Scubapro D400/D350 cracking effort questions

[ericpitar]

DA Aquamaster et al.: Are your references to the cracking effort in this thread based on using a magnehelic or the "field method" (ie. dipping in a bucket of water)?

 

[Couv]

DA Aquamaster et al.: Are your references to the cracking effort in this thread based on using a magnehelic or the "field method" (ie. dipping in a bucket of water)?

Hello Eric,

Manometers and magnehelic gages are just two of the many instruments that can be used to measure pressure. Some units of measurement are inches of mercury, psi, feet of sea water, atmospheres, etc. The cracking pressure (differential) of scuba regulator second stages is measured in "inches of water." Meaning, how much pressure (in inches of water) does it take to force the soft seat and hard seat apart? Because manometer and magnehelic gages are measuring the pressure of a water column they can not be more accurate than the water column itself. So why use a manometer or magnehelic gage? For the same reason we wear a depth gage rather that take a tape measure on a dive….it is just easier to use. Getting a decent scale reading in a sink or bucket is just not as easy as using an instrument, but it can be done and is close enough for a DIYer. Remember, you are really just measuring the distance the diaphragm travels under water to the point where it moves the lever enough to open (cracks) the two seats apart. (It goes without saying not to let water onto the valve side of the diaphragm.)

couv

 

[Luis H]

Hello Eric,

Manometers and magnehelic gages are just two of the many instruments that can be used to measure pressure. Some units of measurement are inches of mercury, psi, feet of sea water, atmospheres, etc. The cracking pressure (differential) of scuba regulator second stages is measured in "inches of water." Meaning, how much pressure (in inches of water) does it take to force the soft seat and hard seat apart? Because manometer and magnehelic gages are measuring the pressure of a water column they can not be more accurate than the water column itself. So why use a manometer or magnehelic gage? For the same reason we wear a depth gage rather that take a tape measure on a dive….it is just easier to use. Getting a decent scale reading in a sink or bucket is just not as easy as using an instrument, but it can be done and is close enough for a DIYer. Remember, you are really just measuring the distance the diaphragm travels under water to the point where it moves the lever enough to open (cracks) the two seats apart. (It goes without saying not to let water onto the valve side of the diaphragm.)

couv

I agree that a Magnehelic is not more accurate than a manometer, but it can be read much more precisely. It is just like an inclined manometer can be read more precisely than a “U tube” manometer or using a magnifying lens to read a precise distance scale.

For a rough measurement, the “field” method you are describing is fine.

I have several new Magnehelic that I have bought from eBay for about $20 each that really help when trying to take more accurate measurements.

I have being working on things like refining double hose regulators mouth piece valves. I need precision of a bit less than 0.1 inWC range to be a meaningful measurement.

Now I need a decent flow meter.

Note:
Precision and accuracy are often confused. A digital thermometer may show temperatures to several decimal places, but they are actually only accurate to about one degree…just like an analog thermometer (which may be hard to read with much precision). The terms are often interchanged (and I am not an English major), but there is a physical deference.

 

[DA Aquamaster]

DA Aquamaster et al.: Are your references to the cracking effort in this thread based on using a magnehelic or the "field method" (ie. dipping in a bucket of water)?

I use a magnahelic. I have never used the field method as it woudl be pretty hard to get any great degree of precision. Being able to tell whether the center of the diaphragm 1" under water, or 1.5" under water would be doing pretty good.

If I were magnehelic-less I'd be more inclined to just test the reg in the water (while diving) and set the reg so that it breathed as light as possible while not freeflowing in any posiiton -which is what in the end tends to be most important. In fact, once you determine that for a given second stage model you can check it with a magnehelic and use the number to form a good baseline for adjustment.

 

[DA Aquamaster]

Precision and accuracy are often confused. A digital thermometer may show temperatures to several decimal places, but they are actually only accurate to about one degree…just like an analog thermometer (which may be hard to read with much precision). The terms are often interchanged (and I am not an English major), but there is a physical deference.

Absolutley. Digital "precision" when applied to thermometers is a lot like taking a number good to only 1 significant figure dividing it by another number and then claiming the results are accurate to 2 or 3 significant figures. You just can't be more accurate than the limiting factor in the system.

The same thing happens with o2 sensors. The other day a very well intentioned and well trained gas blender commented on the MOD label on the tanks I was having filled as being "wrong" as his measurment of the contents on his O2 meter varied from the contents as measured with my meter by a little less than 1%. Most O2 sensors are only accurate to +/-.5% under the best of circumstances and sensors are also suceptible to differences in pressure and flow rate, so even a 1% difference in readings between two different sensors is not unusual and if different calibration techniques are used, you could get nearly that same difference with the same sensor. Having numbers behind the decimal point does not always mean they make the reading any more accurate.

 

[Luis H]

Absolutley. Digital "precision" when applied to thermometers is a lot like taking a number good to only 1 significant figure dividing it by another number and then claiming the results are accurate to 2 or 3 significant figures. You just can't be more accurate than the limiting factor in the system.

The same thing happens with o2 sensors. The other day a very well intentioned and well trained gas blender commented on the MOD label on the tanks I was having filled as being "wrong" as his measurment of the contents on his O2 meter varied from the contents as measured with my meter by a little less than 1%. Most O2 sensors are only accurate to +/-.5% under the best of circumstances and sensors are also suceptible to differences in pressure and flow rate, so even a 1% difference in readings between two different sensors is not unusual and if different calibration techniques are used, you could get nearly that same difference with the same sensor. Having numbers behind the decimal point does not always mean they make the reading any more accurate.

I went to engineering school in the early days of the use of calculators…all of my professors made it clear that they didn’t want to see results with more significant digits than the initial data. Providing answers with more significant digits than the original data can be misleading (about the accuracy or precision of the instruments, etc.) and it can even be considered unethical.

 

[tfsails]

After servicing several hundred regulators, I've noticed that I've become able to guess the cracking effort within about +/- .2" just by how it feels to breathe the reg. I was sorta surprised when I tried it the first time and found out I was reasonably correct. Goes to show that if you beat a dummy on the head long enough he'll get the message, I guess. BTW, I still use the gauge to set cracking effort.