Pressure and Temperature Measurement Instruments
Pressure Gauges
A gauge is an instrument used for measuring fluid pressure, typically the difference between fluid pressure and local mechanical pressure. Pressure is defined as the force per unit area exerted by a liquid or gas perpendicular to a surface.
U-Tube Manometer
 | Pressure gauges are instruments designed to measure pressure. A traditional method for accurate pressure measurement involves a U-shaped glass tube containing a liquid of known density. Mercury is commonly used for high-pressure measurements to keep the tube size reasonable. However, for low pressures, a mercury U-tube gauge lacks sensitivity. This type of gauge measures the pressure difference between the two ends of the tube by measuring the difference in height (length) of the liquid columns. |
Absolute Pressure Gauges
Absolute pressure gauges consist of a set of bellows and a spring opposing a sealed absolute vacuum. The resulting motion from the interaction of the bellows and spring corresponds to the absolute pressure of the fluid. The bellows are typically made of brass or stainless steel. These gauges are used for accurate measurement and precise control of low pressures, which can be affected by changes in atmospheric pressure. For example, when using a vacuum gauge to maintain an absolute pressure of 50 mm of mercury in a distillation column, the set point would be 710 mm, assuming an atmospheric pressure of 760 mm. If the atmospheric pressure changes to 775 mm, the vacuum gauge would read 710 + 15 = 725 mm. The absolute pressure in the column would then be 50 + 15 = 65 mm, which is 30% higher than desired.
Bimetallic Thermostats
Bimetallic thermostats consist of two bonded metal sheets with different coefficients of thermal expansion. When the temperature changes, the combined strip bends, acting upon contacts to close or open a circuit.
These thermostats can be normally open or normally closed, changing state when the temperature reaches a predetermined level.
Manual Thermostats
Manual thermostats require human intervention to reset after activation, such as safety thermostats that trigger a function when the temperature reaches dangerous levels.
Automatic Thermostats
Automatic thermostats return to their initial state without human intervention, operating fully automatically, making them common in many households.
Gas Thermostats
Gas thermostats consist of a gas sealed inside a copper tube. As the temperature rises, the gas expands, pushing a valve that performs a specific function.
Wax Thermostats
Wax thermostats are used to control fluid valves. They contain encapsulated paraffin wax that expands with increasing temperature, pushing a disc that allows fluid passage. When the fluid temperature decreases, a spring returns the disc to its original position, closing the passage. A common example is the thermostat used in the cooling system of internal combustion engines.
Electronic Thermostats
Electronic thermostats are increasingly prevalent due to several advantages:
- They can be designed without moving parts and contacts, which are susceptible to damage.
- They can be configured for both temperature thresholds and minimum times between activations.
- They can be easily integrated into systems with additional features, such as timers and other event triggers.
- With a PID controller, they can provide more intelligent temperature management.
Electronic thermostats can improve upon applications where mechanical thermostats are traditionally used:
- In refrigerators, they can prevent compressor startup due to brief temperature increases, such as when the door is opened.
- In vehicle cooling systems, they can control electric pumps to optimize energy consumption by varying pump speed based on demand.
- In homes, they can be integrated with schedules based on time, day of the week, or other events for increased efficiency.
The temperature-sensing element can be an infrared system or other technologies, but the most common is a thermistor.
Thermistors
Thermistor-based thermostats utilize a thermistor, a device whose impedance changes with temperature.
A control system, typically based on a microprocessor, reads the thermistor’s impedance and is programmed to perform specific actions at certain temperatures.
Numerous electronic thermostat variations exist, but the thermistor is the most common temperature-sensing component. Older versions used gas-filled sensors. Generally, any electronic temperature sensor, such as platinum resistors or semiconductor sensors, can be integrated into a thermostat.
Uses of Thermostats
Thermostats can be used in various devices as sensors within a feedback control system.