Laboratory Tidal Simulator

My sister Lydia is a PHD student at the University of California at Santa Barbara (UCSB), performing research on ocean acidification (link to her page: lydiakapsenberg.weebly.com). In 2013 she mentioned she was interested in an experiment to test the affect various concentrations of CO2 in the ocean water would have on a group of mussels, but that she did not have the experimental facility to do so.

The experiment was to require control of both simulated ocean tides and solar heat flux on a timed cycle for 5 parallel tanks. Each tank would be subject to a different concentration of CO2, while all other factors were to remain constant. Since the experiment would be have to run continuously for a month, this ruled out manual control, and she was on a tight budget. I mentioned that I might be able to design something for her that could accomplish this, and that I could provide her with an accurate budget estimate before having to commit to it.

Lydia accepted my offer and over the course of a week or two, we settled on the design shown in Figures 1-3. Figure 1 shows the design of the fluid flow loop, including tanks, pumps, and valves.

The header tank on the right provides the controlled concentration of CO2, and is set to feed the reservoir and primary tanks at a constant rate set by adjustable valves. This equipment was part of the existing laboratory infrastructure, and was therefore not modified. A pump (12V 3A DC Gear Pump) serves to pump water between the primary tank in which the experiment would take place, and a reservoir tank. During low tide the pump would drain the primary tank to below the level of the mussels, and during high tide would pump water from the reservoir tank back into the primary tank to submerge them. The constant flow of water from the header tank was drained by an overflow valve in both tanks. All three tanks are placed in a large bath of fresh ocean water to maintain a constant temperature.

The "control panel" was chosen to be made from an existing desktop computer tower due to the widespread availability of obsolete desktop computers at most campus IT offices, its ability to enclose the system safely, and the ease at which the sheet metal frame could be modified to mount all the required systems. Additionally, all components were chosen to work on 12V, allowing the use of the computer's ATX power supply to power the system, as the power supply and housing were initially expected to capture a big portion of the budget, this resulted in a substantial cost savings.

The 12V rail powers a timed relay that can be set by the user (THC15 12VDC Digital Programmable Switch). This relay switches a secondary 12V line (shown as red) to control a 4-pole-double-throw relay (McMaster 4PDT Relay). The four channels of the 4PDT relay have the following functions:

• Switch to turn on the heat lamps (one switch pole), drawing no more than 10A total

• Pump H-bridge to control pumping direction (two switch poles)

• Switch to route the pump's power through either upper or lower liquid level switches (Water Level Float Switch, one switch pole)

Here is how it all works:

1. When the timer relay is closed:

   • Heat lamps are turned on

   • Lower liquid level switch is set as the active switch (current runs through blue wire)

   • Pump direction is set to pump water out of the tank until the water level drops below the lower liquid level switch

   • Steady inflow of fresh water from the header tank periodically trips the switch to allow the pump to move the excess water into the reservoir tank

2. When the timer relay is opened:

   • Heat lamps turn off

   • Upper liquid level switch is now active (current runs through yellow wire), and is closed due to the to the low water level.

   • Because the upper liquid level switch is closed, the pump begins pumping water from the reservoir tank into the primary tank thanks to the H-bridge having reversed the direction of current through the pump.

   • Once the water level rises above the upper liquid level switch, the pump shuts off.

   • Excess water introduced by the header tank is drained naturally via an overflow.

All connections were made using either screwed down ring terminals, solder, or connectors to make the system robust for extended use.

The components were laid out as shown in the schematic in Figure 3. The ATX power supply was mounted to the case using the existing clips intended for this purpose, and was therefore quick to snap into place. A screwed down terminal block served to distribute the power and signals to each of the 5 subsystems. The timer relay and 5X 4PDT relays were mounted on a single DIN rail. The specific timer was chosen due to its ability to mount to a DIN rail, and the relays were inserted into DIN rail-mountable sockets. This both made them easy to install, and also avoided having to solder or otherwise connect wires to the relay directly, making the installation both neat, and able to accommodate a quick replacement of a relay should one fail in the future. The dimmer switches and heat lamp power receptacles were mounted in custom cutouts to allow access from the outside.

The entire system was initially assembled in about one day. At first, only enough was ordered and built to control the water level in one tank to prove the concept before ordering the remaining components. During testing of this system however, it was discovered that although the off-the-shelf float switches could handle the continuous current required by the pump, they could not stand up to pump's inductive load when the switch was opened; the switch either fused or broke from arc-over after just one or two cycles. A quick evaluation was done on whether to implement fly-back diodes across the motor to prevent current from being pulled after the pump was switched off, or to just replace the existing switches with high current switches (Subminiature Snap-Acting Switch). The decision was made to upgrade the switches (even though it required mounting them above the water and attaching a float on an extended rod), as it kept the cost and technical complexity of the design to a minimum.

The upgraded switches worked perfectly, and after two more days of work all five systems were built up, the wiring in the case cleaned up, and tested in the lab. The system worked flawlessly! Lydia would later modify her experiment and rule out the use of individual heat lamps, instead requiring the mussels to be placed in an incubator. In addition to this modification, the continuous influx of water from the header tank into the primary tank was discovered to be sufficient enough to not require the pump to refill the primary tank by pumping water from the reservoir tank, thus eliminating the reservoir tank altogether.

The system is currently in use in this configuration, and is expected to produce scientific results this year (2014).