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Holding tank pressure sensor

Can someone explain how this (see photo) tank monitor works? I know that it shows the tank level based on pressure, so when the tank is full there is more pressure and the audit monitor shows 'full'. I know that these sensors can be calibrated and how to do that.
From the Audit wiring diagram (A-2701.11 in my manual), I can see that one wire is ground, one has 8 volts, and the third is the water 'input'. Is the 'input' signal a varying voltage?
I was hoping that I might be able to use this varying signal to control the water flow for my R/O water filtration system so that when the tank is 3/4 full the water to the R/O system turns OFF and when the tank level is 1/4 it turns back ON.
When I try to measure voltage on the terminals, while connected and with the Audit turned ON and displaying a tank level of 1/2, I can't find 8 volts. The most I get about 25 mV, which is about how much my meter jumps around to in open air. I'm using a Craftsman True RMS Clamp meter model 73756. It specs 400 mV as the smallest range, although it auto ranges.
Is it possible to capture a usable signal from this?
Dick, '03 U320 40' Tag, 2 slides, Coach #6075
Full Timers
2009 Honda CR-V

Re: Holding tank pressure sensor

Reply #1 –
We use an RO system to filter water going into the fresh water tank. We installed a float switch to control the high pressure pump and valve for the RO system. We chose to cut off at a full tank. You could put a float switch at the level of your choice. The installation of the float switch was easy. I expect the float switches might be simpler and more reliable than to use input from the pressure sensor.

The vendor of our system recommended using a single switch and allowing the RO system to keep the tank full when it is turned on. We chose to do that. He also sells a "deluxe" system with two switches, one for full and one for low. The controller he sells knows what to do with the switch inputs. We have found the single float switch meets our needs very well.
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: Holding tank pressure sensor

Reply #2 –
Hi Dick,
 
I have the same meter and note the same low current reading instability - it's frustrating. You may have to disconnect or cut one of the power wires to the sensor and then, in series, read the current with a regular VOM. For voltage, it sounds like your meter maybe loading down the circuit or is just not fast enough to give an accurate readings. For a digital signal or pulsed DC, then an oscilloscope might work much better.  I don't know how you system is designed so I'm guessing. Hope this is helpful.
 
Good luck.
 
Jim
2002 U320
Jim Frerichs
2002 U320 42'

Re: Holding tank pressure sensor

Reply #3 –
Jim,

You took me back to my calibration days at TI.  I remembered that the old analog VOMs had an input impedance of 20K ohms per volt but I could not remember what it was for DVMs.  Believe it or not, neither Craftsman nor Fluke listed the impedance of the meters I looked at but a Wikipedia article states one to ten M Ohms which should be enough to not affect the reading.  One of the advantages of an old analog meter is that the needle would swing very rapidly unless it was severely damped and therefore better to read fluctuating voltage than a DVM.
John Cooper
'91 GV 36'
Oshkosh chassis
Cat 3208T 300HP

Re: Holding tank pressure sensor

Reply #5 –
You won't be able to measure the output of the tank sensor with any meter.  The Audit system periodically sends a pulse of voltage to the sensor and reads the output only during that pulse.  A meter will try to read tha average voltage over a period of time.  As you noted, this is very low.

To see what is happening, you will need an oscilloscope.

Dick

Re: Holding tank pressure sensor

Reply #6 –
Aha! Thank you. That makes sense. I will work on using the existing "tank full " sensor which is designed to close the 'tank fill' valve. As mentioned above, I'll get the water shut off device from the RO guys. I guess it is some kind of solenoid water valve. I just need to figure out how to use that sensor.
Dick, '03 U320 40' Tag, 2 slides, Coach #6075
Full Timers
2009 Honda CR-V