Liquid Bore & Elastic Bellows Manometers, PAM and SPM Uses
Liquid Bore Manometer (LBM)
Definition
A Liquid Bore Manometer (LBM) is a type of manometer used to measure small pressure differences of gases or liquids. It consists of a uniform narrow glass tube (bore) partially filled with a manometric liquid (usually mercury or a colored liquid).
Construction
- A long, uniform bore glass tube bent into a U-shape or inclined form
- Filled with manometric liquid
- One end connected to the pressure source
- Other end open to the atmosphere (or connected to another pressure source)
Working
When pressure is applied to one limb of the manometer, the liquid level falls in that limb and rises in the other limb. The difference in liquid levels (h) indicates the pressure difference.
P = ρ g h
Where:
- ρ = density of the manometric liquid
- g = acceleration due to gravity
- h = difference in liquid levels
Working Principle
The operation of the LBM is based on the hydrostatic principle: pressure applied to a confined liquid is transmitted equally in all directions.
Advantages
- Simple construction
- High accuracy for small pressure measurement
- No moving parts, hence low maintenance
- Direct reading possible
- Suitable for low pressure measurement
Disadvantages
- Not suitable for high pressure
- Reading errors due to parallax
- Bulky and fragile (glass tube)
- Slow response to fluctuating pressure
- Requires careful handling
Applications
- Measurement of gas pressure in laboratories
- Used in calibration of pressure gauges
- HVAC systems for air pressure measurement
- Fluid mechanics experiments
- Measurement of small differential pressure
Elastic Bellows Manometer (EBM)
Definition
An Elastic Bellows Manometer (EBM) is a mechanical pressure-measuring instrument that works on the principle of elastic deformation of a bellows when pressure is applied. It is mainly used to measure low and medium pressures.
Construction
- Metallic elastic bellows (corrugated cylindrical element)
- One end of the bellows is fixed; the other end is free to move
- Pressure inlet connected to the bellows
- Spring for restoring force
- Linkage and pointer mechanism
- Calibrated scale
Working
- Pressure to be measured is applied inside the bellows.
- Due to applied pressure, the bellows expand axially.
- The expansion causes movement of the free end of the bellows.
- This movement is transmitted through a linkage mechanism to a pointer.
- The pointer moves over a calibrated scale indicating pressure.
- When pressure is removed, the spring brings the bellows back to the original position.
Advantages
- Suitable for low pressure measurement
- Good sensitivity
- Compact and portable
- Can measure vacuum and positive pressure
- Faster response than liquid manometers
Disadvantages
- Limited to low and medium pressures
- Accuracy affected by temperature variation
- Mechanical wear over time
- Requires periodic calibration
- Not suitable for very high pressure
Applications
- Measurement of gas pressure
- Used in vacuum systems
- HVAC pressure measurement
- Process industries
- Pressure indication in laboratory instruments
Pulse Amplitude Modulation (PAM)
Definition
Pulse Amplitude Modulation (PAM) is a modulation technique in which the amplitude of regularly spaced pulses is varied in proportion to the instantaneous amplitude of the message (modulating) signal, while pulse width and position remain constant.
Working of PAM
- The message signal (analog signal) is applied to a sampler.
- A pulse train (carrier) is generated using a pulse generator.
- The sampler samples the message signal at regular intervals.
- The amplitude of each pulse is made proportional to the amplitude of the message signal at the sampling instant.
- The output consists of pulses whose heights vary according to the message signal.
Types of PAM
- Natural PAM – Pulse tops follow the message signal shape.
- Flat-top PAM – Pulses have flat tops; easier to process digitally.
Advantages of PAM
- Simple modulation technique
- Easy to generate and demodulate
- Suitable for digital communication systems
- Forms the basis for PCM and other digital modulation schemes
Disadvantages of PAM
- Highly susceptible to noise
- Requires large bandwidth
- Signal distortion due to noise and attenuation
- Not suitable for long-distance transmission
Applications of PAM
- Used in telephony systems
- Basis for PCM, PDM, and PWM
- Used in data communication systems
- Used in speech and audio signal processing
- Employed in multiplexing techniques
Special Purpose Machine (SPM)
Definition
A Special Purpose Machine (SPM) is a custom-designed machine developed to perform one specific operation or a set of similar operations on a workpiece with high accuracy, high productivity, and repeatability. Unlike general-purpose machines, SPMs are not flexible and are used mainly for mass or batch production.
Features of SPM
- Designed for a specific job only
- High production rate
- High accuracy and consistency
- Low cycle time
- Reduced manpower requirement
- Automation possible
Example of a Special Purpose Machine
Multi-Spindle Drilling Machine (SPM)
Construction
- Multiple drilling spindles arranged on a single head
- Electric motor with gearbox
- Fixture to hold the workpiece
- Control system (manual / semi-automatic / automatic)
Working
The workpiece is clamped on the fixture. When the machine is started, all spindles rotate simultaneously. Multiple holes are drilled at the same time in one cycle. After drilling, the spindles retract and the workpiece is removed.
Advantages
- Multiple operations in a single cycle
- High productivity
- Accurate hole spacing
- Reduced machining time
- Low labor cost
Applications
- Automobile components (engine blocks, gear housings)
- Pump bodies
- Machine tool parts
- Mass production industries
Limitations
- High initial cost
- Not flexible for design changes
- Suitable only for large-scale production