Temperature & pH Sensors Presented by, Aarthi Balan

Contents Temperature sensors: Types of temperature sensors Working Applications pH sensors: What is pH? Working Applications

Temperature Sensor

Temperature Sensors  Contact sensors Measures their own temperature. Measures their own temperature. The temperature of the object to which the sensor is in contact is inferred by assuming or knowing that the two are in thermal equilibrium. The temperature of the object to which the sensor is in contact is inferred by assuming or knowing that the two are in thermal equilibrium.  Non contact sensors Measure the thermal radiant power of the Infrared or Optical radiation that they receive. Measure the thermal radiant power of the Infrared or Optical radiation that they receive. The temperature of an object is inferred from which the radiant power is assumed to be emitted. The temperature of an object is inferred from which the radiant power is assumed to be emitted.

Contact Temperature Sensors  Thermocouples Seebeck effect  Thermistors Thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature.  Liquid In-Glass Thermometers  Resistance Temperature Detectors (RTDs) Positive temperature coefficient of electrical resistance of metals.  Bimetallic Strip Thermometers Different metals expand at different rates as they warm up.

Bimetallic Strip Thermometers

Non contact Temperature Sensors  Radiation Thermometers (Pyrometers) Measures temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement. Measures temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement.  Optical Pyrometers Using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument. Using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument.  Fiber Optic Thermometers A temperature sensing component is placed on the tip of the fiber optic's "free end". The other end is attached to a measuring system that collects the desired radiation and processes it into a temperature value. A temperature sensing component is placed on the tip of the fiber optic's "free end". The other end is attached to a measuring system that collects the desired radiation and processes it into a temperature value.

Eye lens Field Stop Field Lens Exit Stop Red Filter Lamp Erecting Lens Range Filter Entrance Stop Objective Lens Figure - Optical system of DFP 2000 pyrometer The disappearing filament principle—optical system -

The disappearing filament principle-1. An operator sights onto a hot target, adjusts the range until its image is seen in red. The lamp filament is initially cooler than the target and its image appears as a darker red or black superimposed on the target’s image. Image of Filament (Cooler) Image of Hot Target What the operator sees when looking into the eyepiece; the target in red, its surroundings in black (cooler) or red (hot) and superimposed on the target, the filament. The view is circular because the optical system is made up of circular lenses, apertures etc.

The disappearing filament principle-2. The lamp current is raised until the image of the filament becomes hotter than the target and it appears as brighter red than the target. Image of filament (Hotter) Image of hot target Pointer indicating the center of the filament.

The disappearing filament principle-3. The lamp current is adjusted until the lamp filament’s brightness* temperature equals that of the target. The filament’s image blends into the image of the target. The filament “disappears”. * Brightness or radiance temperature is the temperature that a blackbody would have when it looks as bright as the target. It is almost always a lower temperature than the true temperature because of the effect of the target’s emissivity. However if the target is an object in a furnace or oven of about the same temperature, the true and brightness temperatures are very close to the same value. Also, if the target is in a cooler surroundings and has a relatively high emissivity, the difference between the true and brightness temperatures may be small. The difference for a wide range of conditions can be estimated from a table in ASTM Standard E1256.

Working of Temperature Sensors   In order to collect data from the sensors, the signals from the sensors need to be converted into digital numbers that the processor can handle.   The temperature sensor converts the temperature it is measuring to a voltage that corresponds to the temperature. The ADC measures the voltage and convert it to a digital number. The computer process it to calculate the temperature.   SMART Sensors Smart sensors incorporate microprocessors programmed to act as transceivers for bidirectional, serial communications between sensors on the manufacturing floor and computers in the control room.

Configuring and monitoring a network of smart sensors is easy with PC software and two-way communications. Marathon Support Software from Raytek Corp. can be used to maintain up to 32 sensors in a multi-drop environment.

Applications  Steel Industry was one of the first to use temperature sensors for automatic process control and QA measurements.  Personal computers, mobile phones, automobiles, medical equipment, and gaming consoles.  Freezing temperature alarm.  Freezer over-temperature alarm.  Micro power over-temperature alarm.

pH Sensors  What is pH? Definition: The negative logarithm of hydrogen ions in a solution.  Ratio of Hydrogen ions (H+) and Hydroxyl ions (OH-) determine the pH value of a solution.  By measuring pH a liquid can be categorized as either acidic, neutral or alkaline. The measurement is expressed on a scale of 0.0 to  Water with a pH of 7 is considered neutral (H+ ions = 10-7 and OH- ions =10-7). A solution is considered Acidic when the hydrogen ions (H+) exceed the Hydroxyl ions (OH-) and a solution is considered an Alkaline (base) when the Hydroxyl ions (OH-) exceed Hydrogen ions (H+).

Working of pH Sensors

Working  The pH measuring cell: Hydrogen sensitive glass is blown onto the end of an inert glass stem. The measuring solution has a neutral pH level of 7 or 0 mV. The glass sensor will produce a "Gel Layer" on the inside and outside of the glass membrane. The "Gel Layer" enables hydrogen ions to develop an electrical potential across the pH glass sensor and the millivolt signal varies with hydrogen ion activity on the glass membrane.  The Reference cell: The inert glass prevents hydrogen ion activity from test solutions to influence the reference cells constant millivolt signal. The reference voltage is used as a baseline to compare variations or changes in solution being tested.  Display meter: When the pH sensor is placed in a solution, the pH-measuring cell develops a millivolt signal that reflects the hydrogen ion activity of the test solution.  Display meter: When the pH sensor is placed in a solution, the pH-measuring cell develops a millivolt signal that reflects the hydrogen ion activity of the test solution.

Applications  Acid-base titrations.  Investigations of acid rain and buffering.  Investigations of water quality in streams and lakes  Monitoring pH change during chemical reactions  Monitoring pH changes in an aquarium as a result of photosynthesis.

References  Temperature sensors:  Ph sensors: ftp://ftp.pasco.com/manuals/English/CI/CI-6507A/ A/ A.pdf ftp://ftp.pasco.com/manuals/English/CI/CI-6507A/ A/ A.pdf

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