Measurements

Table of Contents

Pressure

Pressure is a measurement of the ratio between the force applied to the surface area on which that force is applied. The SI pressure units are Pascals which are equivalent to Newtons / Meters² (N/m²). Among many other units of pressure, some of the most common used within engineering include bar, atm, torr, psi, and mmHg. There are many different types of pressure as essentially anything can exert a force on a surface although within the context of chemical engineering fluid pressure is the most common which can be the pressure of either a liquid or vapour.

Liquid pressure and vapor pressure are both very different in the ways they apply force to a container. As shown in Figure 1, liquid pressure is a combination of atmospheric pressure, which is about 101325 Pa / 760 mmHg / 1.01325 bar / 14.696 psi at sea level, and the amount of a fluid in a container applying pressure to the bottom. The height determines the liquid pressure within a container whether they container is as thin as a straw or as wide as a pot, if the height of both containers is 15 cm, the pressure at a point at the bottom of each container will be the same. Vapour pressure is different as the molecules within a gas bumping into the walls of the closed container is the pressure being measured as shown in Figure 2. Vapour pressure depends on factors such as temperature and volume which determine how fast these particles are moving and how many collisions there are with other particles and the container [1].

Temperature

Temperature can be defined as the average kinetic energy of all the molecules in a substance. The kinetic energy of molecules cannot be measured directly therefore temperature is determined by measuring other physical properties which are affected by temperature. There are many different devices that use different ways to measure temperature because of this. A resistance thermometer measures the electrical resistance of a conductor, a thermocouple measures the voltage at the connection point of 2 different metals, a pyrometer measures the radiation of the substance, and the most common type of tool a thermometer measures change in volume of a fluid. Among many different tools of measuring temperature, there are also many different temperature scales. The Celsius and Fahrenheit scales are based off setting values to the freezing and boiling points of water while the Kelvin and Rankine scales are do the same thing although their absolute zero temperature is at zero unlike the Celsius and Fahrenheit scales. The conversions between temperatures can be found in Figure 3 and the key temperatures can be found in Table 1 [1].

Level

Level measurement refers to the level of liquid within a tank as if this is not monitored, the tank could overflow or a pump mechanism could malfunction. There are many different level sensors used within the industry and they can be divided into two categories based on their functionality: point measurement and continuous measurement. Point level measurement sensors will indicate when the level of liquid has reached a specific height and continuous level measurement sensors can always indicate what level the liquid is at, no matter how high or low the liquid level is. Level sensors also often have different roles within a tank. A sensor that indicates when the water level is too low is referred to as a low level sensor and a sensor that indicates when it is too high is a high level sensor. Further classification into high level sensors includes high-high level sensors for example in a water tank a high-level sensor can be installed to indicate simply when the water level is too high and a high-high level sensor can be installed on that same water tank to indicate when the water level is dangerously too high and something must be done [2].

Flow

Flow rate is an essential measurement within many chemical processes and can be used in many different ways. It is usually depicted using a point over a variable for example mass flow rate can be represented as such in Figure 4. Flow rate can be defined as the rate at which a substance travels from one point to another point and can be measured either using volume (volume/time) or mass (mass/time). Mass and volume are not independent measurements as one can be calculating from the other using the density of whichever fluid is being measured. The relationship between mass and volume can be found in Figure 5. There are many different tools that can be used to measure the flow rate of a substance through a pipe although rotameters and orifice meters are the most common. A rotameter measures flow rate by determining the height of an object when the substance is flowing upward as depicted in Figure 6. An orifice meter measures the pressure drop of a substance before an after travelling through a small opening as shown in Figure 7. The large pressure drop indicates a large flow rate while a small pressure drop indicates a small flow rate [1]. Other flow measurement devices include venturi tubes, flow nozzles, velocity flow meters, pilot tubes, calorimetric flow meters, mass flow meters, and thermal flow meters among many other devices [3].

Mass

Mass is simply the measurement of how much matter a substance or object contains. It is often confused with the term weight although there is a distinct difference between the two terms. When someone stands on a scale, the scale is feeling the force from that person along with the force of gravity which means said scale is measuring weight which could be in kg/(m/s2) or similar units. Your mass is then calculated by the scale and displayed to you in kilograms or another unit of mass. Since weight is a measurement of the force of gravity on an object, such as a human in the example given, your weight would not be the same on a different planet although your mass would be. Common units of mass include kilograms (kg), grams (g), pound mass (lbm), ounces (oz), and tons (t). Density is another measurement related to mass as it is the ratio of mass to a unit of volume such as g/cm³, kg/m³, and lb_m/ft³. [1]

Composition

Dissolved Oxygen

The amount of dissolved oxygen within a substance is another measurement often used on water within chemical processes and labs. The three most common techniques of measuring dissolved oxygen are modern-day sensors, the colorimetric method, and Winkler titration. Dissolved oxygen sensors can either be electrochemical sensors or optical sensors with most having an analogue output for the reading. These types of sensors are often used within a lab or field to obtain the amount of dissolved oxygen within a variety of substances. Similar to the sensors, there are two different types of colorimetric methods known as the indigo carmine method and the rhodazine D method. Both of these methods use reagents that react with oxygen, changing the colour of the substance being analyzed for dissolved oxygen. This newly coloured substance is then compared to a chart which will indicate how much oxygen is present similar to pH strips. The Winkler titration is a much older method and is essentially a titration based on the fact dissolved oxygen has an oxidizing property to it. [4]

Salinity

Salinity is a measurement of the amount of dissolved salts within a substance which can include ions such as chloride, sodium, magnesium, sulfate, calcium, potassium, bicarbonate, and bromine. Conductivity and salinity are closely related and often measured together as conductivity is the measurement of a substance’s ability to pass an electrical current which is dependent on the amount of ions within the substance. Determining the amount of ions within a substance directly can be difficult which is why salinity is often measured indirectly by determining the conductivity of a substance. Units for salinity measurements are simply a ratio of ions over the set amount of substance which can be parts per thousand (g/kg), mg/L, or any other ratio of this type. [6]