ME 322: Instrumentation
Lecture 18
February 29, 2016
Professor Miles Greiner
TC signal conditioner, Computer data acquisition
introduction, Lab 7, myDAQ, LabVIEW program
Announcements/Reminders
• Please pick up graded Midterm 1 during lab periods
• HW 7 due Friday
– Upload LabVIEW VI to WebCampus
• Lab 6 (wind tunnel) this week
– See WebCampus for schedule
– Please be on time.
– Bring Excel from HW 6 and use it to process the data
you acquire.
– This will help check the data as you take it and allow you to complete
the data acquisition phase quickly.
Thermocouples
TS
TT
+ V
OUT
-
• Consist of two dissimilar metal wires, A(+) & B(-)
connected to a digital voltmeter (copper terminals C)
• Output voltage VOUT depends on the sensing junction
temperature TS
– However, VOUT’s sensitivity S = 20–50 mV/°C is very small
– Moreover, VOUT also depends on (undesired inputs)
• Terminal temperature TT
• High Frequency Radio Frequency Noise
• Ground loop currents
In ME 322 Labs
+
Constantan (
)
-
• Type J (Iron/Constantan) thermocouples
– + Iron (white insulation)
– - Constantan (red or white with red stripe insulation)
• Omega DRE–TC-J TC Signal Conditioner ; web
TT
– http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Inst
rumentation/Labs/Lab%2007%20Boiling%20Water%20Temperatu
re/Lab7%20Index.htm
– Wiring:
• Iron (white insulation) goes to +Tc; Constantan (red stripe) goes to (-Tc)
• Output Voltage, VSC: Com = -; VOUT = +
• Power: 110 VAC (top)
Transfer Function (Type-J-TC/DRE–TC-J TC)
Transfer
Function
10
Reading
VSC [V]
• For 0℃ < 𝑇𝑆 < 400℃
– 𝑉𝑆𝐶 = 10𝑉
𝑇𝑆
400℃
= 0.025
𝑉
? Out of
range
𝑆𝑆𝐶 =
0
𝑉
℃
0
Measurand, T [°C]
𝜕𝑉𝑆𝐶
𝜕𝑇
400
𝑇𝑆 (linear)
𝑉
𝑆
• 𝑆𝑆𝐶 = 0.025 °𝐶 ; 𝑆𝑇𝐶,𝐽 = 0.00005 °𝐶; G𝑎𝑖𝑛 = 𝑆 𝑆𝐶 = 500
𝑇𝐶,𝐽
• For boiling water 𝑉𝑆𝐶 = 10𝑉
100℃
400℃
= 2.5 𝑉, For ice 𝑉𝑆𝐶 = ?
– To use: Inverted transfer function: TS = (40°C/V)*VSC
• Conditioner Provides
–
–
–
–
–
Reference Junction Compensation (𝑉𝑆𝐶 not sensitive to TT)
Amplification (Allows normal DVM or computer acquisition to be used)
Low Pass Filtration (Rejects high frequency RF noise)
Linearization (Easy to convert voltage to temperature)
Galvanic Isolation (TC can be used in water environments)
Computer Data Acquisition (DAQ)
• Sensors detect measurands and produce signals
– Voltages, currents, resistances, pulses,…
• Conditioners convert those output signals to moderately large voltages
• Multiplexer (MUX) sweeps channel-to-channel and feeds individual signals,
at different times, to the Analog-to-Digital (A/D) converter
• A/D converter samples real voltages (7.674337…V) and converts them to
integers (0, 1, 2,…) that the digital computer can work with.
• Computer programs store and/or process the data
– In ME 322: LabVIEW and DAQmx drivers
Lab 7 Boiling Water Temperature in Reno
•
•
•
•
Elevation 4500 ft, PATM = 86 kPa (101.3 kPa at sea level)
Beaker, water, immersion heater
Type-J TC, DRE–TC-J TC Signal Conditioner
National Instruments myDAQ (USB to computer)
National Instruments myDAQ
• $179 on Studica
– http://www.studica.com/us/en/NI-Hardware-Only/student-ni-mydaq/78132501_2.html?utm_source=google&utm_medium=ppc&kpid=734446&gclid=CJXw956H8LwCF
Ylafgodb3wAMg
• Reads TC Signal Conditioner output (or other device)
– Connect to Analog Input (AI) Channel 0 (could use channel 1)
• COM to 0- and AGND (negative analog input and ground)
• Vout to 0+ (positive analog input)
• This is a Differential Input
– Difference between two channels, gives better common-mode rejection
Check Connections
• Open NI–MySystem-Measurement &
Automation Explorer (NI MAX)
–This program comes with LabView
• MySystem (upper left corner)
–Device & Interfaces (Double Click)
• MyDAQ (Right Click)
– Device Pin outs
– Test Panels
Lab 7 LabVIEW Data Acquisition Program
• Called a VI – Virtual Instrument
– Front Panel – Indications and Controls
– Block Diagram – Devices, connections and logic
• Lab 7 Objectives
– Measure VOUT from AI channel 0 for 3 seconds
• Sample rate 1000 times per second
• Convert to TS = (40°C/V)VOUT
– Plot Temperature versus time on front panel
• Write data to an Exel file
– Find average temperature, TAVG
• LabVIEW and DAQmx drivers are available on campus in
PE2, ECC, and DeLaMare library
LabVIEW Five Main Acquisition Steps
Measurement I/O, NI DAQmx
1) Create a channel
2) Timing
3) Start Process
4) Read Data analog waveform
–
–
–
1 Channel
N-Samples
Output voltage – convert to ̊ C
5) Clear the test
Programming; Dialog and User Interface
simple error handler
45-day LabVIEW free trial: http://www.ni.com/download-labview/
Wait until April 1 so you’ll be able to use it to study for final?
You will need to download and install DAQmx to on your computer (after
installing LabVIEW) find these icons (these processes take a long time).
In this class we give and modify example LabVIEW programs to help you
quickly learn to perform data acquisition. However, we don’t deal with
structured programming.
LabVIEW program (block diagram)
• I will repeat this construction next lecture
LabVIEW Front Panel
MUX
Lab 7 VI Objectives
• Measure atmospheric pressure PATM
– Find TSAT at that pressure
• Allow water in beaker to reach a full, rolling boil
• Measure VOUT for 3 sec
– Sample 1000 times per second
– Convert to TS = (40°C/V)VOUT
• Plot Temperature versus time
– Write to a file
•
•
•
•
•
Find average temperature, TAVG
Repeat for ice/water mixture and room temperature water
Compare to liquid in glass thermometer
Is TAVG,Boil ± wTavg ~ TSAT
Determine effect of salt in the water