With computers such as the older Sherwood Source it’s very easy to change the battery, despite the statement from the manufacturer that the unit must be returned to them for service.
The pressure plate that computes depth is not integral, but rather is mounted on the back of the unit, and the electronics are sealed by an “O ring” inside the unit where the battery is located. If you have one like this, I stongly suggest a spare "O Ring"
I changed batteries in mine several times before I got fed up with forgetting to switch it on prior to getting wet and got a Wisdom, which I love!
It's great to know there is a place in town that can offer a service like this...
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It's rarely rocket science, it's usually just math: then again if you can't do the math.......