Construction

It is generally made of wood, consists of a graduated rule and two arms. One is fixed at rt. angles to one end of the rule and the other movable parallel to the fixed rule. Size more than 120cm in length is rarely used. The use depends on the desired accuracy. Callipers marked are painted to differentiate the reading. In some callipers, the arms are made parallel by screw adjustment. Metal callipers made of alluminium alloy are in use. They are not heavier than wooden callipers and easy to keep clean & adjustment.

Types

1. Flurry Callipers: Arms from aluminium and graduated from wooden and bound with brass of sizes available in lengths of 80, 60, 35cms
2. Fommes Calipers: All aluminum and same size as Flurry.
3. Indian Aluminum Callipers: They are variable in the length of 50 cm, 75cm & 100 cm & they are graduated to show cm & mm.

Uses

To measure diameter of standing trees and logs

Advantages

• Diameters can be reading the directly in cm and mm thus making the instrument applicable for precise scientific work
• The points arms touching the tree are always in sight and irregularities if any can be avoided
• By firmly pressing the arms against the tree bole, the loose swollen bark is crushed out and irregularity from this source is avoided
• It is adaptable for use by unskilled labor
• The errors are both +ve and -ve and therefore the chances are that they may neutralize to give more accurate results than the tape which gives only +ve errors.

Disadvantages

• They are not accurate when not in adjustment
• Callipers sufficient in size to measure large trees are very awkward to carry and handle.
• Two measurements have to be taken on every tree to get the correct diameter. In steep hilly terrain, measurement of second diameter in correct orientation is often very difficult
• Movable arm often sticks when the scale is wet or dirty, thus wasting a lot of time.

Precautions

• The callipers must be placed on the tree with movable arms well-opened and must not be forced on the tree, there by causing stress or damage to the arms.
• The reading must be taken before the callipers is removed from the tree
• If the cross-section of stem is more or less elliptical, it is necessary to measure two diameter which corresponds to the major and minor axes of ellipse. Since axes of an ellipse are at right angles to each other, diameter is then understood to the average of two measurements

Suppose, d₁ and d₂ are two diameters of elliptical cross section of tree, basal area= (π/4) d₁x d₂

Calipers must be placed at right angles to the axis of the tree. If the plane of the calipers is inclined by an angle A from the horizontal plane of tree with diameter D the measured diameter becomes DsecA.

Numerical

If angle formed by the calipers is 60 degree of diameter measuring 30cm. Calculate the error evolved and percentage error.

Solution:

• A=60 degree; D=30cm
• Error involved = d (secA-1) = 0.3 (sec60°-1) = 0.3 (2-1) = 0.3m
• Percentage involved =100(sec60°-1) = 100%