Negative pressure test. Sucking all the gas out of the breathing loop until the crinkly hose collapses (below), then waiting to see if air leaks in. In addition to finding leaks, a positive-pressure test checks that the plug-in oxygen sensors in this Submatix rebreather are firmly secured.

POSITIVE AND NEGATIVE PRESSURE TESTS
One thing a rebreather diver does not want on a dive is a flooded breathing loop. Water in the loop gets in the way of breathing and, worse, reacts with the scrubber chemicals to create an alkaline solution that is not at all nice to ingest or inhale.
Positive- and negative-pressure tests are used to check that there are no leaks in the breathing loop before a dive. The former involves inflating the breathing loop of the rebreather until it is tight and waiting to see if it deflates.
A negative-pressure test is the opposite, sucking all the air out of the loop until it crushes down, then waiting to see if any air leaks in.
A rebreather diver will have performed these tests when assembling the rebreather from scratch, so you are unlikely to see either on the boat.
What you will see as a matter of good procedure is the diver putting a rebreather into a positive- or negative-pressure test while kitting up, as a final check that nothing has worked loose as the boat bounces about.
Which test is used depends on the rebreather and the owner. Rebreathers with counter-lungs inside the casing are easiest to check with a negative-pressure test. Where the counter-lungs are outside the casing, there is a choice.
A positive-pressure test can be performed quickly by screwing down the dump valve and pressing the diluent button.
A negative-pressure test is less likely to get in the way of putting the rebreather on while it is left in the test state. An added advantage is that less oxygen is needed to flush the loop and bring the ppO2 up when the rebreather is turned on.

The display of an Inspiration steps through the self test and calibration procedure.

SENSOR CALIBRATION
Nearly all rebreathers have oxygen sensors in the breathing loop to tell the diver (and any controlling electronics) what the oxygen level is during the dive. Some semi-closed rebreathers can be used without sensors, because the constant flow of gas through the loop maintains oxygen levels.
The sensors are like weak oxygen-powered batteries. The more oxygen they face, the higher the output voltage. The associated displays are really just voltmeters calibrated to read partial pressure of oxygen instead of volts.
As with any battery, the sensors age and the output degrades with use. To make sure they are showing an accurate ppO2 during the dive, the calibration has to be checked and adjusted before use.
Some rebreathers allow the oxygen sensors to be calibrated in the breathing loop while the rebreather is fully assembled. Others need to be calibrated before being inserted into the breathing loop.
So, as with the positive- and negative-pressure tests, you may not see the full procedure immediately before diving. Nevertheless, any rebreather that depends on oxygen sensors to function will have at least two displays. They may be identical; they may be a master and a slave; they may be a comprehensive primary and a simpler secondary.
Whatever the configuration, one of the things a rebreather diver will do more than once during the process of kitting up and getting in the water is to look at all the displays, check that they are switched on, and that they all show the same ppO2 within the loop.
It won’t be a perfect match, because no two sensors are identical, but it will be within a close margin.

Assembling the scrubber lid of an Inspiration. Divers usually remove it overnight to dry the sensors… Inserting the scrubber canister into an Inspiration… Connecting the scrubber canister to the lungs to complete the breathing loop.

WARMING UP THE SCRUBBER
The chemical reaction in a rebreather’s scrubber takes a while to get warmed up and going at full efficiency. As a consequence, either during the process of kitting up, or after kitting up but before getting in the water, rebreather divers will begin breathing off the rebreather for a few minutes before starting the dive.
As part of this process they will turn the gas on, check cylinder pressures, and check the O2 displays several times, to make sure that the sensors and hence displays track the oxygen level as it comes up to the operating level.
They may also make adjustments by pushing buttons or twiddling knobs, depending on which type of rebreather they are using. It’s all part of making sure that a rebreather is working properly while still safely on the boat – a sign of good procedure.
As a buddy, what you would need to be concerned about is a rebreather diver who just puts the mouthpiece in and rolls off the boat, missing this procedure out.

The DSV (Dive Surface Valve) of a Submatix. Many rebreathers have a lever to move this, where up is “open” and down is “closed”. It should be closed whenever the DSV is not in the diver’s mouth.

HOW DOES THE MOUTHPIECE CLOSE?
If the mouthpiece comes out of a rebreather diver’s mouth, it has to be closed. Normally the diver will take care of this, as part of his procedures. But if you have to rescue him, closing the
mouthpiece becomes part of the rescue procedure.
Leaving it open will result in the loop flooding and a big loss of buoyancy.
While he should have a big-enough wing to cope with this, keeping the buoyancy inside the rebreather loop is always preferable.
The technical term for the entire assembly is DSV, for “Dive Surface Valve”. To be accurate, the mouthpiece is just the bit the diver chews on. Most DSVs are a barrel design, where the inner barrel rotates inside the outer barrel to bring an inner hole in line with an outer hole, to which the mouthpiece is connected.
To close it, the inner barrel is rotated through 90° so that the holes no longer line up and are sealed from each other.
It could be done with a lever that sticks out at the front, or by rotating a ring at the end of the barrel.
In either case, you may need to use both hands to close the DSV.
If needed, you can hang on by the crinkly hose.
Part of the CE test for a rebreather is that the crinkly hoses are strong enough to hang on to.

This inspiration owner has clipped an additional pony cylinder to the outside of the casing for bail-out. A common modification to Drager rebreathers is to fit a single cylinder with a valve. One tap feeds the rebreather, while the second tap carries a bail-out regulator.

WHERE IS THE AAS?
As a diver who is buddied up with someone wearing a rebreather, a concern more to do with your personal safety is this: “Where is the alternative air source?”
You may be surprised to learn that while rebreathers look complete straight from the manufacturer, many come with a minimal bail-out that is configured more for the benefit of the rebreather diver than for their buddy. I am all in favour of bail-out that is configured for me, but
it shouldn’t be at the expense of not being able to assist my buddy should it be required.
The one thing that you can’t do with a rebreather diver is to grab the mouthpiece from which he is breathing. This would flood the loop and endanger both of you.
So nearly all rebreather divers have a conventional second stage regulator connected up and placed ready for their own or their buddy’s use. But it is very unlikely to be standard.
It could be connected to the diluent-cylinder first stage, it could be connected to a dedicated first stage sharing an H-valve with the diluent, or it could be connected to a separate pony or side-mount cylinder dedicated to bail-out. There may also be a second stage connected to the oxygen cylinder’s first stage, something you definitely do not want to mistakenly grab at depth.
You need to know where the AAS is that you can use, how to get to it, what gas it is connected to and how much gas is available.

Inspiration with side mount for bail-out.

PAUSE AND CHECK FOR BUBBLES
As a rebreather diver descends the shotline, part of the descent procedure is to pause a few metres down and check for bubbles.
This isn’t unique to rebreather divers, as some open-circuit divers also like to do a bubble check before they get too far down. If you are buddied with a rebreather diver, he may do this as a self-check, or ask you to help.
The trick is to know which bubbles are supposed to be there and which are not. At the start of a dive, there will be little bubbles of air trapped under all sorts of bits of equipment that slowly work their way out and bubble up as a diver moves in the water. Is this just air trapped under the rebreather shell escaping, or a genuine leak? Or perhaps it’s a semi-closed rebreather, and is supposed to trickle bubbles out through the exhaust valve.
Even with no knowledge of rebreathers, there are some parts we can all agree should not emit bubbles – the crinkly hoses and mouthpiece, the DIN threads on the first stages, and any of the HP or IP hoses.