Some Mysteries Solved

http://people.csail.mit.edu/jaffer/SimRoof/Convection/Measurements has been updated with photos of the new insulation, derivation of the model for the back surface, and measurements of natural convection.

In the first version of the electronics the plate and ambient temperature sensors had 3 times the variance of the back surface sensor.  I also noticed that, in most of the runs, the ambient temperature would have an initial ramp up in the first 5 minutes.

The LM35CZ temperature sensor datasheet warns of self-heating for sensors not attached to heatsinks.  So I added circuitry and modified the program to power on the sensor 2ms before it is read and turn it off after.  Surprisingly, it didn't eliminate the ramp, but it reduced the variance to a level comparable with the back surface sensor!

I checked the other components and found that the MPXH6115A6U atmospheric pressure sensor dissipates 30mW (and has a long warm-up time in its specification).  The LM35CZ was mounted next to the MPXH6115A6U on the same header; heat conduction through their leads was responsible for the warmup.  I moved the LM35CZ off of the header and connected it with thin wires, which solved the problem.

I am still left with the mystery of why the LM35CZ in the plate has the large variance.  If the thermal adhesive which fastens it in a hole in the aluminum plate is broken, is self-heating creating the variance?  Disassembling the insulation to examine it may damage the insulation, requiring it to be replaced; so I will leave it as it is for now.

With the upward natural convection measurement looking reasonably consistent with the model, it is time to mount the plate in the wind tunnel.

When I rebuilt the electronics, I improved the sensitivity of the opto-interrupter.  Its sensitivity is now high enough that it responds to reflection of the fan blade.  This allows the led and phtotransistor to be on the same side of the fan, which simplifies mounting.  The photo shows the blue board with the LED adjacent to the board with the phototransistor.

Convection Measurement Electronics

Pictured are two electronics boards constructed for measuring convection from an instrumented plate.  The small board on the left will measure pressure, humidity, and temperature of the air at the wind-tunnel intake.

The larger board contains power supplies, heater control and drive, signal conditioning, a three-digit 7-segment display, and a STMicroelectronics ARM STM32F3 Discovery board.  The ARM computer is the green board.  It will control the heater embedded in the plate and collect the sensor data during experiment runs.

The two yellow blocks connected by black cables to the larger board are receptacles for Lenovo laptop 20 Volt power supplies.  The large black object is a heat sink for the heater drive.

The other photo shows the wiring on the back of this board.  Although there are many soldered connections, most of the components are mounted on 16-pin DIP headers seen on the front side.  These are plugged into prototyping sockets with insulation-displacement contacts.  Connections made using 30 AWG wirewrap wire.  This technology was sold by 3M in the 1990s.  I don't think it is sold commercially any more.

Convection Instrumentation

My first design for the plate heater involved building a 100+ Volt power supply.  Since then I have realized that by stacking two laptop power supplies, a 40.V supply could be had inexpensively.  Each supply is rated for 90.W, so they have ample power for the heater.  The maximum current the heater will draw is 2 Amps; in order to account for wire losses, the voltage across the heater will be sensed by another pair of wires.

The first design had a digital-to-analog converter driving the gate of a SD220 n-channel MOSFET, and its output driving the gate of an IRF9520 p-channel power MOSFET.  The forward gain of the circuit was over 1000.  With such high gain, only a few of the DAC codes would cover the range of planned drive levels for the heater.  So I have ballasted both MOSFET source terminals with resistors to lower their gains.  The SD220 guarantees its threshold only to be less than 3.V, but the STM32F303VCT6 microcontroller DAC output only drives up to 2.8.V, so an op-amp is needed to servo the source resistor to the DAC output voltage.  Servoing the IRF9520 current to be proportional to the SD220 current means that, within its operating range, the current is set by the DAC.  Multiplying that current by the voltage sensed across the heater measures the power delivered to the heater.

The design includes electronic temperature, humidity, and air pressure sensors.  The only relevant condition not being sensed is windspeed in the chamber.  I investigated many windspeed sensing technologies, but techniques for measuring below 4.m/s require laminar flow, affect the flow, are too large, are inaccurate or are prohibitively expensive.

With a variable transformer I measured consistent windspeeds in the tunnel from 4.m/s down to 1.m/s.  The windspeed varies smoothly with the rotation rate of the fan.  Instead of trying to servo the fan's rotation rate, I will measure the rate of the fan blades interrupting an infrared beam between a LED and photo-transistor on opposite sides of the fan.  Calibrating the relation between windspeed (as measured by an impeller anemometer) and the fan's rotation rate will enable tracking of windspeed over small variations in rotation rate.