Metric or Imperial?

[Freak*Flag*Flyer]

As my goal is to take some tech diving classes, and as I'm in the process of updating/improving my scuba gear, I feel compelled to ask...which is more commonly used, Metric? or Imperial? I am currently reading Steve Lewis' book, "Staying Alive" and it just seems at least for me, to calculate in metric is much more conducive. Of course I can only speak for myself. But one of the items I am looking to purchase is a new dive computer. I know some will calculate in metric AND imperial, but if I had to choose one over the other, I'd opt for metric but also, I want to be sure that this would be okay considering most of my diving will be here in the US, so any advice and/or comments will be greatly appreciated.

D

 

[PfcAJ]

Use the local convention.

 

[tbone1004]

what AJ said. Metric is better, it makes more sense, tanks are measured directly instead of indirectly, depth is easier to think about because of atmospheres which makes all the calculations easier. My gear is in metric, but only because my two primary dive buddies are engineers and think in metric, so it works for us, we also dive with "same ocean" principals, so there isn't much communication going on with regards to those units. Their pressure gauges are in imperial because they haven't had a need to change them.

Imperial is certainly easier if you dive with varied buddies, or dive in a team, it is best to stay on the local system, which is imperial on this side of the pond unfortunately.

 

[agilis]

Whichever you are intimately familiar with.

 

[Dhboner]

I live in a metric country but unfortunately I am a wee bit old so I was schooled in Imperial. Diving calculations are SO much easier in metric that I am trying to get comfortable in the metric system

 

[underH2Oman]

In my opinion metric is more widely used. I do not know of any country outside of north america that uses imperial measurements, but I have not visited everywhere. Here in Canada we use the metric system but you will find most divers are using the imperial system as often we get equipment that is designed for the north american market (mostly USA) and as such is built for imperial. I agree that metric is easier to work in.

If you want to use metric go for it.

 

[TMHeimer]

Interesting that most of the Canadians I've dived with use psi and feet of water, but celeius when it comes to temperature. If you're diving mostly in the US use imperial of course for everything. Mexico is a metric country but I don't know what they use for diving. Both they and us are also on this side of the pond.

 

[Lorenzoid]

Anyone who has a little scientific education immediately realizes how much easier metric is than Imperial. If you plan to mostly dive solo, then it wouldn't be unreasonable to buck the US system and do your own calculations in metric. But when you have to plan a dive with a buddy who prefers Imperial, you will find that using metric for your numbers and Imperial for your buddy's, is inconvenient and has the potential to introduce errors and result in miscommunications.

 

[Kevrumbo]

Metric if you plan to do a lot of overseas travel. I'm into WWII Southeast Asia Indo-Pacific wreck diving and get buddied-up with mostly Asian & European Divers. At least start by getting a dual unit SPG: ScubaPro Dual Scuba Pressure Gauge - SPG in Metric and Imperial | SPG

The gas planning both pre-dive & real-time during the dive is much easier in metric; just the simpler arithmetic, in at most three significant digits, is less cumbersome:

For example which is the quicker much easier subtraction to see & figure in your head: 200 bar minus 80 bar equals 120 bar? Or: 3000 psi minus 1080 psi equals 1920 psi? Can you figure this out on the fly how much gas pressure you have consumed and keep a running total of how much remains? Can you quickly figure out a gas plan for a lost Buddy Search using Modified Rule of Thirds?

Can you quickly figure out depth in Atmospheres Absolute (an important factor needed to calculate gas pressure consumption rate at a particular depth)? How many Atmospheres Absolute (ATA) is 70 feet for example? In US Imperial units, 70 feet divided-by 33 plus 1 equals 3.1 ATA

-->versus in Metric:

21 meters divided-by 10 plus 1 is the same 3.1 ATA --->Which evaluation to ATA was easier to conceptualize, and quickly do the arithmetic to use & figure?

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with a given arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers):

​

Cylinder Size | Pressure SCR
11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

-->The point is: How much easier is it working with simple base quantities like "1 to 2bar/min per ATA" in Metric, rather than "14.5 to 29psi/min per ATA" in US Imperial units?

---
Notes: How to Derive a Pressure SCR

Example: Given a volume SCR (also called SAC rate or RMV) of 11 liters/min*ATA, divide it by the Tank Factor Rating of the Cylinder in use. For an AL80 cylinder with a tank factor of 11 liters/bar:

11 liters/min*ATA divided-by 11 liters/bar equals 1 bar/min*ATA.

 

[Phil_C]

Personally I would say get go with metric gear, and also go to a DIN first stage. All three of my dive computers are set to metric, but all three (Suunto, Atomic and a Shearwater) can be set to either with a fairly simple menu change.

I find metric just so much more intuitive and easy to calculate, even 'on the fly', and a DIN to A clamp convertor is easy to carry anytime you really can't get a DIN tank.

With a captive 'o' ring DIN is less likely to suffer an 'o' ring failure, but I have seen many A clamp 'o' rings fail on pressurising tanks, and have had one fail on my buddies tank at 30 metres (100') on descent at the start of the dive - that was spectacular!

Ultimately though what will (or should) be your deciding factor is what will you need to know and be familiar with for the technical training you are aiming for?

Have a look at the training providers you are going to use, and see what they use. Although I would recommend metric, if the centres you plan on using for training are using imperial, then there is little point in going metric unless you really can perform mental gymnastics. You will have enough to learn on a technical course without adding extra stress by incompatible gauges and complicated conversions.

Phil.