Transcript 2/9 Lecture Notes

Chem. 133 – 2/9 Lecture
Announcements
• Return Q1 + HW 1.1
• This week’s seminar: on aerosol air pollution in Asia
• Lab
– Sign up for term project instruments. I hope to have some
better defined STORC term projects soon.
• Today’s Lecture
– Electronic measurement
• digitization in digital voltmeters
• use of voltmeters in measurements
Electrical Measurements
Analog to Digital Conversion
• Performance Measures:
– Number of bits (more bits means analog signal is
converted to more precisely known digital signal)
– To ensure that digitization is NOT the limiting factor
to sensitivity, noise should be seen following
digitization
– Speed (frequency): boards used in class could
operate at up to ~100kHz. High speeds are needed
for fast measurements.
– Input range: the minimum voltage will correspond to
all 0s and the maximum voltage will correspond to all
1s. Voltages greater than the maximum will be read
as the maximum.
Electrical Measurements
Analog to Digital Conversion
• Example of digitization of data (HPLC with fluorescence
detector)
digitization at 0.01 level shown
Flurescence
signalsignal
"Analog"
flurescence
1.21.18
1.181.16
Signal (FUs)
Signal (FUs)
1.241.22
1.22 1.2
1.161.14
1.141.12
"Analog"
1.12 1.1
1.11.08
Digitized
1.081.06
1.061.04
0
1.04
0
0.2
0.2
0.4
0.4
Was digitization o.k.?
0.6
0.8
0.6
0.8 (min) 1
Time
Time (min)
1
1.2
1.2
1.4
1.4
Electrical Measurements
Analog to Digital Conversion
• Second Example:
– A pH meter is used to monitor a process where a solution is
acidified and then neutralized. The pH range that is desired to
measure is 1 to 8.
– The equation for the relationship between voltage and pH is
found to be Voltage (in mV) = 231 – 60.1∙pH
– The analog to digital convertor is a 12 bit convertor with the
useful input range from -250 to 250 mV.
– Answer the following questions:
• Before the solution is acidified, the binary # = 010 001 011 111.
What is the voltage and the pH?
• After acidification, the voltage = 172 mV, what is the decimal # and
pH corresponding to this?
• What is the maximum pH that can be read?
• Can a difference between pH = 7.00 and 7.05 be discerned?
Electronics
Digital Volt Meter (DVM) Measurement
• Use of DVM for V, I, and R measurements
voltage
Shunt
resistor
Current
Thermocouple
+
-
Pair (generates V)
Iout
+
-
transducer
DVM
I = Vmeter/Rshunt
DVM
Multimeter
Electronics
Digital Volt Meter (DVM) Measurement
• Resistance Measurement
thermistor
DVM
Constant
I source
multimeter
example problem: The power source puts out 1.00 mA and the
voltage read is 0.722 V, calculate the resistance
Electronics
DVM Measurements
• Errors in Measurements
– Errors in voltage measurements:
•
can occur if a device also has
"internal resistance"
• in combination with less than infinite
resistance in DVM
– Example: measurement of voltage
from an ion selective electrode or pH
electrode. Calculate the error in
voltage if a pH electrode reads 0.721
V and has an internal resistance of
830 kΩ if the DVM has a meter
resistance of 10.0 MΩ.
– (go to blackboard)
R(cell)
R(meter)
DVM
Cell
Cell = pH electrode
Electronics
Transducers
Definition:
A transducer is a device
that converts a physical
(or chemical) property
into an electrical signal
Classifications:
•
•
•
By output measure (V,
I, R, frequency)
By phenomenon
measured (charged
particle flux,
temperature, light
intensity, surface
modification)
Internally vs. Externally
Amplified
Transducers
Charge Particle Detectors
• Measurement of
electrons, molecular
ions and charged
aerosol particles
• Most common type
for GC and MS
edetectors
Charge Collector or
Faraday Cup
Can detect currents > 10-15 A
I
Transducers
Charged Particle Detectors
Detection Process
Electron Multiplier (MS detector)
1.
2.
M-
e- e
3.
I
Cathode
Dynodes
4.
Charged particle hits
cathode
Electrons emitted from
collision
Amplificaion occurs with
each stage
Current (electron flux)
increases before anode
Example: if each stage produces 6
useful electrons out per ion in,
amplification in current would be x63
or x216. With greater amplification,
single particle detection is possible
Transducers
Measurement of Temperature
• Applications:
– Temperature control (e.g. GC ovens)
– Infrared light
• Resistance based
– Thermistors and platinum resistance
thermometers (both have R = f(T))
• Voltage based
– Thermocouples (voltage generated by metal
junction which depends on T)
Transducers
Detection of Light
1. Vacuum tube types
-
-
-
Based on photoelectron effect
Current based detectors
Photocells (see diagram)
All have minimum energy
(maximum wavelength) where
electron ejection just occurs
Photomultiplier tube
(combination of photocell and
electron multiplier)
Photomultiplier tube allows
detection of single photons
e-
I
Transducers
Detection of Light
2. Solid state types
- Typically less expensive than vacuum tube
types
- Tend to operate better at longer wavelengths
- Based on promotion of electrons to conducting
bands
- Photodiodes (I proportional to intensity)
- Photoconductivity cells (R dependent on
intensity)
- Photovoltaic cells (V dependent on intensity)
- Advanced devices (discussed in spectroscopy
section)
- Arrays (1D or 2D sets of detectors)
1D Photodiode
Array
+
e- e
n
e-
p
-
Reversed-bias
photodiode:
High impedance
until photons
arrive
Some Questions on Transducers
1.
2.
3.
4.
5.
List a transducer with a (primary) current signal.
List a transducer with a (primary) resistance signal.
List a transducer that can be used to measure charged
particles.
What is the main reason that a photomultiplier tube is
more sensitive than a photocell?
Give an example of a transducer that is readily
available in an array form.