This is not exactly a chemistry question, but I suspect the correct answer involves chemistry.
Here goes, I recently had a odd situation occur. I have an IFM pressure sensor installed on my Aquarium Engineering ACR CaRx. Because I am using it on the 'wet' side of the CO2 feed, downstream of the check valve, where CO2 and saltwater could come in contact, I have a Plast-o-matic gauge guard protecting it. The gauge guard separates the sensor from the media using a membrane. On the sensor side of the membrane, you have to fill the void with a liquid to accurately transfer the pressure experienced by the membrane, to the sensor.
I have being running this system for about a year with no issues. I have been using an older style of their gauge guard, filled with mineral oil. Recently, I decided to switch to one of their newer, cheaper models of guards, instead of the industrial version that I had been using. At the same time, I decided to use RODI as the filler for the sensor side. My thinking was if the membrane ever ruptured, only RODI would leak into the tank.
Ok, here is where things get weird. I installed the new gauge guard along with a new, but similar, IFM sensor onto my CaRx. It was working as expected, monitoring and automating the pressure inside the CaRx to ensure proper cycling of the reactor. A normal cycle involves oscillating between a pressure of 4.0 to 5.0 PSI, with different equipment active as preset pressures are reached. Then I noticed that the cycle was slowing, it was taking longer for each 0.1 PSI of pressure change. Because the flow rate of the effluent leaving the reactor determines the rate of change, and since I had not changed the flow rate, I knew something was amiss. After observing it over a few hours, the slowdown increased until it stopped changing all together and stayed at 4.3 PSI. My first guess was that the new IFM sensor I had installed was bad, but when I removed the sensor assembly (with the guard still attached), the pressure still read 4.3 PSI with process side of the guard open to atmosphere. It should have read 0.0 PSI. When I removed the sensor from the guard, the pressure reading dropped to 0.0 PSI. I also observed bubbles in the RODI inside the guard. It almost looked like the RODI was carbonated.
My first assumption was that the gauge guard had a leak and was allowing media to bypass the membrane. I am not so sure of that anymore though. What I think happened is that the Buna-N membrane in the guard is permeable to CO2 and allowed over 3 or 4 days, enough CO2 to saturate the RODI and 'pressurize' it to the average pressure of the cycle (apparently 4.3 PSI). It probably took a few days for enough CO2 to saturate the RODI so that the static pressure could build up and slow the cycling.
I did not see this behavior with the larger industrial guard from the same manufacturer, filled with mineral oil. It's possible that the older guard uses a different membrane material. The newer entry level guards only come with either Bun-N or Viton for membranes. So I am not sure if the difference in fill media (RODI instead of mineral oil) or a difference in the permeability of the membrane to CO2, or if it's just a defective item.
This has me scratching my head as to what the exact cause is. For the moment I swapped the guard for another of the newer style of guard and filled it with mineral oil. I checked with the manufacturer and they only recommend filing with mineral oil for that model of guard.
If the problem does not re-occur with the swapped guard, than I plan to fill the suspected defective guard with mineral oil and test to see if the issue re-occurs. If it does, than it would indicated a bad guard. Otherwise the different behavior of RODI and mineral oil when exposed to CO2?
Any insight or musings welcomed.
Here goes, I recently had a odd situation occur. I have an IFM pressure sensor installed on my Aquarium Engineering ACR CaRx. Because I am using it on the 'wet' side of the CO2 feed, downstream of the check valve, where CO2 and saltwater could come in contact, I have a Plast-o-matic gauge guard protecting it. The gauge guard separates the sensor from the media using a membrane. On the sensor side of the membrane, you have to fill the void with a liquid to accurately transfer the pressure experienced by the membrane, to the sensor.
I have being running this system for about a year with no issues. I have been using an older style of their gauge guard, filled with mineral oil. Recently, I decided to switch to one of their newer, cheaper models of guards, instead of the industrial version that I had been using. At the same time, I decided to use RODI as the filler for the sensor side. My thinking was if the membrane ever ruptured, only RODI would leak into the tank.
Ok, here is where things get weird. I installed the new gauge guard along with a new, but similar, IFM sensor onto my CaRx. It was working as expected, monitoring and automating the pressure inside the CaRx to ensure proper cycling of the reactor. A normal cycle involves oscillating between a pressure of 4.0 to 5.0 PSI, with different equipment active as preset pressures are reached. Then I noticed that the cycle was slowing, it was taking longer for each 0.1 PSI of pressure change. Because the flow rate of the effluent leaving the reactor determines the rate of change, and since I had not changed the flow rate, I knew something was amiss. After observing it over a few hours, the slowdown increased until it stopped changing all together and stayed at 4.3 PSI. My first guess was that the new IFM sensor I had installed was bad, but when I removed the sensor assembly (with the guard still attached), the pressure still read 4.3 PSI with process side of the guard open to atmosphere. It should have read 0.0 PSI. When I removed the sensor from the guard, the pressure reading dropped to 0.0 PSI. I also observed bubbles in the RODI inside the guard. It almost looked like the RODI was carbonated.
My first assumption was that the gauge guard had a leak and was allowing media to bypass the membrane. I am not so sure of that anymore though. What I think happened is that the Buna-N membrane in the guard is permeable to CO2 and allowed over 3 or 4 days, enough CO2 to saturate the RODI and 'pressurize' it to the average pressure of the cycle (apparently 4.3 PSI). It probably took a few days for enough CO2 to saturate the RODI so that the static pressure could build up and slow the cycling.
I did not see this behavior with the larger industrial guard from the same manufacturer, filled with mineral oil. It's possible that the older guard uses a different membrane material. The newer entry level guards only come with either Bun-N or Viton for membranes. So I am not sure if the difference in fill media (RODI instead of mineral oil) or a difference in the permeability of the membrane to CO2, or if it's just a defective item.
This has me scratching my head as to what the exact cause is. For the moment I swapped the guard for another of the newer style of guard and filled it with mineral oil. I checked with the manufacturer and they only recommend filing with mineral oil for that model of guard.
If the problem does not re-occur with the swapped guard, than I plan to fill the suspected defective guard with mineral oil and test to see if the issue re-occurs. If it does, than it would indicated a bad guard. Otherwise the different behavior of RODI and mineral oil when exposed to CO2?
Any insight or musings welcomed.
