help with CMF vs metering valves

OceanOpportunity · 25th November 2006, 16:23

hey all,

working on my O2 injection system...I'm looking for some clear cut info on pros/cons CMF vs metering valves, and the rationale behind using a constant IP 1st stage or not for each

thanks

**Dave Sutton** · 25th November 2006, 23:50

There is no supportable rationale for using a metering valve. There is no rationale to trying to match 02 consumption accurately, as it changes with workload and there is really no possibility of ever matching it accurately. All that an be done from a practical perspective is to buffer out the time period between manual 02 injection cycles. If you shoot for somewhere between 0.6 and 0.8 LPM 02 then you've done 99.9% of what is required. Those who have used metering valves have found that they set them and then forget them... so why bother having one? Use an orifice and forgt about it.

My 2 cents. actual mileage may vary.

Dave Sutton

OceanOpportunity · 26th November 2006, 16:26

thanks Dave,

Can you indulge me with the use of constant IP vs. standard 1st stages with the use of an orifice? there are some conflicting arguements out there. Also, recommendations on an orifice? do you use something from swagelock or McMaster or a dedicated KISS style valve from one of teh dive mfg's?

thanks

**Dave Sutton** · 26th November 2006, 23:03

There have been conflicting ideas posited. Here's what I can say:

(1): We know that the constant pressure regs work fine, and produces a reliable result.

(2): We are assured by "some" users that using non-constant pressure regs "seem" to work OK, but have no personal experience. Many theoretical paragrapsh ahve been written about how orifices behave, sonic flow choking, etc., but not to my personal satisfaction for any definitive answer.

*Personally*, I like to reduce variables, so using a blocked first stage to me is my choice. Simple = Best. Blocking a first stage is easy, produces the desired result, and is proven. End of my investigation into the matter. I suggest sticking to proven designs, and would endorse the orifice/bocked first stage technique as a proven one.

Dave

25th November 2006, 16:23	#1 (permalink)
OceanOpportunity for a world of water Current Rebreather/s: Other CCR Dolphin Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Other CCR Dolphin Home Build Join Date: Nov 2006 Location: Providence, RI USA Posts: 590	help with CMF vs metering valves hey all, working on my O2 injection system...I'm looking for some clear cut info on pros/cons CMF vs metering valves, and the rationale behind using a constant IP 1st stage or not for each thanks
(Online)

25th November 2006, 23:50	#2 (permalink)
Dave Sutton Diveshop of Horrors Current Rebreather/s: Sport Kiss MK 15.X rEvo Other CCR Azimuth Home Build Other Rebreather/s: Evolution Megalodon rEvo Other CCR Azimuth Home Build Join Date: Jun 2006 Location: Narragansett, Rhode Island and Hackettstown, New Jersey Posts: 2,720	Re: help with CMF vs metering valves There is no supportable rationale for using a metering valve. There is no rationale to trying to match 02 consumption accurately, as it changes with workload and there is really no possibility of ever matching it accurately. All that an be done from a practical perspective is to buffer out the time period between manual 02 injection cycles. If you shoot for somewhere between 0.6 and 0.8 LPM 02 then you've done 99.9% of what is required. Those who have used metering valves have found that they set them and then forget them... so why bother having one? Use an orifice and forgt about it. My 2 cents. actual mileage may vary. Dave Sutton __________________ "Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com
(Offline)

26th November 2006, 16:26	#3 (permalink)
OceanOpportunity for a world of water Current Rebreather/s: Other CCR Dolphin Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Other CCR Dolphin Home Build Join Date: Nov 2006 Location: Providence, RI USA Posts: 590	Re: help with CMF vs metering valves thanks Dave, Can you indulge me with the use of constant IP vs. standard 1st stages with the use of an orifice? there are some conflicting arguements out there. Also, recommendations on an orifice? do you use something from swagelock or McMaster or a dedicated KISS style valve from one of teh dive mfg's? thanks
(Online)

26th November 2006, 23:03	#4 (permalink)
Dave Sutton Diveshop of Horrors Current Rebreather/s: Sport Kiss MK 15.X rEvo Other CCR Azimuth Home Build Other Rebreather/s: Evolution Megalodon rEvo Other CCR Azimuth Home Build Join Date: Jun 2006 Location: Narragansett, Rhode Island and Hackettstown, New Jersey Posts: 2,720	Re: help with CMF vs metering valves There have been conflicting ideas posited. Here's what I can say: (1): We know that the constant pressure regs work fine, and produces a reliable result. (2): We are assured by "some" users that using non-constant pressure regs "seem" to work OK, but have no personal experience. Many theoretical paragrapsh ahve been written about how orifices behave, sonic flow choking, etc., but not to my personal satisfaction for any definitive answer. Personally, I like to reduce variables, so using a blocked first stage to me is my choice. Simple = Best. Blocking a first stage is easy, produces the desired result, and is proven. End of my investigation into the matter. I suggest sticking to proven designs, and would endorse the orifice/bocked first stage technique as a proven one. Dave __________________ "Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com
(Offline)

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