About Instrumentation

Thursday, September 27, 2012

Units of pressure, types of pressure

Among many process variables pressure is that parameter which is critical for safe and optimum operation in hydraulic equipment, separating processes (such as filtration, distillation, etc.), vacuum processing, etc. Using pressure measurements we are able to measure level of liquids in tanks, or flowrate of liquids or gases in the pipes. In order to keep pressure within desired range we need to be able to accurately measure this process variable, and then control it. It is not easy to use instrumentation for pressure measurements without knowledge of a fundamental concept of pressure.

Pressure is equal to the force divided by the area over which it is applied. In the International System (SI) the unit for pressure is called Pascal (Pa) and is equal to the force of one 1,N divided by an area of one 1,m²:1,Pa = 1, N/m². There are other units, which are not within SI, namely:

1 bar = 10⁵ Pa = 0.1 MPa;

1 atm = 101325 Pa - (standard or physical atmosphere);

1 kgf/cm² = 98066.5 Pa = 0.0980665 MPa - (technical atmosphere);

1 lbf/in² = 1 psi = 6894.76 Pa = 0.00689476 MPa;

1 mm Hg = 133.322 Pa - (760 mm Hg = 101324.7 Pa » 101325 Pa).

The advantage of the Pascal is that it does not depend on the gravitational acceleration. It means that this unit is the same in places with various values of gravitational acceleration. Even on other planets it does not change.

There are various types of pressures. Figure 1 gives illustration of terms used in pressure measurements (From book Van Wylen G.J., Sonntag R.E. "Fundamentals of Classical Thermodynamics", Sec. Ed.). Absolute pressure in a system is equal to the total pressure of a liquid or a gas which acts on the walls of this system. The difference between absolute and atmospheric pressure is called gage or manometric pressure and is read by ordinary pressure gauge:

P_g = P_abs- P_atm. (1)

If P_abs< P_atm, then the difference between atmospheric pressure and absolute pressure is called vacuumetric pressure and is read by ordinary vacuum gauge:

P_vac = P_atm - P_abs. (2)

Figure 1. Illustration of terms used in pressure measurements.

Instrumentation for pressure measurements may be classified regarding to the operational principle used or type of pressure to be measured.

If we consider operational principles employed, then process instrumentation for pressure measurements may be categorised as follows:

• liquid filled pressure instrumentation: “U”-tube manometers, well manometers, bell-type manometers, liquid barometer, absolute pressure manometer;

• elastic-element mechanical pressure gages: Bourdon tube pressure gages, bellows-type pressure gages, diaphragm-type pressure gages;

• dead-weight pressure gages;

• electrical-type pressure gages: piezoelectric pressure gages, capacitance pressure gages, strain-type pressure gages.

Here is the classification of instrumentation for pressure measurements with respect to the type of the measured pressure:

• pressure gages, for measurements of pressures above atmospheric pressure;

• vacuum pressure gages, for measurements of pressures below atmospheric pressure;

• vacuum manometers, for measurements of both pressures above and below atmospheric pressure;

• barometers, for measurement of atmospheric pressure;

• differential pressure and vacuum gages, for measurements of difference of pressures.

Article Source:: Dr. Alexander Badalyan, University of South Australia

Wednesday, September 26, 2012

Optical and radiation pyrometers

Pyrometer is a device which uses the relationship between the electromagnetic radiation emitted by a body and the temperature of this body. In order to better understand the phenomenon which forms the basis of pyrometry, it is useful to explain the concept of the blackbody, and the differences between it and real objects.

The term blackbody is ideal, and designates a body which radiates more electromagnetic energy for all wavelengths intervals than any other body of the same area and at the same temperature, and absorbs all the radiation it intercepts. Fig.1 presents one of the classical blackbody model.

Figure 1. A classical blackbody model.

The temperature of the blackbody determines the nature and extent of such radiation. Stefan-Boltzmann’s law says, that

the blackbody with a finite absolute temperature (T) emits radiant (ie, in all directions) electromagnetic radiation (EMR) per unit area of this blackbody and per second with intensity which is proportional to T⁴, according to an equation:

E_T = σ * T⁴, (2)

where,

E_T- total EM energy emitted by the blackbody in all directions per unit area (1.m²) and per unit time (1,s),W/m²;

σ - Stefan-Boltzmann’s constant, equal to 5.67051*10^-8, W/(m²*K⁴);

T - an absolute temperature of the blackbody, K.

Fig. 2 shows the relationship between EMR emitted by a perfect blackbody as a function of temperature. The area under these curves is equal to the total energy (emitted by a black body) per second per unit area. This body at low temperatures emits EMR in the region of long wavelengths. This region spreads from far-infrared to microwave region (5 mm < λ < 100 mm, where λ is the wavelength in mm, 10^-6 m). With increasing the blackbody temperature, the emission peaks move into the region of shorter wavelengths. At very high temperatures the blackbody emits in the near visible wavelengths region. Visible region corresponds to the wavelengths from 0.7 mm (red) through 0.62 mm (orange), 0.58 mm (yellow), 0.53 mm (green), 0.47 mm (blue) to 0.42 mm (violet).

Real objects emit and absorb less EMR than blackbodies, and this difference is dependent on the wavelength, so nonblackbodies can not exactly follow relationship shown in Fig. 2. For this purposes corrections should be used, otherwise, the apparent temperature will be lower that the actual temperature. Also, it is necessary to take into account the loss of emitted radiation when it passes through the media between the emitting body and a measuring instrument.

Figure 2. EM radiation emitted by the blackbody at various temperatures.

There are two types of pyrometers: optical (monochromatic or narrowband) and radiation (total radiation or broadband) pyrometers. The last devices originally were called radiation pyrometers, then radiation thermometers, and more recently infrared thermometers. However, the first their name (radiation pyrometers) is still widely used at present. These devices have high accuracy of ±0.01 °C as a standard instruments, and from ±0.5 to ±1% for industrial purposes.

a). Optical pyrometers, sometimes referred to as brightness thermometers, generally involve wavelengths only in the visible part of the spectrum. When the temperature of the body increases, so does the intensity at any particular wavelength. If two bodies have the same temperature, then intensities of those two objects are equal. In this type of a pyrometer the intensity of a certain wavelength of a heated body is compared with that of a heated platinum filament of a lamp (see Fig. 3 ).

Figure 3. An optical pyrometer.

An object 1 which temperature is to be measured, emits electromagnetic radiation with intensity proportional to its absolute temperature. This radiation passes through lens 2 and red optical filter 3. Optical filter picks out only the desired wavelength - red. Then radiation focuses on the platinum filament of a lamp 4, and passes through another filter 5, lens 6, viewing system 7. The viewer 8 sees the platinum filament superimposed on an image of the object 1. When the temperature of the filament is low comparing with that of the object, the viewer sees the filament as a dark line on the bright background image of the object. The lamp 4 is connected in series with an electrical battery 9, a variable resistor 10 and an ampermeter 11. By reducing the resistance of the resistor an electrical current passing through the filament increases. So does the temperature of the filament and its brightness. For a certain value of an electrical current (corresponded to a certain value of an object temperature), the brightness of the platinum filament will match the brightness of the object 1. At this setting the viewer cannot distinguish between the image of the object and the filament. At this time the measurement of temperature is performed. The scale of the ampermeter is calibrated in the units of temperature.

The lower temperature limit for optical pyrometers is determined by the temperature at which objects become visible in red (about 225 °C). However, there are devices which are able to measure even lower temperatures down to -50 °C. The upper limit varies from 600 to 3000 °C, and is limited by the melting point of the platinum filament. An accuracy is typically varied from ±5 to ±10 K.

b). Radiation pyrometers, being very simple and cheap, use an exponential relationship between a total emitted EMR energy and given temperature. In radiation pyrometers (see Fig. 4) EMR energy emitted at infrared (2.5 < l < 20 mm) to visible wavelengths (0.42 < l < 0.7 mm) from an object 1 is focused by a spherical reflector 2 on a series of micro-thermocouples attached to a blackened platinum disc 3. The radiation is absorbed by the disc, which temperature is increased, so does thermal electromotive force U developed by the series of thermocouples. This thermal electromotive force is proportional to the temperature of hot junctions of thermocouples, and, finally, to the temperature of the object 1. The advantage of these pyrometers is that their operation slightly depends on the wavelength.

Figure 4. A total radiation pyrometer

The lower limits for radiation pyrometers vary from 0 to 600 °C, the upper limits vary from 1000 to 1900 °C. The accuracy varies from ±0.5 to ±5 K, depending on cost. They are widely used for temperature measurements in metal production facilities, glass industries, semiconductor processes, etc.

Article Source:: Dr. Alexander Badalyan, University of South Australia

Thermistors Basics

If semiconductors or heat-treated metallic oxides (oxides of cobalt, copper, iron, tin, titanium, etc.) are used as the materials for producing temperature sensitive elements, then these temperature transducers are called thermistors (the name is derived from the term of ‘thermally sensitive resistor’). These oxides are compressed into the desired shape from the specially formulated powder. After that, the oxides are heat-treated to recrystallise them. As the result of this treatment the ceramic body becomes dense. The leadwires are then attached to this sensor for maintaining electrical contact.The following relationship applies to most thermistors:

R_t = R₀*e^{B*(1/T - 1/T}₀⁾(1)

where,

R_T0- resistance of thermistor at reference temperature T_0,K, Ohm;
R_T - resistance of thermistor at temperature T, K, Ohm;
B - constant over temperature range, depends on manufacturing process and construction characteristics, 1/K.

Fig. 3.15 shows relationship between temperature and resistance for a thermistor.Thermistors have negative thermal coefficient of electrical resistance. It means that when temperature increases the electrical resistance of thermistor decreases. They have greater resistance change (this is an advantage) compared with RTD in a given temperature range. For example, if we compare what change in resistance will be caused by variation of temperature in 1 °C for Platinum and Copper RTD and for thermistor (see Fig. 2) in the temperature range from 273.15 to 423.15 K (ie, from 0 to 150 °C), we will obtain the following values:

• for platinum RTD                 - 0.38, Ohm / ̊C;
• for copper RTD                    - 0.04, Ohm / ̊C;
• for thermistor                       - 0.65, Ohm / ̊C;

Figure 2. Thermistor resistance vs temperature curve.

Wheatstone bridge and resistance measuring constant current circuits, similar to that used in the case of RTDs, are used for resistance measurement of thermistors. Despite their high sensitivity, thermistors have a worse accuracy and repeatability (this is the disadvantage) comparing with metallic RTDs. Since the resistance vs temperature function for thermistors is non-linear (although, some modern thermistors have a nearly linear relationship of temperature vs resistance), it is necessary to use prelinearisation circuits before interacting with related system instrumentation. In addition, due to the negative thermal coefficient of electrical resistance an inversion of the signal to positive form is required when interfacing with some analog or digital instrumentation. Therefore, thermistors are not widely used in process instrumentation field, at least at present. However, they have been well accepted in the food transportation industry, because they are small, portable and convenient. Another field of their growing application are heating and air-conditioning systems, where thermistors are used for checking the temperature in flow and return pipes.

All the discussed above instrumentation for temperature measurement refers to contact-type devices, because their sensitive elements are immersed in the measuring media. When dealing with temperatures above 1500 °C, contact-type temperature measuring devices are not applicable, because irreversible changes occur in metals which form their sensitive elements. It is possible to perform non-contact measurement of temperature by optoelectronic transducers.

Article Source:: Dr. Alexander Badalyan, University of South Australia

Friday, August 3, 2012

Resistance temperature detectors : Basics

The principle of resistance temperature detectors (RTD) is based on the variation of electrical resistance of metals with temperature. For this purpose several metals are used, namely, platinum, copper, nickel. When temperature increases the resistance of these metals increases. Temperature function of resistance for metals in a narrow temperature interval can be expressed by a relationship:

(1)

where:

R_ϑ and R₀ - are the values of electrical resistance of a metal conductor at temperatures ϑ and 0,̊C, respectively, Ohm;

α - thermal coefficient of electrical resistance, 1/̊C .

For metals this coefficient is positive. Fig.1 shows relationship between resistance of platinum and copper RTD and temperature.

Platinum RTDs are used for temperature measurements from -220 to 850 ^oC (they are used as reference RTDs, as well), copper RTD - from -50 to 150 ^oC, and Nickel RTD - from -215 to 320 ^oC.

Figure 1: Resistance vs temperature for platinum and copper RTD.

Fig. 2 shows the assembly of RTDs. Sensitive elements of RTDs are made of a thin wire 1 with outside diameter equal to 0.025 mm (platinum RTD) and 0.1 mm (copper RTD) double wounded (non-inductive) on a micaceous or porcelain stem 2. For mechanical strength the sensitive element is placed in the ceramic insulator tube 3 filled by extremely fine granular powder; extension wires are placed in the ceramic insulator 4, and entire assembly is covered by a protective sheath of stainless steel 5. The space between the sheath and ceramic insulator is filled by ceramic packing powder 6. To avoid contact of sensitive element with environment, sensitive assembly is protected by high-temperature hermetic seal 7. The contact between the wire of the sensitive element and the ceramic encapsulation permits a rapid speed of response.

Figure 2: RTD assembly.

For measurements of resistance of RTDs several methods are used. Among them the most widely used is a method employing a Wheatstone bridge (Figure 3). In this case RTD is connected to the bridge by two connecting cables (conductors). The bridge is powered by direct current power supply in the points "a" and "b". RTD is immersed in the media, which temperature to be measured. When this bridge is in balance, then there is no voltage between points "c" and "d", and zero-indicator (ZI) shows no current. For this condition we can write the following equation:

(2)

(3)

where:

R₁ and R₂ - electrical resistance of two invariable resistors, usually, R₁=R₂, Ohm;

R_ϑ - electrical resistance of RTD, Ohm;

R_c - electrical resistance of a connecting cable, Ohm;

R_sr - electrical resistance of a slide (variable) resistor, Ohm.

Figure 3.: Two-conductor connecting. Figure 4: Three-conductor connecting.

As follows from these equations, the certain position of a slide of the variable resistor (ie, certain value of R_sr) corresponds to each value of a measuring temperature (ie, for each value of R_ϑ) at any balance condition of the bridge. Therefore, the scale of the bridge may be calibrated in degrees Celsius.

For a two-conductor connecting scheme (see Fig.3), variation of an ambient temperature will effect values of an electrical resistance of connecting cables R_cand, therefore, the results of measurements will be erroneous.

With all resistance temperature detectors a three-conductor connection scheme is recommended (see Fig. 4). One conductor is common to both sides of the bridge, while other two connect the RTD to each side of the bridge. Any change in the cable temperature (as the result of variations in ambient temperature) will be cancelled because the resistance of both sides of the bridge change by the same value (providing three connecting cables are at the same temperature).

A four-conductor connection scheme is used when very accurate measurements of temperature are required, up to ±0.01 °C of accuracy.

Derivation of a balanced condition for a Wheatstone Bridge

Below I give you the derivation of the balanced condition for a Wheatstone bridge. Figure 4a shows a circuit of five electrical resistors connected to form the Wheatstone bridge.

Figure 4a.: Wheatstone bridge.

Here we use two Kirchoff’s Laws. The first Kirchoff’s law says: the total current flowing into any junction is equal to the total current flowing out of this junction. The second Kirchoff’s law states: the total change in potential around any closed circuit loop is equal to zero.

For the second Kirchoff’s law we should be sure to include the sign of potential energy correctly: the potential decreases around a resistor in the direction of current flow and increases in the direction opposite to current flow; potential increases from negative to positive terminals of the battery.

Below I give you two different methods for the derivation of the balanced condition for a Wheatstone bridge.

Now, let’s consider several closed circuit loops and apply the above law to them.

a. First method

1. Loop e-a-b-c-f-e :

(4)

2. Loop a-b-g-d-a :

(5)

3. Loop b-c-d-g-b :

(6)

Here, U is a dc voltage, and U≠ 0.

So, we have three equations with 3 unknowns - I₁; I₃ and I₅.

(7)

(8)

(9)

Now we combine terms with I₁; I₃ and I₅.

(10)

(11)

(12)

Now we evaluate the determinant of the matrix developed using this set of equations:

(13)

In the case of the Wheatstone bridge the resistance R₅ is substituted by an ampmeter. Ampmeters have a negligibly small resistance, so we can write that R₅= 0. With this condition we can re-write (13) as follows:

(14)

Because all R_i > 0 we have that ∆ > 0 and ∆ ≠ 0.

For current I₅ we have:

(15)

Now we can evaluate the value of the current, which flows through an ammeter as follows:

(16)

We know that a balanced condition is when current I₅ = 0, and because ∆ > 0, and ∆ ≠ 0 and U ≠ 0, we can give an expression for the balanced condition for the Wheatstone bridge as follows: