Mechanical Measurements and Metrology Subject Code: 10ME32B Prepared By: Nikita Department: Aeronautical Engineering November 23, 2018 MMM Notes

Temperature and strain measurement UNIT 8 Temperature and strain measurement

Strain Measurements

Metallic Gauges If you have a conductor of resistivity r, the resistance across that conductor is If you strain this conductor axially, its length will increase while its cross sectional area will decrease. Taking the total differential of R,

Metallic Gauges Gage factor For most strain gauges, n = 0.3. If the resistivity is not a function of strain, then F only depends on poisson’s ratio, and F ~ 1.6.

Strain Gauge F and R are supplied by the manufacturer, and we measure ∆R.

Bridge with 2 strain gages The bending strain on the top gage is equal and opposite of the one on the bottom.

Wheatstone Bridge make R2 = R4 = R

Multiple Gauge Bridge Most strain gauge measurement systems allow us to make 1, 2, 3 or all 4 legs of the bridge strain gauges. There are many reasons to do this that we will talk about now. Going back to our fundamental bridge equations from chapter 6, Say that unstrained, all of these have the same value. If they are then strained, the resultant change is Eo is Eo

Multiple Gauges Make the following assumptions: All gauges have the same nominal resistance (generally true) All gauges have matched gauge factors (must be purchased as set) Then: Eo

Temperature Measurements Thermometry based on thermal expansion Liquid-in-glass thermometers The manner in which a thermometer is calibrated needs to correspond to how it used. Under normal circumstances, you can get accuracy from ±0.2 to ±2°C.

Bimetalic Thermometers If you take two metals with different thermal expansion coefficients and bond them together, they will bend in one direction if the temperature rises above the temperature at which the boding was done and in the other if it gets less.

Resistance Temperature Detectors RTD

Thermistors Usually made of a semiconductor and have the following properties: Much larger dR/dT than RTD’s, so more sensitive Rugged Fast Response

Thermoelectric Effects Seebeck Generates voltages across two dissimilar materials when a temperature difference is present. Peltier Moves heat through dissimilar materials when current is applied.

Thermocouples Thermocouples measure the difference in temperature between two points. One of those points at a known temperature.

Temperature Measurements Conduction: Your probe can conduct heat to/from the environment to/from your desired measurement location

Radiative Temperature Measurements (Pyrometry) Temperatures greater than 500ºC s = 5.67•10-8 W/m2K4