Flow Sensor Electronics

Created May 1, 2004

Last updated May 1, 2004

We will have two separate loops of fluid destined for the laser enclosure:

The laser coolant: de-ionized water running from the laser "cooling group" up through the cabinet and through a thermally protective umbilical sheath and into the laser heads. This fluid runs at about 2-3 gallons per minute (gpm) in the on-state (at 30 °C), and at about 0.5-1 gpm in the off-state at 10 °C (cold-weather only).
The laser enclosure coolant: propylene-glycol solution running to the two Noren heat exchangers within the laser enclosure. This flow will be several gpm in the on-state at about 5 °C below the laser enclosure set-point (set-point will likely be 10 °C winter, 25 °C summer). In the off-state, this flow will either be off while cold outside, or on at 0.5-1.0 gpm when warm-ish.

The potential for the laser loop to freeze is scary, and thus justifies monitoring the flow. The chiller loop is not in danger of freezing, though we nonetheless want to monitor this flow rate as well. This knowledge will:

Alert us if any of the pumps fail
Indicate a leak in the system
Give us quantitative information about cooling rates
Sound an alarm in tha case of freeze danger

The flow sensors are Omega-brand impellor wheel designs, model FP-5300, in conjunction with calibrated FP-5105 tees. The stated calibration of these devices has 481.55 pulses per gallon, so a 1 Hz rate would correspond to 0.1245 gpm, or 15 Hz would be 1.87 gpm. I measured the following parameters in the lab:

frequency (Hz)	amplitude (V_p-p)	V_rms	flow rate (gpm)
15.0	2.36	0.970	1.87
11.8	1.85	0.755	1.47
8.7	1.36	0.555	1.08
2.8	0.38	0.170	0.35

The sensor is said to only work for flow rates > 0.3 m/s = 6 Hz (19.7 Hz per m/s flow rate), corresponding to 0.75 gpm. But it seemed to work robustly down to 0.35 gpm.

The goal of the flowrate electronics will be to convert the sensor signal frequency into an analog voltage—perhaps one volt per gpm. This can be fed to our A/D PCI card. It would be nice to also mount a couple of analog voltmeters to be able to visually inspect the flow, as well as some smarts to light an alarm LED or make a racket when the both the flow and temperature are low on the water loop.

APOLLO To Do Task List.