Using the Sable Systems Respirometry Multiplexer
Sable Systems makes a very nice 8-channel multiplexer to switch airflows. This unit works quite well with the LabHelper software, but for maximal effectiveness there are a few points to keep in mind:
The multiplexer (and other Sable solenoid valve devices)
expect their digital control signals to be coded in "3-bit word"
mode, not "one line per device" mode. Select this option
in the top right corner of the EXTERNAL DEVICE CONTROL window.
You also need to make sure the wiring is correct. The Sable unit comes
with a connecting cable terminating in several color-coded wires that correspond
to particular bits in the 3-bit coding scheme (along with a digital ground
wire). Make sure these are connected to the appropriate outputs from
your A to D device:
Because the Sable unit is controlled with 3-bit word coding,
only one of its devices (solenoid valves) can be switched on at a given
time. Thus you might expect that you could only monitor one animal
at a time even if you have more than one analyzer available. However,
the design of the Sable multiplexer is quite clever: each solenoid actually
switches TWO air streams. There are 8 connectors that are switched
through the 'IN' port, and another 8 connectors that are switched
through the 'OUT' port. These two sets of plumbing are independent
of each other (except that a given valve's IN and OUT connections are both
activated or deactivated simultaneously). Therefore...... you
can sample from two respirometry chambers simultaneously as long
as you insure that both are attached to the same solenoid on the multiplexer
(one to the IN port and one to the OUT port).
It's quite straightforward to connect just two animals,
since a given analyzer will only be recording data from one of them (or
a reference). But why have an 8 (or rather, 16) channel multiplexer
if you can only do 2 animals at a time? In theory, you could connect up
to 14 animals and 2 references air streams and, in periodic references
mode, sequentially sample from them two at a time. This has
the potential for creating serious confusion during data analysis unless
you take steps to insure that LabHelper records data only for channels
that are actually being sampled by the analyzer.
In 'one line per device' mode, LabHelper switches the two valves independently and each channel is associated with a different valve, so it knows which channels are active and which are not. It can then enter values of zero into channels that are not being sampled at a particular time. However, when using the Sable multiplexer with two channels connected through a SINGLE valve, LabHelper has no direct way of automatically identifying the SECOND channel associated that valve. You need to provide this information using the zero linking option. Click the 'extra zero linking' button, and when you exit the EXTERNAL DEVICE CONTROL window, the program will open a window showing the directly switched channels. Click the appropriate 'additional channel' buttons to 'zero link' other channels.
In this example, there are seven data channels. Four of these (#s 1, 5, 6, and 7) record oxygen consumption of different birds. A dual-channel oxygen analyzer is in use, so we can measure two birds simultaneously. Note that the hardware A-D inputs for the two 'halves' of the analyzer are 1 (connected to data channels 1 and 6) and 6 (connected to data channels 5 and 7). Finally, two valves are in use. Valve 1 handles airflow from birds 1 and 2 (i.e., data channels 1 and 5). Valve 2 handles airflow from birds 3 and 4 (i.e., data channels 6 and 7).
The primary data channels (in other words, the ones LabHelper knows are linked to valves) are 1 and 6. When the program switches from Valve 1 to Valve 2, it automatically zeroes channel 1 and activates channel 6. But without further input it does not know that Valve 1 also switches data channel 5 and Valve 2 also switches data channel 7.
When valve 1 is on, the oxygen analyzer is reading gas streams from birds 1 (via one sensor) and 2 (via the other sensor). When valve 2 is on, the analyzer is reading gas streams from birds 3 (via one sensor) and 4 (via the other sensor). Thus when valve 1 is on, both data channels 6 and 7 should be set to zero. Similarly, when valve 2 is on, both data channels 1 and 5 should be set to zero. This is indicated by selecting the appropriate buttons in the "additional channel" rows.