Abstract
This article discusses a variety of methods which are used in the day to day measurements that are taken in the field of engineering. This is important in familiarizing the engineers with the respective methods, which are unavoidable in the design of any component that requires utter precision. Therefore, this discussion promotes quality in engineering products by advocating for accuracy through precision in measurements.
Introduction
Measurement is the length, amount or size of something, as determined by taking the measure of the respective part or object (Diley & McConnell). It is enabled by the laws of physics which make certain materials behave in specific ways under certain conditions. To determine the measurement of an object, the amount or size of an object must be compared to certain values. Measurement is taken in the form of particular groups of units which are recognized within certain geographical regions. However, there also exists the standard units which are recognized worldwide and are more widely used (Diley & McConnell).
In this article, various methods of measuring different elements such as temperature, fluid pressure and liquid concentration will be discussed. Every method of measurement has its own strengths and weaknesses, as well as advantages and disadvantages over other related methods. Therefore, this article puts into detail the features of each method while at the same time analyzing the underlying physics and comparisons between the respective methods.
Temperature
Temperature is the degree of hotness or coldness of a body or a place. It is measured by sensors which operate either through contact or radiation, depending on the type of instrument. Contact sensors usually assume thermal equilibrium with the element of interest, while non-contact sensors operate through thermal radiant power.
Fluid thermometer
Fluid thermometers usually make use of thermal expansion of liquids. However, not all liquids can be thermometric. Certain features have to be met for a liquid to be considered thermometric. These include opaqueness for better visibility, the ability not to stick to glass and the regular expansion and contraction with respect to temperature change. The respective liquid expands along a column of the thermometer with rising temperature and contracts when the temperature drops. This column is preferably as narrow as possible for the purpose of greater accuracy. The apparent height of the column is measured with respect to a graduated scale (Kirk).
∆l=∝lo ∆T where ∆Tis the change in temperature, ∆l is the change in height of the liquid column, and ∝ is the coefficient and lo is the initial length before the temperature change (pg. 10). Fluid thermometers exist in a number of varieties, for example, clinical thermometer, mercury-alcohol thermometer, and the indoor-outdoor thermometer. The clinical thermometer has a constriction unlike the rest, which enables measurements to be taken easily when the body is withdrawn. This makes it useful especially for medical purposes. The mercury-alcohol thermometer on the other hand, utilizes mercury and alcohol as thermometric liquids at the same time while also making use of two small magnetic metal plates that are adjusted to maximum or minimum depending on the rise or fall of the liquids with respect to change in temperature (pg. 12).
Fluid thermometers, however, have a few drawbacks. The reading is hugely dependent on the position of the observer from the glass. Furthermore, it must be immersed properly into the body for the purpose of attaining more accurate results. Besides, if the glass manages to lose heat to the environment by any chance, the accuracy of the measurement is compromised (Diley & McConnell).
Bimetallic contact thermometer
This instrument relies on the physics concept of thermal expansivity of metals, where some metals have more expansion capabilities
(Kirk)
Two metal strips are used, where one metal expands and contracts more with respect to the other piece. The bimetallic strip is usually coiled in a helix for greater stem sensitivity, where it is attached to a needle at an angular position to a graduated scale
ρ=[31+rh)2 + 1+rh rere 2- 1rerh h6∝1-∝2(1+rh)2 ∆T (Diley & McConnell).
Where rh = thickness ratio of strips, ρ is the radius of the curvature and re is the modulus ratio of strips. The bimetal thermometer has the ability to measure temperature ranging between -185 and 540ºC. This thermometer is more durable compared to the liquid thermometer. However, it is slower in response to change in temperature. More so, it is reliant on contact, thus, it is less sensitive in the environment.
Resistance thermometer
The resistance thermometer is another contact thermometer which utilizes the resistivity of a thin strip or wire of metal that is hugely temperature dependent
(Kirk)
The wire, most preferably standard platinum due to accuracy, is coiled severally around a ceramic rod to improve sensitivity, and it is coated using glass. The high purity platinum wire is located near the tip of the closed protective tube to make a probe that is insertable into the environment to be measured. The sensor has two emerging lead wires, as shown above in the picture above. However, some sensors have a three wire connection while others have a four wire connection. The latter is the most accurate since two wires each is used for the current and the other two for passing voltage.
∝ = R100-R0R0(1000C) (Diley & McConnell).
∝ is the temperature coefficient of the resistance, R0 is the resistance of the sensor at 0 degrees Celsius and R100 is the resistance of the sensor at 100 degrees Celsius. Thus, the resistance changes linearly with temperature as per the above equation. Resistance thermometers use the electrical resistance with respect to a connected power source. Some of the advantages of this instrument include: high accuracy and low drift. However, the thermometer cannot pass certain temperatures, e.g. 660, for fear of being contaminated by impurities that arise when the material is subjected to too much heat that may cause structural changes. Furthermore, they have a small temperature range, and they also have a slower response.
Pressure
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. It is measured in bars, Pascals or Newtons per square meters (Diley & McConnell). There are three main approaches to measuring pressure namely: mechanical, electrical and thermal. The mechanical approach involves the measurement of the movement or position of an object from a reference point. That can be either aneroid, where the elastic deflection of a metallic diaphragm is measured, or hydrostatic. On the other hand, the electrical approach utilizes direct or indirect transduction to current, voltage and resistance.
Bourdon tube
This is a flattened thin walled tube shaped into letter C or coil that winds and unwinds elastically. It is attached to a needle that rotates about a graduated scale. As fluid pressure enters the tube, it becomes reformed. Due to the available free tip, the tip travels in open space, and the tube either coils or uncoils depending on the conditions. This coiling and uncoiling causes the deflection of the needle through mechanical linkage hence a reading is taken (Awazpost.com).
(Awazpost.com)
This instrument is only operational under the principle of elasticity. However, they have a very high range of almost 100,000 psi (700MPa) (Awazpost.com). There are a few drawbacks of this instrument. First of all, the mechanical properties of the tube are also dependent on the change in temperature. Thus, this might affect the general pressure readings. Furthermore, an instant surge in pressure is likely to deform the coil or the needle. Besides, the working parts are exposed to environmental processes such corrosion and contamination.
Aneroid – Diaphragm barometer
(Efunda.com)
The aneroid diaphragm is a thin metal plate or membrane covering the end of an elastic capsule that moves axially as the ambient gas pressure increases or decreases (Efunda.com). As gas pressure leaves or enters the elastic capsule, the amount of pressure results in the movement of the upper part of the capsule, which is attached to two cogwheels that rotate the needle on the calibrated scale. This instrument is small and thus cheap to acquire and maintain. Despite the fact that the aneroid-diaphragm has a small range, it is very sensitive and hence more efficient. Besides, it also works well with viscous fluids (Efunda.com).
Manometers
(Engineeringtoolbox.com)
Manometers are instruments used to measure the pressure of fluids with reference to a known value of pressure from another liquid. The most common structure of the device mainly has a u-shape which contains two fluids on either side of the u-tube (Engineeringtoolbox.com). The forces balance on either side: the one of the applied pressure as well as the force due to gravity from the columns of fluids. The amount of pressure is usually equal on both sides of the u-tube, and it is calculated by multiplying the resultant height of the liquid from the point of reference (h) by the density of the respective fluid (α) and the gravity (g) (Engineeringtoolbox.com).
hx ∝x gx= hy ∝y gy
Where x and y are both liquids on different sides of the u-tube manometer. Perhaps the absence of a calibration in the tube makes the instrument less user-friendly compared to other graduated pressure gauges (Engineeringtoolbox.com). Furthermore, it can measure slight pressure differences but not large ones. Besides, the diameter of the tubes may cause a surface tension effect which could compromise the readings. Moreover, the manometer usually has s slow response which makes it not suitable for fluctuating pressures. A manometer has different variations from the usual u-tube shape. The most common are the well manometer, inclined manometer, and the combination manometer. The well-type manometer is used in situations requiring the reading of only one leg. In this way, higher accuracy is achieved. The inclined tube manometer is enlarged with its measuring leg inclined to the vertical axis at a certain angle. This enables it to measure the smallest of pressure differences. Lastly, the combination manometer, a hybrid instrument that includes the well and the inclined thermometer is used to measure fluid velocity alongside pressure. This multi-purpose feature makes it more efficient and also cost effective (Engineeringtoolbox.com).
Liquid concentration
Liquid concentration refers to the amount of liquid molecules present in water. It can be either mass or molar depending on the dimensions used when calculating or measuring. Some of the main methods used to measure this concentration are sonic velocity, refractometry, conduction, and density.
Sonic velocity
This method involves the sending of an ultrasonic signal through the subject liquid from a sender to a receiver. The speed of the signal is then determined by measuring the time taken for the signal to travel from the sender to the receiver (SensoTech).
(SensoTech)
Since the distance between the sender and the receiver is always given, the sonic velocity is determined by
v=st
Where v is sonic velocity, s is the distance between the sender and the receiver and t is the signal delay (pg. 2). The velocity is then compared with other databases to determine the molar concentration with respect to velocity. Perhaps the ease of measurement is the greatest advantage of this instrument, alongside the fact that there are few factors within the device that could cause interference (par. 3).
Refractometry
This method involves the use of a refractometer, which determines liquid concentration by calculating the refractive index (SensoTech). The refractive index depends on the reflection of light that is scattered or reflected by a liquid. Light scatters differently based on the type and concentration of the dissolved solid. On the refractory is an optimal sensor window that measures the reflection of a light beam from an LED light when it reaches the liquid sample (Pg. 3).
(SensoTech)
Density
The measurement of density is based on a spring-mass system where a tube is made to oscillate (SensoTech). The frequency is dependent on the spring constant and the mass of the tube together with its contents. Since mass and volume of the tube is constant, the density is determined by measuring the frequency (pg. 3).
(SensoTech)
However, the spring rigidity of the tube is dependent on temperature. Therefore, the temperature is measured in the device to compensate for its dependency.
References
Awazpost.com. “Bourdon Pressure-gauge”. 12 June 2013. Web. 23 February 2016. http://www.awazpost.com/bourdon-pressure-gauge/88152/
Dally, W., McConnell, W. “Instrumentation for Engineering Measurements”. Iowa State University, 1993
Efunda.com. “Introduction to the diaphragm pressure gauge”. 13 May 2014. Web. 23 February 2016. http://www.efunda.com/designstandards/sensors/diaphragm/diaphragm_intro.cfm
Engineeringtoolbox.com. “Temperature Measurement”. 20 April 2011. Web. 24 February 2016. http://www.engineeringtoolbox.com/temperature-measument-t_50.html
Engineeringtoolbox.com. “U-Tube Manometer”. 8 August 2013. Web. 24 February 2016< http://www.engineeringtoolbox.com/u-tube-manometer-d_611.html>
Kirk, D. “Heat Transfer with Applications” Prentice-Hall, 1999.
SensoTech. “Measuring Concentration of Liquids”. SensoTech GmbH, Magdeburg-Barleben, 2015.
