Fly Incubator Safety
Fly Incubators
Most fly workers keep their stocks and crosses in incubators, rather than in temperature controlled rooms. Incubators are far cheaper to install, and, obviously, take less space. A typical 30 cubic foot model has more than enough room for most stock collections, assuming that the flies are maintained in vials. With two 10x10 trays per shelf and eight shelves, one can store 1600 vials, with room to spare. A 15 cubic foot model will hold up to 1200 vials; these smaller models are typically much cheaper than the 30 cu. ft. ones, although they take up almost as much floor space in the lab.
Incubators have an additional advantage over temperature controlled rooms if one ever needs to eradicate an outbreak of mites. The mites will hide out in moist and moldy cracks in either case, ready to re-infest stocks you might have cleaned up. With an incubator, one can first chill the entire unit to nearly -20º C for a few days. The mites slow down in the cold, making it difficult for them to beat an escape. One can subsequently cook the interior at 50-60º C for good measure, although some interior components may not be designed for extended periods of high temperature. Temperature controlled rooms are seldom able to reach such extreme temperatures.
The major downside of an incubator is that it will eventually break, and that could be catastrophic for a stock collection. The most common failure is death of the refrigeration compressor. Many incubators (especially the more affordable ones) are modified from standard freezer units. The modification involves the addition of heater coils; the compressor is typically run continuously, and the heaters are turned on intermittently to keep the interior at the set temperature. The compressors on standard freezers are not meant to run continuously, and so the compressors in incubators have shortened life spans (perhaps 5 years). Another problem with this design is that it is quite energy inefficient, especially when the incubator is maintained at 25ºC. A 30 cubic foot incubator consumes 1000-2000 watts, which is a major heat load, especially if the unit is placed in a small room. The second most common failure event is the loss of refrigerant from leaks in the cooling coils. The growing yeast in fly cultures generates acetic acid that may condense on the coils and induce corrosion. The problem is more severe if your fly food recipe calls for phosphoric and propionic acid to inhibit molds. Many manufacturers offer to coat the coils in plastic to impede corrosion--this option is definitely worth the extra cost. Another failure mode is the lock up of a switching relay. Mechanical relays eventually fuse their contacts due to the arcing that happens at every switch event; solid state relays (more common in modern circuits) last longer, but can still fail. Such failures have been known to freeze stock collections.
Incubators typically have low and high safety limits, which should, in theory, shut off the unit when the interior temperature goes below or above the set range. However, almost all commercial units continue to provide power to the internal air circulation fans (also called evaporator fans) even when the safety limits are exceeded. These fans generate enough heat by themselves to raise gradually the internal temperature to 37º or more. If the compressor fails and one doesn’t notice it right away, the flies are cooked.
The first and best line of defense is to keep valuable strains in duplicate, one copy in each of two incubators. The next best strategy is to modify the incubators to shut off the circulation fans in safety mode, and to cycle the compressor. Modifications of the wiring may well void any manufacturer’s warranty on an incubator. If one is purchasing a new incubator, it is advisable to have any modifications pre-approved, or, better yet, to convince the manufacturer to implement the modifications.
If one can obtain a wiring diagram for the incubator, one should be able to find the wires that power the fans. This circuit should be connected in series with the high-temperature safety switch or relay (Figure 1). The high temperature safety switch, which is normally closed, is also in series with the heater coils. When the safety temperature is exceeded, the switch opens, and the heater (and now the fans) loose power. Note that the high temperature safety switch is downstream of the heater in the circuit shown in Fig. 1. If the incubator has this safety switch placed in the circuit between the main temperature control and the heater, the safety switch should be rewired to be downstream.
Cycling the compressor is most easily done with the help of an additional temperature control device that will power the refrigeration compressor only when needed. The idea is to turn on the compressor for a short time to drop the temperature a few degrees, and then to let the temperature slowly drift up due to heating by the circulation fans. The heating coils never need to be powered, and so the power consumption is minimal. An effective temperature controller is the Dyna-Sense MkI, made by Scientific Instruments, Inc. (VWR catalog #61324-811, plus temp sensor, catalog #61324-866; total cost about $600). The “On/Off” version of the Dyna-Sense controllers include an adjustable hysteresis or deadband feature, which is essential for the periodic cooling described above.
The power circuit to the refrigeration compressor should be interrupted by the Dyna-Sense controller (Figure 2). This controller measures the incubator temperature with its own thermistor probe, which must be placed inside the incubator chamber. The controller has a 110V switched output, but it is only rated for 10 Amps, which is insufficient to run the refrigeration compressor directly. Thus the controller is used to power a solid state relay (25A,110V input/110V output, normally open; McMaster-Carr #7456K13), which then switches the power for the compressor. The relay should be mounted (with silicone heat-sink compound) to a large metal surface, to dissipate heat from the relay. As shown in Fig. 2, the output end of the solid state relay can be connected to an external bypass switch (ideally, a toggle switch rated for 25 A, mounted on the front panel). When this switch is manually closed, the control by the Dyna-Sense unit is bypassed, and the incubator functions just as it did without the modification.
If one wished to maintain stocks at 18º C, for example, with this setup, one would first set the normal temperature control on the incubator to about 16º, with a low safety point of 15º and a high safety of 22º. The Dyna-Sense controller should then be set to 17º, with a 3º deadband. With these settings, the compressor runs until the internal temperature drops to 17º, and then it shuts off. The temperature drifts up due to heating by the circulation fans. When the internal temperature reaches 20º, the compressor kicks in again. Such a cycle might repeat every 30 min., with 5 min. of compressor time; the cycle times will vary depending on the amount of heat generated by the fans, the outside temperature, and the heat capacity of the stored fly vials. The temperature experienced by the flies inside a plastic vial is about 18º, and varies by less than 1º throughout such a cycle. If the Dyna-Sense controller should fail, the safety limits of the incubator are still functional, and should prevent any catastrophe.
With the cycling compressor as described, the heater is never used, and the cooling coils never stay cold for very long, so that condensation-induced corrosion is reduced. If the incubator has a defrost cycle, it should be manually switched off, if possible, since there is never any frost buildup. However, the cooling coils will not be able to remove excess humidity by condensation, which could be a problem if the ambient humidity is high. Likewise, if the incubator is meant to be humidity controlled, the compressor cycling scheme cannot be used, since the condensation on the cooling coils is the only way the system can reduce humidity. Some investigators use humidified incubators for maintaining large population cages to harvest gram quantities of embryos. Humidified incubators are expensive, and they must be connected (with high pressure fittings) to a source of filtered, deionized water. There must also be some provision for draining the condensation. An effective alternative to active humidity control is to set in the incubator a large shallow dish, holding water and some sponges. Evaporation from the sponges is usually sufficient to keep the laying cages humid and productive.
Some Incubator vendors:
Barnstead International (Lab-Line) www.barnsteadthermolyne.com
Binder www.binder-world.com
BioCold Environmental www.biocold.com
Fisher Scientific www.fishersci.com
Harris / Puffer Hubbard (now Div. of Thermo Electron) www.harrisphq.com
Nor-Lake Scientific www.norlake.com
Percival Scientific www.percival-scientific.com
Precision / Napco (now Div. of Thermo Electron)
Revco (now Div. of Thermo Electron) www.revco-sci.com
Sheldon Manufacturing www.shellab.com
Terra Universal www.terrauniversal.com
VWR Scientific Products www.vwrsp.com
