Reference no: EM133662082

Sensors and Sensing for Scientists

Wheatstone Bridge, Salinity sensor, Phototransistor/LED & Turbidity Sensor

Introduction

This practical consists of three parts.

Part A, you will determine an unknown resistance using a Wheatstone Bridge circuit.

Parts B1 and B2, we will breadboard a simple phototransistor/LED circuit and then use a commercial turbidity sensors that utilises a similar circuit to make some turbidity measurements.

Part C1 and C2, we will assemble a freshwater salinity sensor, do a one-point calibration and then measure the conductivity of three saline solutions.

General instructions

You will work in pairs for this experiment - It might be easier to detach the results/question pages from the lab notes whilst doing the prac.
All students should start on section C1, assembling the salinity sensor as the silicon sealant requires time to dry. Students should then proceed to do parts B1 and B2 with Part A being completed at any time in the lab when the equipment is not in use by another group. The last part to be completed will be section C2

PART A: The Wheatstone Bridge

Aim - To determine an unknown resistance using a Wheatstone bridge circuit.

PART B1 Phototransistor/LED operation

Aim
To demonstrate operation of an IR phototransistor and IR LED in active (analogue) and switch mode (digital). The circuit we are using is typical of that found in many commercial turbidity sensors.

PART B2 Turbidity Sensor Transfer Function

Aim
In this practical, we will wire a commercial turbidity sensor (that uses a phototransistor and LED as section D1) into a sensing circuit, record the sensor output against solutions of known turbidity, plot the transfer function and determine the operating range.

Part C Freshwater Salinity Sensor:

C1 Build a Freshwater Salinity Sensor part1

You will be provided with a salinity sensor kit comprising an end cap, tube, 2 x carbon electrodes, wires with alligator clips. You only need the end cap and the two carbon electrodes - we will prepare the sensor for use later in the prac.

Ensure the carbon electrodes are clean and free from silicone as they are reused from year to year.

Push the two electrodes through the two pre-drilled holes in the end cap to give a protrusion of about 1 cm. (you can eyeball this - it doesn't need to be exact)

To keep the electrodes reasonably straight you may need to insert a cardboard spacer back from the end cap.

Silicone the electrodes to the end cap. We want to SEAL the end cap so it doesn't leak but we don't want to cover the exposed electrodes with silicone.

Leave for 2 hours to dry

C2 Complete Freshwater Salinity Sensor build and make Measurements

In this section, we will complete the build of a salinity sensor, do a one-point calibration and compare our results to the literature conductivities for salt solutions in range 0.1 - 0.5 % by mass.
Get the salinity sensor kit that you started earlier in the prac.
Attach alligator clips and wires to the carbon electrodes
Put the sensor housing over the wires and carbon electrodes and push the housing into the end cap.

We will now connect the sensor to a waveform generator and DMM as shown below:
The cable from the waveform generator BNC output splits into a red and black cable. The red output from the waveform generator should connect to a 120 Ohm resistor and then to one of the wires of salinity probe. The other wire from the salinity probe will connect back to the black cable going to the signal generator. Then connect the DMM across the fixed resistor.

The waveform generator will have been previously set by the technician for a sinusoidal 1 Volt peak to peak supply at 50Hz.
Note, we are connecting a ~120 Ohm resistor in series with salinity sensor. The two resistances are in series and each resistor is part of a voltage divider.
You must measure the exact resistance of your"120 Ohm" resistor
Once your set up has been checked by a demonstrator, switch the waveform generator on. The screen should look like this:

Set your DMM to read the rms (root mean square) voltage over the fixed resistor. This may be done differently on different DMM's, so consult your demonstrator.
Record the voltage drop over the fixed 120 Ohm resistor in Table 1, when the tip of the carbon electrodes are immersed in:

0.1% sodium chloride solution
0.3% sodium chloride solution
The 5000 mS/cm conductivity standard
0.5% sodium chloride solution

Part A Wheatstone Bridge Questions:

Calculate the unknown resistance along with its uncertainty here. Show workings

PART B1 Phototransistor/LED operation

What mode of operation would suit the PTR set up like this?

What mode of operation would suit the PTR set up like this?

PART B2 Commercial Turbidity Sensor

Take a picture of this table or record this data elsewhere - you will need to take the data away for the online hand in for prac 3 next week.

PART C Salinity Sensor Questions (Hand in when leaving)

Why is the voltage kept low (<1.0 V) across the electrodes in the salinity sensor? (Remember your basic chemistry)

Why is an alternating power supply used? (Remember your basic chemistry)

PART C2 Questions

Follow the method in the Appendix to convert your measured voltage values for the three saline solutions and conductivity standard solution to resistance and then conductivity values. (You need to use equation 6 in the appendix with a predetermined value of K)

To calculate conductivity for your saline solutions, you must calculate the constant, K. This is calculated from the standard solution whose conductivity is known. See example 1 and 2 in the Appendix.
Do what you need to make this table Journal Quality.

You must include the literature conductivity/% saline data below on the graph.
In your Figure caption include your K value, determined from the conductivity of the standard.

II. Method for Calculating Conductivities

For all solutions, determine the solution resistance from the formula below. We did not measure the solution resistance directly and the solution resistance in series with a fixed "120-Ohm" resistor and a hidden internal resistor inside the waveform generator of 50 Ohms.

Where:

R(soln) = resistance of saline solution
R(fixed) = resistance of the ~120 Ohm resistor. Use your exact value e.g. 117.8 ?
V(supply) = 0.707 V
V(measured) = DMM voltage measured over the fixed resistor

Determine the constant, K from the data for the standard solution (5000 mS/cm) as Example 1. This represent a one-point calibration.
Use this value of K along with the measured resistance to find the conductivity of each saline solution as example 2. Include also, the standard solution in your calculation:

Note, your value of K and the measured resistance of the standard solution will necessarily give you 5000 mS/cm if you have done the calculations correctly!

Attachment:- Wheatstone Bridge.rar