Depth differences

[bs63366]

I was doing my AOW not to long ago and while I was doing the deep dive we checked our gauges and our comuters said 94fsw and his depth gauge read about 103fsw. Now which one is more accurate and is there a way to check the accuracy of your computer or depth gauges?

 

[DevonDiver]

The dive computers are possibly going to be more accurate. The best way to be sure is to take 3 depth gauges... and try use that to identify if 2 are closer in readings.

The only 100% way to be sure of depth accuracy is to check your gauges against a known depth. If you have a reel and float... measure out 10m, 20m, 30m etc on the line. Compare your depth gauge against those measured distances.

 

[SeaCobra]

remember first that the depth reading is not a measurement of depth, but a measurement of pressure expressed in fsw. Next all dive computers and DDG's have a plus or minus error factor in them, usually pretty small, like 1 foot. Also, were both computers set for fresh water or for salt water? I have forgotten to reset mine to fresh water after diving in the ocean.

Jeff

 

[bs63366]

I know that my computer is set to salt water and I would not see why his was not as well. He lives and teaches in Guam so all that is there is salt water. The concensus was that the 2 computers were the same and the depth gauge was off. Thats a good idea to check with a line will have to try that the next time that I get deeper.

 

[Crowley]

A correctly functioning computer will generally speaking be more accurate than an off-the-shelf analogue depth gauge, mostly due to the difference between the way they are constructed, in the same way that a digital watch will keep better time than an analogue watch.

That is of course a broad generalisation but at the end of the day, an analogue gauge is subject to the rigours of wear and tear, whereas computers tend to either work or not work. Analogue gauges are designed withing a set +/- tolerance and unless you have them specifically calibrated, may well give readings that are a few feet / 1 metre or so inaccurate. This is one reason the PADI AOW deep dive, for example, includes a depth gauge comparison, in order to demonstrate this.

Practically speaking, following a conservative dive profile on a recreational dive, a few feet of inaccuracy makes little difference, but when it comes to diving where you are pushing limits and accuracy is more important, then it can be a problem. Ask at your LDS or contact the manufacturer - they should be able to assist you in getting your computer or gauge properly pressure tested and in the case of analogue equipment, properly calibrated.

Hope that helps

C.

 

[gryder]

I did a chamber dive to 50m at the Wesley Hospital in Brisbane. It was pretty cool (actually - it was very very hot!) I love the concept of being able to talk and eat pizza on the deco stop...

Anyway, I digress, we took our dive computers with us to see how accurate they were. I have a Uwatec SmartCom and it read 51.5m! I was pretty surprised with the variance, my friends mostly dive with Galileo's and they were within +/- 1%. I figured I'd rather have it read deep than shallow, that makes it a little more conservative per dive, but it was a good way to gauge the accuracy.

 

[theduckguru]

I have both a 2 year old wrist computer and a mechanical depth gauge in my console. Both are rather bottom end as far a price, but both agree exactly on the indicated depth. Of course both could be exactly wrong.

 

[sambolino44]

One of my instructors introduced me to the concept of "Scuba Math". You round off, you extrapolate, you fudge, you give yourself a margin of error, and you don't spend a lot of effort calculating things out to eight decimal places because:

No-Decompression Limits are not absolutes that can be calculated like the number of beans in a jar.

I'm no expert, but I understand NDLs to be something that will result in an "acceptable" number of DCI incidents. Every scuba class I've ever taken has emphasized that, even if you stay within the NDLs and ascent rates, you can still get bent, and nobody really knows why. One factor is that people are so different and unpredictable, you can never be absolutely certain that what works for one person will have the same result for everybody else every time.

So it doesn't make sense for our instruments to be all that accurate or precise; that would make them more expensive than they need to be.

This goes against my nature; I'd rather have a depth gauge accurate to 1%, and a watch accurate to 1 second a month, and a dive computer that's better than that. Shoot, I'm the kind of guy who'd rather have a car with gauges for everything instead of idiot lights, and discrete switches for everything instead of consolidated, automated controls. So it took a while for me to accept the value of "Scuba Math".

The main thing to take away is: this is another good reason not to push the limits.

 

[DandyDon]

I suppose you were in salt water, at sea level?

My Oceanic computers do not have a switch for salt or fresh so I can see some possible confusion there, maybe 2%?

They do auto adjust for altitude if activated in air pressure reflective of a certain altitude; I do not recall the change level off hand, but I wonder if they do adjust for freshwater at altitude. Their altitude reading are + 1,000 feet so there is another variance.

We need to do a comparison next time we go out.

 

[40Fathoms]

From my experience, it isn't all that uncommon to have two depth measurement tools (one computer, one analog gauge; two computers, etc.) show slight differentiating depths. From the (few) test I've done, computers tend to be slightly more accurate than analog gauges, though this is not a 100 % guarantee. My tests also showed that all gauges we tried tended to show a slighter greater depth than the actual depth, making your dives slightly more conservative than what the depth gauge reads. This was usually within the +/- 1 foot range (so don't start thinking you can go further down because "oh well, the gauge is a bit off, anyway" ).

However, almost 10 feet of difference is a bit much. Check if any settings in the computer might be to blame (salt vs. fresh water, if it has that option, etc.). And if you have the opportunity, I'd recommend doing a test of both against a known depth. Or simply hand both in to a certified shop, tell them the problem and let them look at it.