LI-850 multiplexer

A multiplexer that lets a single Licor LI-850 CO₂/H₂O gas analyser serve up to six gas-tight chambers in turn, parallelising metabolic-rate measurements of small invertebrates. BeeHive routes air one chamber at a time, then logs the analyser's readings to CSV.

Ingredients

BeeHive boards:

Other components:

How it works

Measuring metabolic rate one chamber at a time wastes most of the session waiting for readings to stabilise and swapping animals. Here, six chambers stay loaded and BeeHive cycles through them.

Each chamber has an inflow and an outflow solenoid valve, controlled independently by its own solenoid control board. To measure a chamber, the mainboard:

1. Closes every other chamber's valves and opens the air path through the selected chamber (its inflow and outflow valves).
2. Waits for the flow and gas concentration to stabilise.
3. Pings the LI-850 over serial using its documented command grammar and reads back CO₂ and H₂O.
4. Appends the reading, with chamber ID and timestamp, to a CSV file.
5. Moves to the next chamber.

Because chambers are pre-loaded and cycled automatically, the per-experiment stabilisation dead-time and manual animal-swapping are cut sharply. Validation showed that multiplexed stabilisation matches the single-chamber system — routing through the manifold does not degrade the measurement.

Wiring

• Each of the 6 solenoid control boards to a mainboard data line, plus 12 V power and ground. Each board switches its chamber's inflow and outflow valves.
• LI-850 serial (UART) to the mainboard UART lines (respect the analyser's logic levels; add a level shifter if needed).
• Air is plumbed from each chamber's inflow, through the chamber and its outflow, to the analyser via a shared manifold.

Code

Two parts: switching each chamber's valves (standard BeeHive code) and polling the LI-850 over serial (specific to this setup).

Valve control uses the solenoid daughter-board pattern from the BeeHive solenoid driver example — one output per valve, driven high to open:

from machine import Pin
import time

# BeeHive solenoid-driver outputs; one Pin per inflow / outflow valve.
sole1 = Pin(2,  Pin.OUT, drive=Pin.DRIVE_3)
sole2 = Pin(15, Pin.OUT, drive=Pin.DRIVE_3)
sole3 = Pin(16, Pin.OUT, drive=Pin.DRIVE_3)
sole4 = Pin(17, Pin.OUT, drive=Pin.DRIVE_3)

sole1.on()              # open a chamber's valve
time.sleep_ms(1000)
sole1.off()

Polling the LI-850 and logging to CSV is specific to this rig and isn't published upstream. The sketch below shows the shape — confirm the serial grammar against the LI-850 manual:

# ILLUSTRATIVE — not upstream code; the serial grammar is a placeholder.
from machine import UART
import time

uart = UART(2, baudrate=9600, tx=25, rx=26)   # link to the LI-850

def read_li850():
    uart.write(b"<li850><rs>?</rs></li850>\r\n")   # placeholder query
    time.sleep_ms(500)
    line = uart.readline()
    return line.decode().strip() if line else None

# For each chamber in turn: open its valves, wait to stabilise, read + log.

Results / notes

The multiplexer turns one gas analyser into a six-chamber respirometry system, removing most of the per-chamber stabilisation dead-time and the manual animal-swapping that dominate single-chamber runs. Validation showed multiplexed stabilisation matching the single-chamber baseline, so throughput rises without loss of measurement quality.