Biomedical Engineering Reference
In-Depth Information
FL). Alternatively, one can use separate control and heating/fan units (e.g., see products
from Farnam Incorporated, Arden, NC). We build our temperature control units from parts
obtained from Omega Engineering (Stamford, CT). We have had good results with
proportional drivers for the heating elements that clip the waveform of the AC current
driving the heating element and relay-driven “on-off” power sources. For the latter, we
recommend against control units with mechanical relays because they eventually fail.
Instead, we use solid-state relays that are sized to more than adequately handle the
maximum wattage of the heating elements. We also recommend against using high thermal
mass temperature sensors, such as fluid-filled bulbs that lead to a mechanical relay through
a metal tube. These have slow response times and consequently produce long on-off cycles
that lead to temperature cycles. This, in our experience, can result in dimensional changes
in the microscope and cyclical movement of the specimen.
In selecting an air heater, care must be taken to ensure safety and durability. If the
microscope is to be left unattended for significant periods of time (e.g., overnight and
weekends), we recommend against consumer hair dryers rewired so that the heating coil is
driven by the control box. These devices are noisy and the fans will eventually fail with
prolonged usage. When the fan fails, the box cools and the heater is driven constantly
without air flow thereby risking a fire. Commercial grade heat guns designed for heat
shrink tubing applications also work well but should be replaced after approximately a year
of continuous use to avoid fan failure. We use custom heaters built by our machine shop
that consist of a metal box containing a steel tube enclosing a 250 W heater element. At
one end we mount a ball bearing Muffin style fan and at the other we have a snout over
which we slip a 2
3 inch aluminum heater hose. In addition, we mount a cut out
thermostat (Omega Engineering, Stamford, CT) on the steel tube that cuts power to the
heating element should the fan fail and the tube becomes overly hot. Lastly, we use a
double pole single throw switch to control power so that the fan and power feed to the
heater are coordinately turned on or off. That way the heater cannot be energized unless the
fan is on. Powering the heating element without the fan running poses obvious fire dangers.
We favor using lightweight flexible aluminum air ducts because they can be bent to the
desired shape and they will hold the shape without support. Also, we have found that
sufficiently hot air will extract oils from plastic hoses and deposit them on the microscope.
3.2.3 Other Considerations
For the use of sealed preparations (chambers described later, mineral oil capped dishes, or
Rose chambers), a single warmed air feed to the microscope enclosure works well. For open
dish preparations, one must control CO
2
and humidity. To do so we configured the heater
unit with inlet and outlet snouts to allow for delivery and recovery air ducts. Recirculating the
warmed air maintains the humidity and less CO
2
is expended. At the heater box, we tapped in
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