Every 2 minutes initially tie white leads of rtds to

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Unformatted text preview: 17.5 cm cube Winter 2008 Lecture 4 R Temperature measurement 23 – – – – thermistor RTD (Resistive Temperature Device) AD-590 thermocouple • We’ll use platinum RTDs for this purpose We’ – – – – small: very short time constant accurate; no need to calibrate measure with simple ohm-meter R = 1000.0 + 3.85 (T 0° C) • so 20° C would read 1077.0 Winter 2008 24 6 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Random Notes Random Notes, continued • Rig fan and resistor in parallel, running off 5V • Measure temp. every ~2 minutes initially – tie white leads of RTDs to common DVM all together – label red lead so you know where it goes – fan can accept range: 4.5–5.5 V – if you want independent control, d on’t rig together • After equilibrium is reached, measure skin temperatures • Use power supply current reading (plus voltage) to ascertain power (P = IV) being delivered into box • Make sure all RTDs read same thing on block of thermally stabilized chunk of metal – hold in place with spare foam (not finger or thermal conductor!) – best to note time of each digit change • allows extrapolation to final (slow otherwise) • We have only 30 RTDs, so two per group will be standard RTDs, – account for any offset in analysis • Use adhesive spray and foil to line walls, if applicable • Make sure you have no air gaps: tape inside and out of seams – but need to leave top accessible – nice to tape fan to top (avoid heat buildup here) – can hang resistor, RTD from top as well (easy to assemble) Winter 2008 25 – locate inside RTD in fan exhaust, so representative – use external RTD for ambient, skin – some experiments will want 3 RTDs (gradients) • Once equilibrated, go to configuration B – turn on external fan, coat with foil, poke a hole, cold finger Winter 2008 26 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Random Notes, continued Example Series • Send your data points to me via e-mail so I can present the amalgam of results to the class – use format: hh:mm RTD1 RTD2 RTD3 etc. – example: 11:43 1088 1155 1152 1228 – include a description of what each column represents • Also include basic setup and changes in e-mail so I know what I’m plotting I’ • Also include in the message temperatures you measure only once, or occasionally (like skin temp.) • I’ll make the data available for all to access for the write-ups Winter 2008 Lecture 4 27 Winter 2008 28 7 Thermal Considerations 01/17/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Temperature differences References and Assignment • Useful text: – Introduction to Heat Transfer: Incropera & DeWitt • Reading in text: – Chapter 7: skim 7.1.1 & 7.1.2 for interest (at least read first several paragraphs of each); all of 7.1.3, 7.1.5; peruse tables 7.4 and 7.5 Winter 2008 Lecture 4 29 Winter 2008 30 8...
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