Speed, Temperature, and Pressure Transducers
Speed Transducers
The measurement of angular velocity in industry is commonly achieved using tachometers, which can be either mechanical or electrical. The measurement result directly expresses the angular velocity in revolutions per minute. Tachometers can measure the average speed over a time interval or the instantaneous velocity.
Mechanical Tachometers
A revolution counter uses a screw shaft coupled to the engine whose speed is to be measured. This screw is rotated through a gear system with two concentric calibrated dials, allowing the average angular velocity to be determined over a period of time. Centrifugal tachometers use two blocks that, due to centrifugal force, move away from the axis as angular velocity increases. This causes the compression of a spring attached to a device with a pointer that indicates the instantaneous angular velocity on a scale.
Electrical Tachometers
These transducers convert shaft speed into an electrical signal (e = KBW). Types include: T. Trawl, T. Induction, T. AC, T. DC, and T. Frequency.
Temperature Transducers
Thermorresistances (RTDs)
Thermorresistances consist of a wire whose resistance varies with temperature according to the expression: Rt = ro [1 + alpha (T-To)]. The measuring circuit used is usually a Wheatstone bridge, which consists of four resistors arranged in a box. Two non-consecutive vertices (A and D) are connected to the terminals of a generator, and the other two (B and C) are attached to each other, interspersed with a galvanometer (G), which is very sensitive. By changing the resistance value of the potentiometer, it can be achieved that no current passes through the galvanometer (balanced Wheatstone bridge). In a balanced Wheatstone bridge, the product of two opposing resistances is equal to the product of the other two: R1 = R2 Rx Rpt, which determines the value of resistance at 0 degrees.
Thermistors
Thermistors are variable resistors whose resistance changes with temperature, but they are made of semiconductors rather than metal conductors like RTDs. Thermistors can be NTC (Negative Temperature Coefficient) or PTC (Positive Temperature Coefficient). The measuring circuit generally used is a voltage divider, keeping the voltage V fixed. The voltage obtained at the output allows the temperature to be determined.
Thermocouples
Thermocouples are based on the Seebeck effect, which states that when closing a circuit with two different metallic conductors and maintaining a hot junction and a cold junction, an electrical current is established due to the electromotive force produced by the temperature difference. If the circuit is interrupted by one conductor and a millivoltmeter is inserted, it shows a potential difference that depends on the temperature difference between the hot and cold junctions.
Radiation Pyrometers
The function of radiation pyrometers is based on the Stefan-Boltzmann law, which can be stated as follows: The intensity of radiant energy emitted by the surface of a body is directly proportional to the fourth power of its absolute temperature: W = E (round) T Oo (high to 4).
Pressure Transducers
Mechanical Pressure Transducers
- U-tube Manometer: Used to measure atmospheric pressures and consists of a U-shaped tube usually containing mercury. One branch is open to the atmosphere, while the other is connected to the pressure to be measured. If the pressure is different in the two arms, there is a drop of mercury, and pressure is given by: p = patm + or – pgh. The positive sign corresponds to the case where the level of mercury in the open branch is higher than in the other, and negative if the opposite happens.
- Bourdon Tube: A tube of elliptic section closed at one end and curved in an almost perfect ring. By applying pressure to the fluid contained inside, the tube tends to straighten, transmitting the movement to a needle that moves on a graduated scale.
- Diaphragm: Consists of one or more capsules or circular diaphragms welded together at their edges. By applying pressure, each capsule is deformed, and the sum of all these small deformations is amplified and transmitted to a pointer.
- Bellows: Similar to a diaphragm, but consists of a flexible piece in the direction of its axis, which can expand or compress considerably due to the pressure.
Electromechanical Pressure Transducers
Electromechanical pressure transducers use an elastic mechanical element combined with an electric transducer that generates the corresponding electrical signal. Examples include resistive, capacitive, strain gauge, and piezoelectric transducers.