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Measuring Temperature of Fluids

Temperature is the most important variable in milk processing. It is the thermal state of the product that determines whether the physical conditions of the manufactured product are correct or whether the desired chemical reaction will take place or not.

i. Temperature scales

A temperature scale represents the temperature of a body quantitatively. Designating two thermal equilibrium points by numbers forms the temperature scale. The equilibrium point of ice and water at standard pressure is called ice-point. The equilibrium point between water and steam at standard pressure is called steam point. There are a number of temperature scales each assigning a different unit values to these points.

The Fahrenheit Scale (abbreviated as °F) – This scale assigns 32° F to the ice point and 212 F° to the steam point.The Centigrade Scale (abbreviated as °C) - Also known as Celsius Scale. This scale assigns 0° C to the ice point and 100°C to the steam point. This scale is commonly used in scientific calculations.

The Kelvin scale (abbreviated as °K) - Also known as Absolute Celsius Scale.This scale assigns 273°K to the ice point and 373°K to the steam point. This scale is widely used in technical calculations.

The interrelations between these scales can be worked out as below:

The equation relating Kelvin and Celsius scale is:

°K = °C + 273

The relationship between Celsius Temperature and Fahrenheit Temperature is:

°C = (5/9) (F- 32)

ii. Indicating Thermometers

Glass Thermometer: The mercury in glass thermometer is one of the simplest temperature measuring devices.

Principle of operation: It utilizes the volumetric expansion of mercury with temperature as a mean of indicating temperature.

Construction: Construction of a thermometer

It has a bulb formed by a glass envelope. This bulb contains mercury. Bulb is attached to the stem, which contains a fine capillary tube in it. Bulb of the thermometer is inserted into the hot medium whose temperature is to be measured. As the heat is transferred from hot medium into the mercury in the bulb, the mercury expands. This expansion pushes a thread of mercury into the capillary. The glass of the thermometer is generally shaped as to magnify the apparent width of the thread of mercury. A temperature scale is put on the stem of thermometer, which indicates the temperature being measured.

Installation: The industrial thermometer is installed in such a way that it is protected from damage or breakage. The bulb of thermometer is inserted in a metal thermal well. The heat is transferred through the metal well into the bulb. A metal scale is mounted behind the upper end of thermometer and glass cover is provided over the scale. This type of installation provides a complete protection to the thermometer and makes it strong and rugged. Thermal well is made up of brass, steel or aluminum. Space between bulb and the well is filled up with a conducting medium such as mercury or oil, to increase the rate of heat transfer. The thermometer bulb is installed in such a manner that it is immersed to a sufficient length in the medium.This is to attain temperature equilibrium with the medium whose temperature is to be measured.

Pressure-Spring Thermometer: Based on the principle of operation and the material used this thermometer could be of any of the following types:

Principle of Operation

a) Liquid Expansion Thermometer: - It utilizes the volumetric expansion of liquid caused by temperature changes to measure the temperature.

b) Gas Expansion Thermometer: - It operates on the principle that the pressure of gas varies directly as the temperature, if the volume is kept constant.

c) Vapour Pressure Thermometer: - It operates on the principle that the ‘vapour pressure’ of a liquid increases with temperature.

Expansion Material: The commonly used expanding material in the liquid expansion type thermometer is ethyl alcohol. In gas expansion thermometer it is nitrogen gas and in vapour pressure thermometer it is ethyl ether. Of all the above thermometers the vapour pressure thermometer is widely used because it is less costly and simpler to maintain.

Construction: The construction of all of the above mentioned pressure spring thermometers are similar. The basic construction of these thermometers. It consists of a bulb which contain either a liquid or gas or liquid-vapour fluid. A metal capillary is connected to the bulb on one end and to a receiving element at the other end. The receiving element is usually a bourdon tube or pressure spring. A pointer is attached to the pressure spring through appropriate linkage. The whole system containing bulb, capillary and pressure spring is a sealed unit.

Working: The bulb of thermometer is inserted into the medium whose temperature is to be measured. The bulb comes in thermal equilibrium with the medium and transfer heat to the fluid inside the bulb. With this heat a pressure is developed in the fluid and the capillary connected to the bulb transfers this pressure to the receiving element, that is, the pressure spring. The pressure spring converts this pressure into a motion that moves the pointer on a scale to indicate the temperature.

Installation: The bulb of pressure spring thermometer is installed with a thermal well. The bulb and the capillary are made up of stainless steel or copper. For protection against damage the capillary is enclosed in another tubing or protective covering.

iii. Electric Temperature Indicators

Amongst the electrical temperature indicators the electrical resistance thermometer is being widely used in recent years in industries because of its accuracy and simplicity. It also makes it possible to detect very small increments of temperature to be detected. An electrical resistance thermometer is useful in wide range of temperature limits, that is, -180° to 650° C.

Principle of operation: The resistance thermometer is based on the principle of change in electrical resistance of a substance with temperature. In metals, the electrical resistance increases with the increase in temperature, where as, in semiconductor materials their resistance decreases with increase in temperature.

Construction: An electrical resistance thermometer bulb could be made up in many forms. The element may be made up of a strip of very thin foil or a coil of very fine wire wound on a frame. The industrial resistance thermometer is of a probe type . The connection to the wires of the resistance bulb is carefully made.

Material: Usually the material of a resistance thermometer is metal but non-metallic material may also be used. The industrial resistance thermometer employs platinum, nickel or copper. In processing industries the platinum-resistance element is used.The accuracy of a resistance thermometer is better than the expansion thermometers.Measurement of temperature with resistance thermometer reduces to the measurement of electric resistance and the techniques of resistance measurement are well advanced.

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