Ok, time for an errata. I need to highlight the connection method used for the IFM sensor (and it's importance).
When I first started using the IFM sensors, around the v2 era of controllers, I was under the belief that the sensors used a 1/4" NPT connection. For my first builds I used a 1/4" NPT nipple with the gauge guard side sealed with the o-ring and the IFM side sealed with teflon tape. This appeared to work fine at the time.
Later, when I was using a different IFM sensor model (PN7007) for the in-development v4 controller, I ran into some difficulties. At the time, I was thinking the cause of the problem was the model of gauge guard that I was using. I had switch to using the 'mini' gauge guards that are available for purchase new, and are reasonable affordable.
At the time, the IFM had installed, a PN7007 that I had purchased as NIB (New In Box) was dropping to negative values during the auto venting procedure. This was quite unexpected. While trying to diagnose the issue, I found that if I removed the IFM from the controller and left it powered up, it would slowly over time, creep back to a reading of 0 PSI.
What I hypothesized was happening was that CO2 was diffusing through the gauge guard membrane and pressurizing the IFM side. Of course this was completely wrong, lol. While posting about the issue on the forum, I had my first hint as to where I had went wrong. Someone pointed out that the IFM sensors use 1/4" G threads. Sadly, what should have been a eureka moment, didn't register with me, and it was not until months later while assisting a form member with a build (
@Cali9dub), that they pointed out the existence of adapters that used 1/4" G thread. As it turns out, G thread is another name for BSP, which is a thread used widely everywhere except North America.
There is a handy adapter available from
Amazon that has 1/4" NPT and G ends along with a flat for sealing the IFM using an o-ring that IFM sells in packages of 10.
@Cali9dub was nice enough to send me some o-rings from the package he got from IFM.
Here is the adapter on
amazon.
Metalwork 304 Stainless Steel Pipe Fitting，Hex Nipple, Connect G 1/2" Male Thread to 1/2" NPT Male Pipe, 1 Pcs: Amazon.com: Industrial & Scientific
a.co
Here is what it looks like (hand model is
@Cali9dub 
)
This issue came back on the radar because recently, my development v4 controller started showing the symptoms of going negative during the venting cycle. This is the same pattern that I had seen before, and concluded at the time, that the IFM sensor was faulty. This IFM was still using the incorrect method of using a 1/4 NPT nipple with teflon tape as the means of attaching the IFM sensor.
I decided to replace the adapter with the proper one with G threads on the IFM side, and use the IFM supplied o-ring for sealing. What I discovered while replacing it was that the adapter was almost certainly the cause of the current issue (going negative during venting) and most likely the cause of the issue with the original IFM that was replaced as faulty.
I found that the membrane in the gauge guard had been 'cupped' in towards the IFM. When I pressed on the membrane using the flat end of a toothpick, it popped back towards the non-IFM side. This discovery pointed me to the actual cause of the issue all along.
OK, here is what was happening. When sealed with the incorrect type of adapter, a nipple using NPT threads instead of BSP, though the seal was tight enough to prevent the filling liquid from leaking, in this case RODI, it was not tight enough to prevent water vapour from escaping. So over time the fill was slowly depleting and the membrane in the guard was being stretched by the constant pressure being applied on the wet (ACR) side with a lack of filling liquid on the IFM side. That is why when diagnosing the original problem (the IFM going negative), it would slowly creep back to a reading of 0 PSI over time as the IFM side slowly sucked in air through the non sealed enough NPT threads. So the 'faulty' IFM sensor is almost certainly fine, and it was just down to a bad seal.
So how did I get this so wrong? Well as it turns out, the IFM sensors come in both 1/4" G threads and 1/4" NPT threads. I have sensors using both thread types in my collection of IFM's. The original ones I used were in fact 1/4" NPT models and only the later ones that I sourced for the v4 builds use 1/4" G threads. It would appear that the units intended for the North American markets used NPT threads, while those coming from Europe used G threads.
The thread type for the port is displayed on the label of the IFM. So make sure to check to see what thread type you have for any purchased sensors.
