How to convert Garmin PSi/min to Suunto CF/min SAC rate

[Mcdonned23]

I have a Garmin MK2i with T1 transmitter and Suunto Eon Steel Black with transmitter. I get two very different readings on each computer for SAC rate. How do I do an apples to apples comparison/ convert for them to show the same number.

 

[Landau]

Load both into MacDive or Subsurface and either of these software packages will convert for you. Assuming you have the correct tank sizes loaded.

 

[inquis]

Convert psi/min to cuft/min by multiplying by the tank factor (or ratio):
Rated cuft / rated pressure
For an AL80, that would be 80/3000.

ETA: ensure your Suunto is using the correct tank in its calculations.

 

[tursiops]

I have a Garmin MK2i with T1 transmitter and Suunto Eon Steel Black with transmitter. I get two very different readings on each computer for SAC rate. How do I do an apples to apples comparison/ convert for them to show the same number.

What size tank are you diving? The conversion requires that info, and your Suunto must already know it.

 

[Subcooled]

While the surface volume (suunto) is pressure (in atmospheres) times actual volume, plus some coefficient to convert between US and metric systems (psi vs. bar/atm and cf vs. litres), compressibility may be taken into account too.

Pressure and amount of gas are no longer linearly dependent when the pressure is greater than, say, 240 atmospheres. Something like 300=270, 270=250, 250=240, 240=235... Just to give you a rough idea.

You need to convert that PSI used into atmospheres used times cylinders volume(s).

 

[broncobowsher]

As they have stated, the conversion factor requires knowing the tank size and operating pressure.
Also matters what the Suunto is programmed to (is it even correct?)

1 PSI consumed on a tank rated for 3000 PSI filled is 1/3000th the tank capacity.
If you consume 30 PSI per minute, 30/3000 reduces down to 1/100. So 1/100th of the tank capacity.
If you have a little 50 cu/ft tank, that is 0.50 cu/ft minute
If you have a big 100 cu/ft tank, that is 1 cu/ft minute.
If you have double 100s, 200 total, that is 2 cu/ft minute.

So you really need to know what the tank size is to make the calculation.

The Suunto is more subject to error as you need to put in the right tank info to get the correct data.
The Garmin just takes pressure and time. Any size tank can be used and the data will always be correct. If you change tank sizes there is no setting that needs to be programed for a correction.

 

[Mcdonned23]

They are both set for an 80 cu/ft cylinder with a pressure of 3000 psi. My Suunto says 0.43 cu/ft per minute but my Garmin gives me the reading in psi/minute.

 

[tursiops]

They are both set for an 80 cu/ft cylinder with a pressure of 3000 psi. My Suunto says 0.43 cu/ft per minute but my Garmin gives me the reading in psi/minute.

Read the Manual for the Mk2i, page 12:

Selecting the Gas Consumption Rate Metric
1 Hold MENU.
2 Select Dive Setup > Dive Network & Air Integration > Display Setup > Gas Consumption Rate.
3 Select an option.

NOTE: The watch can estimate volumetric surface air consumption (SAC) or respiratory minute volume (RMV) only if the air volume of the tank is entered in the watch (Transceiver Settings, page 19). The watch can estimate pressure-based surface air consumption (PSAC) with or without the air volume of the tank.​

 

[Mcdonned23]

You are awesome thank you