VERNIER CALIPER
An ordinary vernier caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
The basic steps are as follows:

1. Preparation to take the measurement, loosen the locking screw and move the slider to check if the vernier scale works properly. Before measuring, do make sure the caliper reads 0 when fully closed. If the reading is not 0, adjust the caliper’s jaws until you get a 0 reading. If you can’t adjust the caliper, you will have to remember to add to subtract the correct offset from your final reading. Clean the measuring surfaces of both vernier caliper and the object, then you can take the measurement.
2. Close the jaws lightly on the item which you want to measure. If you are measuring something round, be sure the axis of the part is perpendicular to the caliper. Namely, make sure you are measuring the full diameter. An ordinary caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
3. How to read the measured value:

1), Read the centimeter mark on the fixed scale to the left of the 0-mark on the vernier scale. (10mm on the fixed caliper)
how to use vernier calipers
how to use vernier calipers

2). Find the millimeter mark on the fixed scale that is just to the left of the 0-mark on the vernier scale. (6mm on the fixed caliper)
how toread vernier calipers
how toread vernier calipers


3). Look along the ten marks on the vernier scale and the millimeter marks on the adjacent fixed scale, until you find the two that most nearly line up. (0.25mm on the vernier scale)
how to use vernier calipers
how to use vernier calipers

4). To get the correct reading, simply add this found digit to your previous reading. (10mm + 6mm + 0.25mm= 16.25 mm)

hot toread vernier calipers
hot toread vernier calipers

4.Maintenance
Clean the surface of the vernier caliper with dry and clean cloth (or soaked with cleaning oil) and stock in a dry environment if it stands idle for a long time.
http://www.tresnainstrument.com/how_to_read_a_vernier_caliper.html


MICROMETER SCREW GAUGE

The micrometer screw gauge is used to measure even smaller dimensions than the vernier callipers. The micrometer screw gauge also uses an auxiliary scale (measuring hundredths of a millimetre) which is marked on a rotary thimble. Basically it is a screw with an accurately constant pitch (the amount by which the thimble moves forward or backward for one complete revolution). The micrometers in our laboratory have a pitch of 0.50 mm (two full turns are required to close the jaws by 1.00 mm). The rotating thimble is subdivided into 50 equal divisions. The thimble passes through a frame that carries a millimetre scale graduated to 0.5 mm. The jaws can be adjusted by rotating the thimble using the small ratchet knob. This includes a friction clutch which prevents too much tension being applied. The thimble must be rotated through two revolutions to open the jaws by 1 mm.
external image vfig10a.jpg
Figure 10: The micrometer screw gauge
In order to measure an object, the object is placed between the jaws and the thimble is rotated using the ratchet until the object is secured. Note that the ratchet knob must be used to secure the object firmly between the jaws, otherwise the instrument could be damaged or give an inconsistent reading. The manufacturer recommends 3 clicks of the ratchet before taking the reading. The lock may be used to ensure that the thimble does not rotate while you take the reading.
The first significant figure is taken from the last graduation showing on the sleeve directly to the left of the revolving thimble. Note that an additional half scale division (0.5 mm) must be included if the mark below the main scale is visible between the thimble and the main scale division on the sleeve. The remaining two significant figures (hundredths of a millimetre) are taken directly from the thimble opposite the main scale.
external image vfig11a.jpg
Figure 11: The reading is 7.38 mm.
In figure 11 the last graduation visible to the left of the thimble is 7 mm and the thimble lines up with the main scale at 38 hundredths of a millimetre (0.38 mm); therefore the reading is 7.38 mm.
external image vfig12a.jpg
Figure 12: The reading is 7.72 mm.
In figure 12 the last graduation visible to the left of the thimble is 7.5 mm; therefore the reading is 7.5 mm plus the thimble reading of 0.22 mm, giving 7.72 mm.
external image vfig13a.jpg
Figure 13: The reading is 3.46 mm.
In figure 13 the main scale reading is 3 mm while the reading on the drum is 0.46 mm; therefore, the reading is 3.46 mm.
external image vfig14a.jpg
Figure 14: The reading is 3.56 mm.
In figure 14 the 0.5 mm division is visible below the main scale; therefore the reading is 3.5 mm + 0.06 mm = 3.56 mm.
Try the following for yourself.
external image vfig15a.jpg
Figure 15: The reading is 5.80 mm
external image vfig16a.jpg
Figure 16: The reading is 3.09 mm
external image vfig17a.jpg
Figure 17: The reading is 0.29 mm
Taking a zero reading
Whenever you use a vernier calipers or a micrometer screw gauge you must always take a zero reading i.e. a reading with the instrument closed. This is because when you close your calipers, you will see that very often (not always) it does not read zero. Only then open the jaws and place the object to be measured firmly between the jaws and take the open reading. Your actual measurement will then be the difference between your open reading and your zero reading.
Recording the result of your vernier measurement
Let us say you take a reading with an object between the jaws of a vernier calipers and you see the following:
external image vfig04a.jpg
Say that you decide that the best estimate of the reading l 1 is 37.46 mm.
What about the standard uncertainty u(l1) in this reading?
Using a triangular probability density function, you might decide that you are 100% sure that the reading is not 37.42 mm and 100% sure that the reading is not 37.50 mm.
Then
eq1
eq1
mm = 0.0163 mm
When you remove the object and read the vernier calipers with the jaws closed, you might decide that the best estimate of the "closed" reading l0 = 0.04 mm with standard uncertianty u(l 0) = 0.0204 mm
What should you then record as the best estmate of the length of the object you are measuring?
The best estimate of the length l = l 1 - l0 = 37.46 - 0.04 = 37.42 mm
with a standard uncertainty
eq2
eq2
=
eq3
eq3
= 0.0261 mm
Therefore l = 37.420 ± 0.026 mm (65% level of confidence).
from http://www.complore.com/using-vernier-calipers-and-micrometer-screw-gauge-0