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SANDBATH


INTRODUCTION

Temperature is the physical property which provides information about the energy content of a system and thereby describes the heat energy content (degree of heat, heat status).

Temperature plays an important part in determining the conditions in which living matter can exist. Birds and mammals demand a very narrow range of body temperatures for survival and must be protected against extreme heat or cold. Aquatic species can exist only within a narrow temperature range of the water, which differs for various species. Thus, for example, the increase in temperature of river water by only a few degrees as a result of heat discharged from power plants may kill most of the native fish.

In fact, the properties of all materials are markedly affected by temperature changes. At arctic temperatures, for example, steel becomes very brittle and breaks easily, and liquids either solidify or become very viscous, offering high frictional resistance to flow. At temperatures near absolute zero, many materials exhibit strikingly different characteristics. At high temperatures, solid materials liquefy or become gaseous; chemical compounds may break up into their constituents.

TEMPERATURE MEASUREMENT
As with other physical quantities, temperature measurement begins with the definition of a unit. Historically, in the Celsius (centigrade) system the unit was based on the so-called ‘fundamental interval’ of 100 degrees between the melting point of ice and the boiling point of water, both at standard atmospheric pressure.
There are three temperature scales in use today, Fahrenheit, Celsius and Kelvin.
Fahrenheit (F) temperature scale is a scale based on 32 for the freezing point of water and 212 for the boiling point of water, the interval between the two being divided into 180 parts, the Celsius (C) temperature scale also called centigrade temperature scale, is the scale based on 0 for the freezing point of water and 100 for the boiling point of water, and the Kelvin (K) temperature scale is the base unit of thermodynamic temperature measurement in the International System (SI) of measurement. It is defined as 1/ 273.16 of the triple point (equilibrium among the solid, liquid, and gaseous phases) of pure water. The kelvin (symbol K without the degree sign [o]) is also the fundamental unit of the Kelvin scale, an absolute temperature. Such a scale has as its zero point absolute zero; the theoretical temperature at which the molecules of a substance have the lowest energy. The Kelvin scale has been adopted as the international standard for scientific temperature measurement. The Kelvin scale is related to the Celsius scale. The difference between the freezing and boiling points of water is 100 degrees in each, so that the Kelvin has the same magnitude as the degree Celsius.

The relationship between the temperature scales
[°F] = [°C] × 9⁄5 + 32
[°C] = [K] − 273.15
[K] = ([°F] + 459.67) × 5⁄9

Accurate temperature measurement is very important especially in today’s highly automated industries. Temperature measurement in today’s industrial environment encompasses a wide variety of needs and applications. To meet this wide array of needs, the process controls industry has developed a large number of sensors and devices to handle this demand.

Temperature is a very critical and widely measured variable for many industrial processes which must have either a monitored or controlled temperature. This can range from the simple monitoring of the water temperature of an engine or load device, or as complex as the temperature of a weld in a laser welding application. More difficult measurements such as the temperature of smoke stack gas from a power generating station or blast furnace or the exhaust gas of a rocket may be need to be monitored.

Much more common are the temperatures of fluids in processes or process support applications, or the temperature of solid objects such as metal plates, bearings and shafts in a piece of machinery. Thus, today’s industrial processes rely on a wide variety of temperature measurement devices with varying ranges. There are a wide variety of temperature measurement probes in use today depending on process is being measured, how accurately it needs to be measured, if it is to be used for control or just man monitoring, or even if the process can be safely handled physically. For an objective and reproducible measurement of the temperature of a body, a suitable measurement instrument is required. Examples of these industrial temperature measurement devices are thermistor, resistance temperature device (RTD), thermocouple and temperature switches etc.

TEMPERATURE MEASURING DEVICE

Temperature measuring devices are devices used to measure the temperature of a medium. There are two kinds on temperature sensors: contact sensors and noncontact sensors.
Contact Sensors: Contact temperature sensors measure the temperature of the object to which the sensor is in contact by assuming or knowing that the two (sensor and the object) are in thermal equilibrium, in other words, there is no heat flow between them. Examples are Thermocouples, Resistance Temperature Detectors (RTDs), Full System Thermometers, Bimetallic Thermometers.

Non-Contact Sensors: Most commercial and scientific noncontact temperature sensors measure the thermal radiant power of the Infrared or Optical radiation received from a known or calculated area on its surface or volume within it. An example of noncontact temperature sensors is a pyrometer.

SAND BATH CALIBRATOR

A sand bath is a piece of laboratory equipment made from a container filled with heated sand.

Exceptional temperature stability and uniformity make fluidized baths the ideal choice for critical heat treatment procedures. Fluidized Baths are used for laboratory, industrial, process, quality control and instrument shop which provides rapid heat transfer and precise temperature control, enabling calibration and maintenance of temperature sensitive instruments efficiently, economically, and safely. Fluidized Baths offer outstanding advantages of being dry, inert and non-corrosive, as well as being non-abrasive to anything placed in them.

Aluminium oxide particles serve as the heat transfer medium and have no effect on shape or size of immersed objects. This medium consists of a loosely-packed mass of solid particles which are agitated by a vertical flow of gas. In the fluidized state, the aluminium oxide particles become mobile and the bath as a whole displays many of the properties of a liquid. Visually when fluidized, the aluminium oxide looks like liquid boiling vigorously or molten lava bubbling. The bed of levitated articles presents a very large surface area through which heat is transferred to immersed objects.

Fluidized solids have no melting or boiling point, thus solidification which occurs in cooling salt baths and fumes from hot oil baths are eliminated. Heat transfer characteristics between the fluidized bed and the solid interface are similar to those of an agitated liquid - the key to fluidized bath calibration efficiency.

BLOCK DESCRIPTION:

Master Sensor: The master sensor is the temperature sensing element that monitors the temperature of the sand bath during calibration and sends its output to the microcontroller through the analog-to-digital converter.

ADC: The analog-to-digital converter converts analog signals (continuous quantity) into digital signals (discrete time digital representation). The analog signal is a continuous sinusoidal wave form that cannot be read by the microcontroller, hence the need for conversion. By converting the analog signal, data can be amplified, added or taken from the original signal.

Microcontroller: The microcontroller (AT89C52) takes the digital signal from the output of the analogue to digital converter (ADC), processes it and displays the corresponding value on the display circuit.

Display: The display used for this design is the liquid crystal display (LCD). It displays whatever has been processed by the microcontroller. The display also indicates when the processes are above or below the set point designed for each process.

Switching Circuit: The switching circuit switches the sets of relays that powers the heaters depending on the signal received from the controller on the selected mode of operation of the sand bath.

Heater: The heater converts electrical energy to heat energy which is transported by the air from the fan to the sand to raise the temperature to the required level. The heater is powered from the sets of relays.

Air Source: The air is drawn in from the surrounding by the fan and transports the  heat from the heater to the sand and also acts as the agitating means that maintain even heat distribution in the sand bath.

Instrument Under Test: The instrument under test is that which we wish to calibrate using the sand bath calibrator example Thermocouple, RTD etc.

Sand: The Sand is the medium that retains the heat from the heater needed for calibration of the instrument under test. It is in constant agitation by the fan during the period of calibration for even heat circulation.

CALIBRATION

Calibration may be defined, in general, as the process for determination, by measurement or comparison with a standard, of the correct value of each scale reading on a meter or other measuring instrument; or determination of  the settings of a control device that correspond to particular values of voltage, current, frequency, pressure, flow or some other output.

CALIBRATION METHOD

Two types of methods are used for calibration of temperature sensors, one with fixed point cell and another by comparison method.

Comparison: In this method highly accurate PRT, T/C or any other standard (if required) is taken, both standard and UUC (unit under calibration) are kept at the same thermal environment after stabilization of the calibration bath readings of standard and UUC are taken and by comparison of standard and UUC, deviation is found.

Fixed Point Cell: For the very highest accuracy, comparison calibration is replaced by primary or fixed point calibration, fixed points cells are designed to realize the liquid-solid equilibrium temperatures of certain high purity metal elements, for calibration of thermometers at the ITS-90 fixed points.

PRE-REQUISITE FOR TEMPERATURE CALIBRATION

Temperature Calibration provides a means of quantifying uncertainties in temperature measurement in optimize sensor and/or system accuracies. In Temperature calibration three (3) basic things are required viz:

  • Temperature sensor
  • Measuring instruments and
  • Temperature source

Temperature Sensor: For lower temperature calibration, Platinum Resistance Detectors is used. These are very stable and accurate. For higher Temperature calibration, Noble Metal Thermocouple is used like R, S type thermocouple. These sensors must have good Accuracy and repeatability.

Measuring Instrument: These instruments measure the output of the sensors, they must have high resolution and good accuracy.

Temperature Source: There are two types of sources used for calibration they are secondary source and primary source. Fix point cell is a primary source while stirred liquid bath, dry block bath, fluidized bath etc. are secondary sources. These entire sources must have good stability and homogeneity.


CALIBRATION FREQUENCY:

Temperature sensors can be calibrated on following basis:

Manufacturer Recommended Calibration Interval: Every Manufacturer specifications indicate how often their instruments are to be calibrated, but instruments for critical measurements may require different intervals.

Before A Major Critical Measuring Project: Instruments used for any critical and highly accurate measurements are required to be calibrated before that measuring test to get accurate results.
After A Major Critical Measuring Project: Instruments kept for a particular critical test are required to be tested after completion of task to know whether the instruments are still reliable or repair is required.

After An Event: Sometimes if the instrument hits or something knocked out the internal overload or the unit absorbed a particularly sharp impact, it should be calibrated to check the safety integrity.
Per Requirements: Some measurement jobs require calibrated, certified test equipment — regardless of the project size.

Monthly, Quarterly: In cases where mostly critical measurements are done a regular calibration is required because a shorter time span between calibrations means less chance of questionable test results.

Annually: In cases where a mix of critical and non-critical measurements are taken annual calibration tends to strike the right balance between prudence and cost.

Biannually In cases where less critical measurements are done then calibration should be done biannually or after long duration can be cost-effective

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