I'm asked this question pretty regularly. Rod guides are used to correct for a side effect of the way a variable area flow meter
works. We know that a variable area flow meter's measurement tube is narrowest at the bottom, and gradually increases in diameter as it reaches the top of the tube. When flow passes through the measurement tube, the float stops at an "equilibrium" point that shows the user the flow rate through the meter.
Flow-savvy readers out there know there's a bit more to variable area meter operation than that, but I'll leave those details for future posts. As the diameter of the measurement tube increases, there is a point when the diameter of the tube becomes too large in relation to the diameter of the float. At that point the float will no longer just move up and down, but can also move side to side. A rod guide holds the float in the center of the measurement tube to prevent side to side movement. As an added bonus, the rod guide introduces a small amount of friction against the float's vertical movement. This extra friction stabilizes the float and makes the flow meter easier to read.
Do you find yourself trying to guess the actual flow rate because of a bouncing float? Can you hear the float banging around inside one (or several) of your flow meters? Specifying a variable area flow meter with a rod guide is an easy way to overcome these problems.
If you'd like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google