ME 322: InstrumentationLecture 26

March 27, 2015

Professor Miles Greiner

Radiation temperature errors, Lab 9.1 Sensors and instructions

Announcements/Reminders• Next Week: Lab 9 Transient Temperature Response• HW 9 is due Monday– Ch 6(86a), Ch 11(6, 10, 11, 14), Ch.9 (37), L9PP add Ch.9(43,

42 (but assume thermocouple conductivity is modeled as iron k = 68 W/mK)

• Midterm II, Wednesday, April 2, 2014– Review Monday– Marissa Tsugawa review sessions: WebCampus?

• Two Extra-Credit Opportunities – Both 1%-of-grade extra-credit for active participation– Open ended Lab 9.1 (described in this lecture)– “Possible” LabVIEW Computer-Based Measurements On-line

Seminar• Time and Place TBA

Radiation Error: High Temperature (combustion) Gas Measurements

• Radiation heat transfer is important and can cause errors• Convection heat transfer to the sensor equals radiation heat

transfer from the sensor– Q = Ah(Tgas – TS) = Ase(TS

4 -TW4)

• s = Stefan-Boltzmann constant = 5.67x10-8W/m2K4

• = e Sensor emissivity (surface property ≤ 1)• T[K] = T[C] + 273.15

• Measurement Error– DTCond = Tgas – TS = (se/h)(TS

4 -TW4)

QConv=Ah(Tgas– TS)

TS

QRad=Ase(TS4 -TW

4)

Tgas

TW

Sensorh, TS, A, e

Conduction through Support (Fin Configuration)

• Sensor temperature TS will be between those of the fluid T∞ and duct surface T0

– Support: cross sectional area A, parameter length P, conductivity k– Convection heat transfer coefficient between gas and support h

• Fin Temperature Profile (from conduction heat transfer analysis):– (dimensionless length)

•

• Dimensionless Tip Temperature Error from conduction– , (want this to be small, )– Decreases as

• L, h and P increase• k and A decrease

T∞

h xLA, P, k

T0

TS

Example

• A 1-cm-long, 1-mm-diameter thermocouple (whose conductivity is k = 20 W/mK, stainless steel) is mounted inside a pipe whose temperature is 350°C. The heat transfer coefficient between gas in the pipe and the support is 100 W/m2K, and a sensor at the end of the support reads 500°C. What is the gas temperature? Assume esensor = 0

• Steady or unsteady• Radiation or Conduction error

Solution

• Sensor temperature: •

• What is given and what must be found?

• What if esensor = 0.2?

Extra Credit Lab 9.1• 1% of grade, April 6-10, 2015– Not Required

• Use a low-cost chip to make a measurement – Open Ended– Turn in a one paragraph proposal summarizing your test

plan, and the supplies you need by Friday, April 3, 2015

• Some Possibilities– Get a sample from www.ti.com– Available in lab (See Lab 9.1 website)• Photo Diode, Hall Effect (magnetic field) Chip, Accelerometer

Chip, LM35 temperature sensor chip• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/

Lab%2009.1%20Extra%20Credit/Lab9.1%20Index.htm

+ 5

AI0DAQ

GND

Needs 200Ω Resistor across output. Use referenced signal EWD (RSE) because VS & Vout use the same ground.

200 Ω

LM35 precision temperature chip

LM35 Data Sheet

• Calibrated directly in ˚ Celsius (Centigrade)• Linear + 10.0 mV/˚C scale factor• 0.5˚C accuracy guaranteeable (at +25˚C)• Rated for full −55˚ to +150˚C range• Suitable for remote applications• Low cost due to wafer-level trimming• Operates from 4 to 30 volts• Less than 60 µA current drain• Low self-heating, 0.08˚C in still air• Nonlinearity only ±1⁄4˚C typical• Low impedance output, 0.1 Ω for 1 mA load

-0.55V

1.5V

-55 C

150 C

Possibilities• Measure boiling water temperature using an LM35• Photo diode output voltage versus distance from a light

source (florescent or incandescent) • Hall effect chip output voltage versus distance from a

magnet• Vibration of a weighted, cantilevered steel or aluminum

beam• There are three “Lab-in-a-Box” setups available for check

out from the DeLaMare (Engineering) Library, which can be used at home if you like.– Measure outdoor light and temperature levels during a 24 hour

period– Dominant car frequency on a bumpy road– Kitchen oven temperature stability using a thermocouple

Problem 9.39 (p. 335)

• Calculate the actual temperature of exhaust gas from a diesel engine in a pipe, if the measuring thermocouple reads 500°C and the exhaust pipe is 350°C. The emissivity of the thermocouple is 0.7 and the convection heat-transfer coefficient of the flow over the thermocouple is 200W/m2-C.

• ID: Steady or Unsteady?• What if there is uncertainty in emissivity?

Power 4 – 10 watts VS & GND

Output Sensitivity

LM 35

For RSE

Absolute Voltage Accuracy:

AVA = 14.7mV = 0.00147 V

Absolute Tem Accuracy: