between notches. The front beam is a sliding scale graduated in grams. The rider on this beam can be positioned anywhere on the scale. Masses on a triple-beam balance can be read to tenths of a gram and estimated to hundredths of a gram.

 Using the Triple Beam Balance The picture at the upper left shows two different models of triple beam balances commonly used in the biology laboratory.    The picture at the lower left shows the measurement of a mass in progress.    Without estimation, the mass of the object appears to be 373.3 grams (g).

Length Measurement
Most measurements in biology will involve metric units of measurement.  It is good to start at a whole number increment that isn't 0.   Many times the end of a ruler will be worn away by student/teacher use or is inaccurate due to the manufacturing process.   It is important to remember to take away the whole number increment one has moved in on the ruler (in the example below 1 cm) from the measurement obtained.

 Using a Ruler to Measure Length Problem:  How long is leaf A? The tip of the leaf is at about 6.5 cm, but note the measurement started at 1 cm.   Therefore, Leaf A is 5.5 cm or 55 mm. in length.

Microscopic Measurement
The magnifying power of most objectives and oculars is engraved on them. On the ocular, the marking can be found on the top edge or on the smooth cylinder that fits inside the body tube; on the objectives, magnification is on the side of the cylinder. For example, a marking "10x" means that the particular lens forms an image ten times larger than the object being viewed.  The total magnification of a microscope is equal to the power of the eyepiece (ocular) X power of the objective used.   For example, if a student is using a microscope with a 10 X ocular and a 43 X high power objective, the total magnification of the specimen the student is viewing is equal to 10 X 43 or 430 X (times).

Formula for Total Microscope Magnification

 Total magnification = Power of the eyepiece x Power of the objective

The size of a microscopic field of view can be determined on low power using a device called an optical micrometer.   An economy version of this can be made by placing a clear metric ruler on
the stage of a microscope and using it to estimate the field of view.  The light microscope is used to look at cells or other similarly sized microscopic objects, so small units of measure such as millimeters or micrometers are used.  It is important to remember that there are 1,000 micrometers in 1 mm (millimeter) and 1000 millimeters in a meter.

 Finding the Size of a Microscope Field of View In the pictured field of view at the left, it can be observed that there are approximately 3 1/2 divisions equal to a length of 3.5 mm.   Therefore this field of view is equal to 3.5 mm or 3,500 micrometers.

 Finding the Size of Multiple Cells in a Field of View The two cells in this field take up a field of view of one millimeter.  Therefore, the size of the specimen is equal to 1 mm/2 cells or 0.5 mm per cell.  There is 500 micrometers in 0.5 mm., so the average size of each cell is 500 micrometers.

 Estimating Cell Size When the Field of View is Known It is often difficult to approximate the approximate size of the field of view, but this ameba considered lengthwise appears to occupy approximately 1/3 of the field of view.   The field of view in the left image is 3 mm.   Given that the ameba in the image takes up about 1/3 of that field, we can find its approximate length by multiplying the 3 mm  X 1/3 = 1 mm length or 1,000 micrometers for the approximate length of this ameba. The student is viewing the same ameba in the field of view at the right on a higher power.   The field of view gets smaller which makes the ameba appear larger in this field.

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