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TRIBHUVAN UNIVERSITY

INSTITUE OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

PULCHOWK CAMPUS

Experiment Setup for Low Pressure Measurement

(Assignment on EG851EE Instrumentation)

Date: Dec. 6, 2010

Submitted by: Submitted to: Santosh Sharma (066MSR515)

Prof. Dr. R. R. Staphit Department of Mechanical Engineering

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Measurement of Low Pressure using McLeod Gauge.

A. Objective of Experiment Intellectual Skills:

Able to understand different pressure measuring instruments. Able to understand working principle of McLeod gauge and analyze it from generalized

measurement system. Able to compare different pressure measuring instruments. Able to select appropriate pressure measuring instrument for different situations.

Motor skills

Ability to measure low pressure by using McLeod gauge. Ability to observe and record the reading. To be familiar with pressure measuring device.

B. Condition for your experiment

When low pressure measurement is required. (less than atmospheric pressure)

C. Instrumentation 1) McLeod Gauge 2) Low pressure System 3) Any other instrument available in lab to measure low pressure .

Specification : Pressure measurement tutor using McLeod gauge. Range : 0.01micron to 50 mm of Hg. Bulb, Reservoir, capillary should be made of transparent material. Provision of cut off and different valves at proper places. All Joints must be leak-proof..

D. Planning of experiment

McLeod gauges operate by taking in a sample volume of gas from a vacuum chamber, and then

compressing it by tilting, and infilling with mercury. The pressure in this smaller volume is then

measured by a mercury manometer, and, by knowing the compression ratio, the pressure of the

original vacuum can be determined. It should be noted that McLeod Gauge is an absolute

measuring device.

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E. Theory A slug of mercury moving in a tube is used to isolate a volume of gas at the pressure to be

measured. The gas in the volume is then compressed by a known amount, and the final pressure

is obtained with a manometer. Boyle's law is then used to find the initial pressure from the final

pressure, and the initial and final volumes. This is a relatively slow and laborious process, and a

liquid nitrogen cold trap must be used to prevent the vapor pressure of mercury from perturbing

the pressure measurements. However, the readings of the McLeod gauge are absolute, and it is

used to calibrate other gauges, such as ionization gauges.

Figure 1 Schematic Diagram of McLeod Gauge

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F. Stepwise procedure:

Locate the position in the system of which pressure has to be measured.

Connect unknown pressure source to the instrument at point 'P' as shown in figure 1.

Mercury level is adjusted to fill the volume represented by darker shading (Under these

conditions, unknown pressure fills the bulb B and capillary C. Volume of unknown

pressure is volume of bulb and capillary.)

Mercury is then forced out of reservoir D up into the bulb and reference column E.

Continue the forcing of mercury till it reaches to cutoff point F. (A known volume of gas

is trapped in the bulb and capillary).

Mercury level is then further raised until it reaches a zero reference point in E.

Volume remaining in the capillary is read directly from the scale.

The difference in the height of two columns is the measure of trapped pressure.

Initial pressure is then calculated by using equation P1 = P2V2 /V1 (Boyle's Law)

Carry out the same procedure for other readings.

G. Observations:

Volume of bulb= ------------------cm3

Volume of capillary = ------------------cm3

Initial volume = Volume of bulb + volume of capillary = ----------------- cm3

Area of tube, At = ------------------ cm2

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H. Observation Table:

S.No. Difference in height of

two columns (h) cm

Final Volume

V2 = At × h cm3

Initial Pressure in cm of Hg

P1 = P2V2/V1

I. Result: The pressure measured by McLeod gauge is ------------cm of Hg.

J. Possible errors in measurement

Accidental release of the mercury into the test chamber. The gauge is also easily contaminated. Deviation from Boyle’s Law due to the non-ideal nature of gas. The presence of condensable vapors, or absorption or desorption effects on the walls may

easily produce systematic errors. Use of clod trap in the tube connecting the McLeod gauge to the gauge to be calibrated

may lead to gain of pressure.