Is Your Dive Computer Correct?

Your diving computer monitors and calculates your decompression schedule for each dive, based on continuous measurement and recording of pressure and time, and the depth displayed on your computer is an interpretation of the pressure measured. In the past, studies have discussed how the conversion of a pressure-to-depth estimate can be affected by environmental factors, like altitude. But what about factors like temperature and salinity?

The European standard used in many dive computers largely overlooks the fact that total accuracy with regard to depth can only be achieved through converting pressure readings in a combination of measured physical parameters, mostly water density and temperature. Only Cochran computers are capable of automatically adjusting for salinity changes. Most dive computers don't have that capability -- they have assumed calibrations for water density built in, letting the user switch between "freshwater" or "seawater." And almost no manuals don't explain their assumptions for pressure-todepth conversions.

Elaine Azzopardi and Martin Sayer of the UK National Facility for Scientific Diving in Scotland wanted to investigate how dive computers display depth when exposed to a number of pressures, and when set to seawater and freshwater modes at typical densities in both water types. Many computers also display water temperature, so those recordings were also studied.

The Testing

They bought 47 models made by 14 different manufacturers (Apeks, Beauchat, Buddy, Citizen, Cressi Sub, Delta P, Mares, Oceanic, Scubapro, Seeman, Suunto, TUSA, Uemis and Uwatec), and each one was immersed in a tank of either seawater of freshwater. The tanks were placed in a standard recompression chamber, which was compressed to a simulated depth of 165 feet. Then pressure was released to the depths of 130 feet, 100 feet, 65 feet and 30 feet before surfacing. Five to eight trials of each test were carried out in both freshwater and seawater. After each test, the stored dive profile of each computer was downloaded for analysis.

At the same time, the temperature tests were carried out over a simulated range, in water ranging between 50 and 60 degrees Fahrenheit. As before, the computers were downloaded after each test; however, downloaded temperature data was not recorded or displayed in a uniform manner between different brands of computer, or even between different models of the same brand, because not all downloads gave the maximum and minimum temperatures recorded during a dive. A number of brands -- Mares, Oceanic, Citizen, Beuchat, Seeman Sub, Scubapro and the Aladin Pro Ultra -- only gave the minimum temperature recorded during a dive. Suunto downloads gave the temperatures at the start and end of the dive, as well as at the maximum depth, although occasionally supplemental information could be obtained from the temperature readings in the profile list. Cressi Sub, Tusa and Apeks computers only gave the water temperature at the maximum depth. Most of the Uwatec models, except the Pro Ultra, gave a temperature profile throughout the dive, as did the Delta P and Uemis computers. No temperature reading was obtained from the Buddy Nexus downloads, although this isn't to say that the temperature was not displayed and recorded during dives.

In some cases, there were differences in the values given in the downloaded data. For example, in Uwatec's SmartTrak software program, temperature readings occasionally differed between the downloaded logbook and the downloaded dive profile display; this was also the case with some Oceanic models in the Oceanic series.

The Results

While in the early days of computers -- the mid to late 80s -- many dive computers erred in their depth measurement; faulty measurement in the Orce EDGE computer led to many cases of divers getting bent.

The good news from this study is that today most computers gave estimated depths in seawater that were very close to the nominal values. Taken as a percentage of the nominal depth, the difference for the overall mean estimate values ranged from -0.8 to 0.1 percent in freshwater, and -0.1 to 0.9 percent in seawater. The overall maximum depth estimates for each nominal depth ranged from 4.7 to 5.9 percent in freshwater, and from 3.2 to 4.1 percent in seawater. Minimum values were -2.7 percent to -8.8 percent in freshwater, and -0.8 to -8.4 percent in seawater.

Some units gave estimated depth values that were consistently deeper than nominal, such as the Apeks Quantum, which at 30 meters read 31.4 meters. Some tended to read low over certain depths, like the Beuchat Voyager, which at 30 meters read 29.3 meters. But the majority of models produced relatively consistent and accurate results (mostly within 1percent of nominal, and less than one meter across the depths tested and between the two water types. The Buddy Nexus unit tested did not produce useable depth data on download.

The testing showed varying ranges of estimated depth from the same model of computer. No model tested produced perfect, repeated depth estimates for every depth/trial combination; there was always some variation, either within depth or between the depths tested.

Overall for the five depths tested, 41 out of the 46 units that gave depth estimates in freshwater trials produced maximum ranges of replicate displayed depths of less than .6 meters; in the seawater trials, there were 42 out of 46 units. However, of those, only the Uemis SDA produced maximum ranges that were less than a foot in the freshwater exposures, compared with 22 of the computer models in seawater. Only the Oceanic Veo 250 was able to produce maximum ranges of the depths displayed more than one meter, and did so both for freshwater and seawater.

On the other hand, temperature measurements were far less accurate, as any diver might expect when comparing their readout of water temperature with other divers' computers. The measured nominal temperature was 62 degrees, but the computers' measurements ranged from 51 to 66 degrees. In general, there was little, if any, standardization in recording or displaying temperature, meaning that was probably not a primary design factor for most dive computers.

What This Means

Because only single samples of each model were tested, the two researchers admit the lack of replication in computer models. Pressure measurement is the only barometric parameter used in decompression algorithms to calculate and manage dive profiles. This means that totally accurate depth information is not an essential component for decompression monitoring. If a diver is using the computer depth display to compute decompression obligations, then this study's results show that dive computers should be accurate enough for most table depth intervals.

Temperature measurement is a different story. Some manufacturers claimed accuracies for theirs. For example, Tusa, Apeks and Mares computers, with the exception of the Mares Nemo Sport, all claim an accuracy range of 4 degrees Fahrenheit in temperature recording. Suunto also claims that same accuracy, but only within 20 minutes of the temperature changing, whereas the Cressi Sub accuracy claim was for within a 10-minute change. Citizen claimed its models were accurate within a 4.5 degree envelope.

- - Vanessa Richardson