Temperature Measurements

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Temperature Measurements

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Temperature Measurements

(Temperature MeasurementsLecture NotesSystems & Biomedical Engineering Department Faculty of Engineering, Cairo University2011Prof. Bassel TawfikBiomedical Measurements1/1/2011)

Lecture Outline

1. The Concept of Heat

2. Types of Temperature Sensors

3. Comparison of Different Types

4. Applications in Medicine

1. The Concept of Heat

(Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material. Therefore, temperature is a measure of the kinetic energy of the material in question.)Temperature is an indication of the thermodynamic state of an object or system. It is a macroscopic description of the collective microscopic kinetic energy in a given material. If two bodies are at the same temperature, they are said to be in thermodynamic equilibrium with each other. This implies that if they were connected to each other, there would be no net flow of heat from one to the other.

Interestingly, temperature is not a measure of the unit thermodynamic energy of a body; unit masses of differing materials can require differing amounts of energy to be added or removed to change their temperature by a given amount. Identical temperature of two bodies merely implies there would be no transfer of heat between the two, regardless of the actual energy stored as heat in each body.

Most temperature measuring devices use the energy of the material or system they are monitoring to raise (or lower) the kinetic energy of the device. A normal household thermometer is one example. The mercury in the bulb of the thermometer expands as its kinetic energy is raised. By observing how far the liquid rises in the tube, you can tell the temperature of the measured object.

2. Types of Temperature Sensors

2.1 RTD (Resistance temperature Detectors)

(Figure 1)Introduction

RTDs are electrical resistors whose resistance increases with temperature. RTDs are manufactured using different metals as the sensing element. The most commonly used material is Platinum. Platinum is preferred to other materials because of (1) its high temperature coefficient, (2) excellent stability, and (3) repeatability (reproducibility). Other materials used to make RTDs are nickel, copper, and nickel-iron. These materials are becoming less common now that the cost of platinum RTDs is coming down. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal. Figure 1 shows the internal construction of an RTD.

Characteristics

(Figure 2)Since RTDs resistance increase as the temperature increases, they are referred to as a positive temperature coefficient devices. RTDS are manufactured with a base resistance at some temperature point. This temperature is most commonly 0C (32F). The most common base resistance is 100, which means that if the RTD is at 0C, the resistance would be 100. There are other base resistances at different temperatures.

The relationship between the temperature and the electrical resistance is usually non-linear and described by a higher order polynomial:

R(t) = Ro(1 + At + Bt2 +C(t 100)3)

Where: Ro is the nominal (base) resistance, t is the temperature, and the coefficients A, B, C depend on the conductor material and basically define the temperature-resistance relationship. For temperatures above 0 C, the C coefficient equals zero. Therefore, for temperatures above 0 C, this equation reduces to a quadratic.

(The temperature coefficient, called alpha (), is defined as the change in RTD resistance from 0 to 100C, divided by the resistance at 0C, divided by 100C: (//C) = (R 100 - R0)/(R0 * 100 C)Where R100 is the resistance of the RTD at 100 C, andR0 is the resistance of the RTD at 0 C.For example, a 100 platinum RTD with = 0.003911 will measure 139.11 at 100 C.)Another common term used with RTDs is temperature coefficient. This refers to the rate of change of resistance with respect to temperature. The most common platinum RTD has a temperature coefficient of .00385 //C. This means that a 100 platinum RTD will increase in resistance .385 for every 1C increase in temperature.

The maximum temperature rating for RTDs is based on 2 different factors. First is the element material. For instance, Platinum RTDs can be used as high as 650C (1202F). The other determining factor for temperature rating is probe construction. Finally, the tolerance or accuracy for RTD sensors is stated at one point only, which is usually 0C (32F). ASTM publications recognize 2 grades of platinum RTD elements while DIN (Europes version of ASTM, also called IEC/DIN) recognizes 2 classes of elements. They are as follows:

ASTM E-1137 grade B = .10% @ 0C (32F)

ASTM E-1137 grade A = .05% @ 0C (32F)

DIN 43760 class B = .12% @ 0C (32F)

DIN 43760 class A = .06% @ 0C (32F)

An RTD does not produce any voltage and so it requires a source of power for its operation. On the other hand, RTDs are generally more expensive to manufacture or purchase than thermocouples because of the expensive nature of Platinum. Yet, Platinum is not without defects. RTD elements become quite fragile at temperatures above 320C (600F). An RTD sensor will not hold up well at these elevated temperatures if there is any vibration present. Finally, it has been observed that the tolerance (accuracy) of an RTD generally decreases as temperature increases.

(Figure 3: Variable shapes of RTDs.)

RTD Circuitry

(1) Two-wire voltage-source (or current source) configuration

(Figure 4b: 2-wire voltage source RTD in a Wheatstone bridge.) (Figure 4a: Simple RTD connection to a voltage source.)Since the RTD is a resistance which varies with temperature, if we pass a current through it, the voltage drop across it will reflect the amount of resistance and hence the temperature. There is a catch however! As you can see from figure 3 above, the RTD sensor is usually placed at the end of a long wire to avoid placing the entire electronics near the source of heat, thereby avoiding harming these sensitive components. Since the long wire has a significant non-zero resistance, the following situation arises when the RTD is connected to a voltage source directly (as shown in figure 4).

Here, we are measuring the voltage across the RTD at points R, R (Red and Red) at the tips of the 2 long wires connected to the RTD. If the RTD resistance is 100 and each wire has a resistance of 5 (actual value is unknown), then the total resistance of the 2 wires and the RTD will be 110 . Ideally, it should be just 100 . Now there is an error of 10% in measured resistance which translates to another error in the estimation of temperature. If the relationship between temperature and resistance is linear, the error in temperature estimation will also be 10% but this is strictly not true. This 2-wire voltage-source circuit produces the largest error in temperature measurements. Notice that the same problem occurs when using a Wheatstone bridge in which one arm of the Bridge is the RTD and the 2 lead wires.

(2) Three-wire voltage-source configuration

(Figure 5b: 3-wire RTD connection to a voltage source using Wheatstone bridge.) (Figure 5a: 3-wire RTD connection to a voltage source. R= Red, W=White.)To reduce the error incurred by the 2-wire voltage source method, a 3-wire circuit is constructed as shown in figure 5a. The idea is to add a third wire with a resistance and material equivalent to the first and second wires. By measuring the resistance between the red and the white leads and then subtracting the resistance between the two reds, we end up with the RTD resistance only (100 ). The main assumption here is that all three wires have exactly the same values all the time.

The above circuit is fine except that it requires mathematical subtraction in order to obtain just the RTD true value. By using the Wheatstone bridge shown in figure 5b, the effects of the lead wires cancel each other electrically. Using this method the two leads to the sensor are on adjoining arms. There is a lead resistance in each arm of the bridge so that the 2 resistances are cancelled out (so long as the two lead resistances are exactly the same). This configuration allows up to 600 meters of cable. It remains to say, however, that the Wheatstone bridge shown in Figure 5 creates a non-linear relationship between resistance change and bridge output voltage change.

(Figure 6a: Another configuration of the 3-wire RTD circuit using a current source. The circuit details are shown in figure 6b. The assumption is that the resistances of the two lead wires are equal.)(3) Three-wire current-source configuration

Another way to connect the 3-wire configuration using a current source is shown in figure 6a. The RTD element is shown to the left with two terminals: Hi and Lo, while the signal conditioning (SC) is the box to the right. The 2 lead wires are called W1 and W2, while the compensating (third) wire is called W3. The constant current Iexc flows from ehi to RTD Hi through wire W1.

(Figure 6b: Details of the circuit in figure 6a. The circuit to the right is a bit more simplified than that on the left but they are essentially the same. The objective is to measure the voltage across the RTD. This is equivalent to measuring Va Vb (the right hand circuit). Va and Vb are accessible for measurement because they are at the Signal Processing end not the RTD end.)

(4) Four-wire Current-Source Configuration

(Figure 7: Four-wire configuration. Theoretically, the current through RT is constant. Consequently, no current flows through L1, L2. Hen