Cobra 1 Questions

[Howie411]

Went out to the Quarry today to test my new 7mm wetsuit, I'm always confused about some info my Cobra 1 gives me when I connect it to the computer.


For instance.

Dive 1:

Max Depth 36 Feet
Temp at Start - 86 degrees
Water Temp Max Depth - 54 degree
Water Temp End of the Dive -70 degrees

Dive 2:

Max Depth 50 Feet
Temp at Start - 86 degrees
Water Temp Max Depth - 63 degree
Water Temp End of the Dive -70 degrees


How is the water temp warmer on the 2nd dive which we were a lot deeper? And why wouldn't the water Temp at the end of the dive be = or higher to the start of the dive? As it got later in the day I would think it would be hotter.

Also, how do you read the "Slow" or Surface warning messages correctly?

For instance on Dive 1, at the end I see both the SLOW and the SURFACE red marks. But on Dive 2, I don't see them at all (mind you Dive 2 was only 10 mins and Dive 1 was 36 minutes).

 

[Scared Silly]

The end temps are when the dive ended and you measured the surface water temp. When you got in the water the unit was hot and you probably immediately descended as the unit was in the process of cooling down so you got a transition temperature.

As for the temps during the dive. It all depend on where there are thermal clines. On your second dive you might have been deeper but closer to shore.

The slow means you got a warning for ascending too fast.

 

[Hatul]

It's normal for the end water temp to be lower than at the start because as my co-poster stated the first temp is closer to air temp, which is usually warmer.

You have to consider that there is a lag time between the temp of the water and the temp of the sensor. It takes time for the two to equilibrate. Besides the different position with respect to the thermocline, you may have quickly descended and ascended on the second dive, so the sensor is reflecting a temp of the shallower water.

Adam

 

[CMAN]

The Cobra takes a Temp reading as soon as it senses the water and records that as the "start" temp, which could be warmer or cooler than the actual surface temp. The Cobra also only records the temp every five minutes so your actual "coldest temp" may not be recorded. The Cobra also records the temp at the last 20 seconds of the dive so that is usually the surface temp. As for the "Slow" warning, if the computer senses that you are accending faster than 30 feet per minute it will beep and record a warning. This can happen if you are holding on to an up-line and there are waves pulling on the line or if you go to read your computer and raise it up too fast. I see lots of divers do a three minute safety stop at 15 ft. and then shoot right to the surface (which will give a warning) when they should take a minimun of 15 seconds to accend to the surface after the stop. Also the "surface" reading is not a warning, it just indicates that you were at the surface at that time.

 

[Howie411]

Thanks for clarifying the temperature stuff, also I wasn't aware its only taking a reading every 5 minutes.


Below is a picture of my first dive with the 2 warnings. Our 3 minute stop was at about 18-20 feet, (ya I know I can't keep still) so I guess I just came up to fast after that.

 

[Scared Silly]

Yeah came up too fast. The greatest pressure differential is closest to the surface so that is where you want to go the slowest. As said above most people think after they finish there safety stop they can blow to the surface. I usually try to take a minute from there to the surface.