Reference no: EM133682319

Prac - Soldering Techniques: Temperature Sensors, Thermal Response and Bandwidth

Aim:

In Part A, you will be instructed in, and practise, basic soldering techniques.

In part B of this practical, we will investigate the temporal response time of a number of different thermometers for a step change in temperature and use this to determine the effective bandwidth of the sensor.

PART A

Soldering is an essential basic skill necessary to build and prototype sensor circuits. For a scientist, it can also prove extremely useful if repairs to a sensor board are required, especially in the field e.g. a dry joint or loose connection is spotted. You will be given instruction in soldering techniques and the opportunity to perform practical soldering yourselves. After some instruction and practise, you will be given a light chaser circuit boards kit to solder and assemble. The light chaser circuits that will give you practise in soldering will be shared between 3 people. Each students should not spend more than 15 minutes of their time soldering the light chaser board. Once the group has soldered all components onto the board they should present their light chaser board for inspection and test.

Part B

Background

All sensors have a response time - some are quick whilst others are slow to respond to a change in the measurand. The response time is related to the bandwidth of the device. In this practical, we will take a selection of temperature sensors and measure their temporal response to a step change in temperature. By plotting the temperature response against time, an approximate operational bandwidth for each of the sensors in Hertz will be calculated.

In the figure below, the signal is changing from 0 - 5V. The 63.2 % signal equating to the time constant tc is shown along with the rise time when the signal rises to 99.35% of its final value. The time to rise to 99% of the final value is normally equated to five time constants.

We will estimate the final reading and measure the time it took for the output to either reach 63% of this (t) or 99.5 of its final value (5t). The bandwidth of the sensor can then be determined (approximately) by:

Bandwidth Δf » 0.35 / tc
or bandwidth Δf » 5*0.35 / 5t

In this experiment, we will be using K-type thermocouples connected to a digital multi meter (DMM). Thermocouples are a very common way of measuring temperature. They are cheap, easy to use. Thermocouples employ a junction between two dissimilar metals.

Two junctions are used - one is the sensing element and the other is the reference junction that is held at a known temperature. A variety of thermocouples exist that relate to the choice of dissimilar metals making up the junction e.g. J-type, K-type. Each junction type has its own performance parameters that includes sensitivity, temperature range, stability, resistance to oxidation and suitability for high temperature measurement. The accuracy of thermocouples is typically 0.1 - 1.0% of full scale reading. In this practical, we will be using K-type thermocouples. A voltage difference (DV) develops across the two junctions that is proportional to the temperature difference between them:
DV = (SAB) DT
where SAB is an effective Seebeck constant. We will assume a linear transfer function for the thermocouple in the range in which we are operating.
To convert your readings in mV to °C, you can either use an applet such as https://au.flukecal.com/Thermocouple-Temperature-Calculator
or determine the effective Seebeck coefficient (SAB) (i.e., DV /DT) using data from the ITS-90 table for a K-type thermocouple for the limited temperature range in which you are working (you are using the table values to work out an effective gradient of the slope of the transfer function).

The ITS-90 values are calculated from high-order polynomials using up to ten constants. If we plotted the data in these tables for the full temperature, range over which the thermocouples can operate (e.g., K-type, -270 - 1370 °C), we would note that the transfer function is close to, but not perfectly linear, leading to a linearity error. However, in a smaller span we can satisfactorily approximate the transfer function as being a straight line accepting the very small linearity error that the straight line fit produces.
Either method used requires you to input the temperature of the reference junction - this will be close to 25 °C.
For comparison, the operating frequency of various thermometers vs temperature has been published in the literature and reproduced below

Materials and Equipment
IR camera
K type thermocouple with DMM adapter Digital multi meter
Iced water, 250 mL beaker
Hot water @ ~70, 250 mL beaker Plasticine
Phone camera Digital Timer

Sensors:
Alcohol thermometer
K type thermocouple plugged into DMM reading millivolts
K type thermocouple with marble sized piece of plasticine moulded on tip, plugged into DMM reading millivolts
IR camera

Method

Prepare ice water in 250 mL beaker.
Prepare hot water in 250 mL beaker (~ 70°C is OK)
Put the measuring sensor into the ice water
Allow the sensor reading to stabilise.
Use a phone to film the temperature with time for the step change. You can film the counter/timer in the same frame with the thermometer but you can usually get the times in seconds on the camera playback itself, as the video is time coded.
Start the clock and perform the step change by moving the measuring device from the cold to hot beaker. The time t = 0 secs will be at the moment you immerse the sensor in the hot water
Keep recording until the temperature has reached an equilibrium. The alcohol thermometer will change quickly at first and you will need to make sure that the meniscus of the alcohol is visible in the camera frame.
Repeat for the all three temperature sensors except you will also perform the reverse operation for the K type thermocouple with the plasticine bulb (i.e. hot to cold).
On completion of all data collection, go through the video and extract sufficient temperature/time points to draw the rise time graph (temperature vs time).

Answer the following questions and hand in before you leave the lab

The temperature sensor with the highest operating frequency given in Fig 1 is that of irreversible indicators (thermochromic sensors) that change colour irreversibly. Postulate what might be the sensing mechanism involved to give such a high bandwidth?

Is an irreversible sensor inconsistent with having an operating frequency? Explain.

Aside from a long rise time and limited bandwidth, what other problem might a high thermal load sensor create?

From your result does bandwidth depend on the direction of the step change?

Analysis and online Hand In. (By the due date)

Submit journal quality graphs of temperature vs time response for sensors a, b and c. Annotate with the calculated bandwidth. For c) include hot-cold and cold-hot step change. (i.e. 4 graphs total)

Refer to figure 1 above that shows the operating frequency of temperature sensors vs operating temperature. Use this diagram to complete the table below to estimate the operating frequency (bandwidth) for infrared imaging, liquid-in-glass and thermocouple thermometers.

Method
Step through your videos and produce tables of temperature vs time for the three temperature sensors a - c for the cold-hot step change and the hot-cold step change for sensor ( c).
From each graph determine the time for a rise in temperature to 99% rise equivalent to 5t.
We will not plot the rise time for the IR camera and assume that this is limited by the frame refresh rate of the image (10 Hz).
Calculate the bandwidth in Hz for each temperature sensor.