Turista 2.0: ¿El experto como cronista de viajes?

University to significantly reduce noise pickup in the Earth’s Field NMR probe was to place the probe inside an open ended rectangular tunnel made from thick copper plates.

A quotation from a local company for 12mm copper plate was around $2000/m2

(Mike Christie Sheetmetals Ltd [2]). Constructing a similar but larger copper box for the Mole probe was not an option due to expense, so aluminium plate was considered instead. 5083 grade aluminium with 0.4% iron content was available so a sample was obtained and tested for magnetic properties using a 1T magnet but none was evident. A quote from the same company for six pieces of 5083 aluminium plate 500mm×500mm×12mm was $490 with cold-cutting to a resolution of 0.1mm an additional $240.

While the cost of aluminium was significantly less than copper, the conductivity of copper was higher (6.45×107_S·m−1 _{compared to 4.00×10}7 _S·m−1_{) thus providing}

greater attenuation against incident electromagnetic radiation. The attenuation is measured in terms of skin depth δ (Ludwig and Bretchko (2000) [96]):

δ = √ 1 πf µσcond

(2.1)

where f is the frequency of the incident electromagnetic wave in Hz, µ is the permeability of the metal in H·m−1_{, and σ}

cond is the conductivity of the metal in

S·m−1_{. The value ofµis 4π×10}−7 _H·m−1_{since the dimensionlessµ}

r is close to unity

for both paramagnetic aluminium and diamagnetic copper (Young (1979) [161]). At the Mole’s operating frequency,δ= 0.035mm for copper andδ = 0.044mm for aluminium.

While copper would have provided better shielding than aluminium, the dif- ference was not important given the ratio of δ to the thickness of the plate. Also important was the density of copper which is significantly greater than aluminium making it less suitable for portable equipment. Aluminium has a density of 2.7×103 _kg·m−3 _{compared to 8.9×10}3 _kg·m−3 _{for copper (Young (1992) [162]).}

The Mole box dimensions above would require around 17.5kg of aluminium com- pared to 57.5kg of copper. 12mm aluminium plate was readily available and suitable for making a sturdy box, although thinner aluminium plate would have sufficed for shielding purposes.

2.3.2

Investigating temperature control options

Assuming a Mole box built from the materials described above, the next problem to solve was how to maintain a lock between the proton Larmor frequency and the B1

tuned circuit frequency. One method for achieving this was to lock the temperature of the Mole probe magnets by maintaining a constant temperature inside the aluminium box. An alternative method could have been to allow the magnet

38 Chapter 2. Experimental apparatus

temperature to drift and automatically adjust the B1 tuned circuit frequency to

track the B0 frequency. This could possibly have been achieved by designing a

tuning capacitor with temperature coefficient to match the magnet drift. This was an interesting option but better suited to a future research project.

To lock the temperature of the Mole’s magnets, the first technique considered was locating the Mole box in a water bath. A water bath was available from the chemistry department with a milli-Kelvin temperature control system. While this may have provided excellent temperature control the apparatus would have become non-portable and uncomfortably close to water. An alternative technique was to use a console-style air-bath unit. Water would not be problematic but the available unit was large and the electrical power components and pumps were located near the location of the noise-sensitive Mole. The air bath was also non-portable.

Heating the aluminium Mole box using copper tubing and a small commercial water bath was considered. The tubing would have been attached to the outside of the Mole box using 3M DP190 Grey Epoxy Adhesive or 3M 9713 Electrically Conductive Tape. Copper piping was readily available and could be bent to the required shape. Water would then be passed through the pipes and the Mole could be heated and cooled over a wide the temperature range. The water bath could be located in another room resulting in minimal electrical or audible interference. The downsides of this technique were introduction of water and piping, maintenance of the water bath, and loss of portability.

An alternative to the previous solutions was to use electrical components to control the temperature. Electronic temperature controllers were widely available from manufacturers such as Omron [113] and Cal [40] and came with many options. For example, they could be powered from 12V, 24V or 240V. Output controls for switching heating and cooling elements on and off included relay contacts, solid state relays (SSR), and voltage and current outputs. Temperature sense inputs were available for Pt100 platinum resistance thermometers and thermocouple sensors. An RS-485 serial communication port was available on some controllers thereby allowing the controller to be configured and data logged using a computer. Temperature regulation in these controllers is typically performed using built-in proportional-integral-derivative (PID) software algorithms, and built-in auto-tuning could be used to determine the optimal PID coefficients for a given environment. These controllers could be purchased locally at a cost of several hundred dollars.

Various heating and cooling components that could be controlled by the tem- perature controller were also available. The first electrical heating component to be considered was an incandescent lightbulb mounted inside the Mole box. This would have provided a heat source, but wiring entering the Mole box was observed in other situations to behave as an antenna to the sensitive MoleB1 coil. A better