0.1 Temperature measurement and first-order dynamic response  (Page 3/5)

In this section, you will multiply the data by the conversion scalar“k”that you found experimentally. You will also add an offset“b”.

1. Delete the Tone Measurement ExpressVI. Delete the Frequency and Amplitude indicators.

2. Delete the wires that connect the DAQ Assistant to the thermometer, the Waveform Graph, and the Write toFile ExpressVI. Press Ctrl+b to remove any broken wire remnants.

3. Place a Multiply and an Add on the block diagram.

4. Create a control and connect it to the y terminal of the Multiply icon. Rename the control“k”.

5. Create a control and connect it to the y terminal of the Add icon. Rename the control“b”.

6. Wire your block diagram as shown in Figure 4.

7. On the front panel, enter the values you found experimentally for the“k”and“b”values.

8. When you run the VI, your measurements should now be displayed as temperatures.

Part 3: time constants

The calibration curves you found represent static calibrations. In the following section, you will considerthe dynamic behavior of the sensors. The dynamics of all the temperature sensors used in this lab can be modeled accurately asfirst-order systems. The natural response of a first-order system can be characterized by a single parameter—the time constant.

Time constants can be found by observing the response of the instrument to a step input in temperature. Notethat the second medium governs the physics of the transient response. For each of the sensors, you will determine the timeconstant for transfers from ambient air to ice water and from hot air to ambient air.

An ice water bath will be provided for your tests. A heat gun will be used to generate hot air. Caution: Keepall temperature sensors (particularly the thermometer and the thermistor) at least 9 inches away from the heat gun. Holding themcloser to the heat gun will result in temperatures outside the operating range of the sensors.

Four methods will be used to calculate various time constants. Table 1 summarizes the tests to be performed. Thetesting methods are described below.

Table 1. Time constant calculation methods.

Method 1- time elapsed at 63.2% sensor accuracy

Thermometer: Perform this test for ambient air to ice water. Repeat for hot air to ambient air.

1. Record the temperature of the initial medium, T0, and the temperature of the final medium, T_inf.

2. Calculate the intermediate reference temperature, Tr that is 63.2 percent of the way from T0 toT_inf.

3. With the instrument at steady state (reading T0) in the initial medium, move it quickly to the finalmedium.

4. The time from when the step in temperature began to when the instrument reads Tr is the time constant, . Thistime can be measured using a stopwatch.

Method 2 - mathematic definition of tau

Thermometer: Perform this test moving from ambient air to ice water.

1. Record the temperature of the initial medium, T0, and the temperature of the final medium, T_inf.