Md.Kausher ahmed Electrical department

Biomedical engineering Code:6875

Lesson declared The operation of biomedical measuring instrument

Learning outcomes After finished this lesson student will able to # Say. about Types and operation of dosimeter. # Say about LVDT. # Say about Thermistor operation.

Dosimeter A radiation dosimeter is a device that measures exposure to ionizing radiation. It has two main uses: for human radiation protection and for measurement of dose in both medical and industrial processes Personal dosimeters The personal ionising radiation dosimeter is of fundamental importance in the disciplines of radiation dosimetry and radiation

Continue,,,,, health physics and is primarily used to estimate the radiation dose deposited in an individual wearing the device. Ionising radiation damage to the human body is cumulative, and is related to the total dose received, for which the SI unit is the sievert. Workers exposed to radiation, such as radiographers, nuclear power plant workers, doctors using radiotherapy, those in laboratories

Continue,,,,, using radionuclides, and HAZMAT teams are required to wear dosimeters so a record of occupational exposure can be made. Such devices are known as "legal dosimeters" if they have been approved for use in recording personnel dose for regulatory purposes.Dosimeters can be worn to obtain a whole body dose and there are also specialist types that can be worn on

Continue the fingers or clipped to headgear, to measure the localized body irradiation for specific activities. Types Common types of personal dosimeters for ionizing radiation include

Continue,,,,, Electronic personal dosimeter (EPD) The electronic personal dosimeter (EPD) is an electronic device that has a number of sophisticated functions, such as continual monitoring which allows alarm warnings at preset levels and live readout of dose accumulated.

Continue These are especially useful in high dose areas where residence time of the wearer is limited due to dose constraints. The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times. MOSFET Dosimeter MOSFET dosimeters are now used as clinical dosimeters for radiotherapy radiation beams. The main advantages of MOSFET devices are:

Continue 1. The MOSFET dosimeter is direct reading with a very thin active area (less than 2 μm). 2. The physical size of the MOSFET when packaged is less than 4 mm. 3. The post radiation signal is permanently stored and is dose rate independent.

Fig : Dosimeter

Linear variable differential transformer

Continue LVDTs are robust, absolute linear position/displacement transducers; inherently frictionless, they have a virtually infinite cycle life when properly used. As AC operated LVDTs do not contain any electronics, they can be designed to operate at cryogenic temperatures or up to 1200 °F (650 °C), in harsh environments, under high vibration and shock levels. LVDTs have been

Continue,,,,,,,,,, widely used in applications such as power turbines, hydraulics, automation, aircraft, satellites, nuclear reactors, and many others. These transducers have low hysteresis and excellent repeatability. The LVDT converts a position or linear displacement from a mechanical reference (zero, or null position) into a proportional electrical signal containing phase (for direction) and

Continue,,,,,,,,,, amplitude (for distance) information. The LVDT operation does not require an electrical contact between the moving part (probe or core assembly) and the coil assembly, but instead relies on electromagnetic coupling.

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Thermistor A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush curren limiter, temperature sensors (NTC type typically), self-resetting overcurrent protectors, and self- regulating heating elements.

Continue Basic operation Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then: where, change in resistance, change in temperature

Steinhart–Hart equation In practice, the linear approximation (above) works only over a small temperature range. For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The Steinhart–Hart equation is a widely used third-order approximation:

Continue,,,,,,,,,, where a, b and c are called the Steinhart–Hart parameters, and must be specified for each device. T is the absolute temperature and R is the resistance. To give resistance as a function of temperature, the above can be rearranged into:

Continue where

Continue The error in the Steinhart–Hart equation is generally less than 0.02 °C in the measurement of temperature over a 200 °C range. As an example, typical values for a thermistor with a resistance of 3000 Ω at room temperature (25 °C = K) are:

Fig: Digital tachometer

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