Meet new scuba divers, maintain a virtual dive log, participate in our forum, share underwater photos, research dive sites and more. Members login here.

SMACO S400, An Alternate Air Source?
scubadivingnomad - 3/14/2023 6:36 AM
View Member Articles
Category: Equipment
Comments: 0
SMACO S400, An Alternate Air Source?SMACO S400, Will It Be Your Alternate Air Source?

You may be wondering how I came about reviewing the SMACO S400. Life has some interesting twists at times. As the end of the year approached, I started to plan what type of vacation I wanted to take in 2023. One of the trips I decided on was a scuba diving liveaboard. Looking over the conditions of the trip, I saw they allowed solo diving for properly qualified divers. Therefore, I added getting certified as a solo diver to my goals for the year. Each accreditation agency that offers it has a different name for it, however, the requirements are basically the same. Redundancy is a key factor, including an alternate air supply. I started to research what was available, and develop a very short list of alternate air supplies that I was interested in. Much to my surprise and good fortune, the manufacturer of one of my shortlist items the SMACO S400 saw my review of an underwater scooter and asked me to review their product.

Developing My Short List that included the SMACO S400

Before jumping into my review, I think it is best to explain why I felt the SMACO S400 deserved to be on my list. A few years ago, I wrote an article titled “Buyer Beware”, that focused on items used in scuba diving being sold to the general public. Before internet sales became such a major factor, buying items such as scuba tanks was in a manner restricted. Few dive stores would sell you a scuba tank if you were not certified. If you did own a tank, getting it filled without being certified was nearly impossible. However, over time things have relaxed, partly due to paintball enthusiasts using scuba tanks to charge their guns. Today there are many companies making scuba tanks that are sold online, not all of which I would consider safe. Many of these tanks are small and marketed as an alternative to becoming a certified diver. They gloss over the dangers of using compressed gases underwater. I do accept that these tanks do fill a need for people that are using them for activities that are not recreational scuba diving, such as boat inspections, and emergency air for surfers. However, users must understand the dangers involved.

Size Matters
The logical point to start qualifying an item to purchase is to determine your needs and what size tank will meet that needs. I am looking for a pony bottle, that will serve as an alternate air supply in the case of an out-of-air situation. I will be within decompression limits, so I do not need one large enough for extended decompression stops. Generally, a pony bottle used for this purpose is smaller than a staging bottle used for a multi-gas decompression dive, while there may be some overlap. You will want something large enough to provide ample air, while still small enough not to interfere with your diving and trim. I also like the concept of being able to hand over my alternate air source to a dive buddy that has run low on air. Having them use my octopus is fine, however, it is also the air I am using. Pony bottles used for this purpose are generally in the 6 cf (cubic feet), 13 cf, or 19 cf size. There are a number of 3 cf tanks as well which are designed for quick ascents in an emergency. These smaller sizes are often preferred by surfers for extra insurance against being wiped out by a larger wave and pushed under. You will also see cylinders in 1-liter or 2-liter sizes as well. Comparing a tank size between cubic feet and liters can be confusing because they are measuring different things. You can not directly calculate between them. Tanks being measured in cf is a calculation of how much air the tank will hold when pressurized to its operating pressure. The most common scuba tank is the aluminum 80. This will hold approximately 80 cf of air when pressurized to 3,000 pounds per square inch (PSI) or using the metric system 200 bar. If you use a simple conversion of cf to liters it gives you slightly over 2,265 liters of air. However, the aluminum 80 cylinders in the European market is an 11.1-liter tank. This is due to a difference in the type of measurements. A tank measured in liters represents the volume of water the tank will hold and compression is not considered. To give a rough guide to compare a cf tank with a liter tank, a 6 cf tank is near the size of a 1-liter tank. The 13-cf tank and the 2-liter tank are similar as are the 3-cf and the .5 liter tank. Balancing out the air reserve available and the physical size of the cylinder, with my needs, I decided to focus on the 6 cf and 1-liter size tanks.

DOT Accepted?
The scuba industry is mostly self-regulated, however, scuba tanks are just special-purpose compressed air tanks. Therefore, they must adhere to the regulations that control those items. In the United States, to fill a tank with compressed air, the tank must meet Department of Transportation (DOT) requirements. Approved tanks will have a code stamped into the metal showing it has been approved. Canada and Europe have similar standards. If you take your tank to a scuba dive center or a compressed gas company, they are required to verify the tank has the proper markings before they will fill it. When I went to the dive center to test the mini-scuba tank in the pool, the owner verified that the tank had a DOT code stamped on it before I was allowed to fill it from one of their tanks. She told me that she had to turn away other individuals who had similar types of tanks that were not approved. Also, she told me that even if the tank is approved, she can not fill the tank unless the person getting the fill can show a scuba diving certification. Some companies that sell online do not meet these requirements and use alternative means to fill the tanks. There is nothing wrong with these alternative means in themselves. Special hand pumps and small electrical compressors can be used to fill the tanks both DOT-approved and those that are not. For my own safety, I want a tank that has been manufactured to the DOT standard. When I started to narrow my focus for a tank, I eliminated any without DOT approval from my search. I was surprised at first, how many were found online without any mention of DOT approval.


My kit included an adapter for filling the S400 from a full-size tank. That is not included with all combinations. It also had a harness to hold the cylinder while using it. While just using the S400, I found the harness very comfortable. I could position the tank either behind me or across my chest. The S400 itself has a 1-liter dive cylinder. If you are diving solo, then you must have an independent source of air. That is what you can use this tank for.   Also provided is a first-stage regulator (the manual refers to it as the primary pressure-reducing valve) which will attach to the dive cylinder. The purpose of the first stage is to lower the pressure of the air as it leaves the cylinder. The first stage and the second stage are connected by way of a hose. The second stage has a mouthpiece that you put in your mouth to breathe from. The second stage reduces the air pressure further so that the air you take in is at the same pressure as the water around you. Assembling the SMACO S400 You need to be careful assembling the SMACO S400, still, it is very simple to do. Remove the end cap from the first stage and verify that the “O” ring is in place. Then carefully screw the first stage onto the high-pressure cylinder. The first stage should be hand-tight. Remove the end cap from the medium-pressure hose. Connect this to the port on the first stage and tighten. Use the supplied wrench to insure a tight connection. Do not try to over-tighten the connections. Generally, you can leave the mini-scuba tank assembled unless you are transporting it by air. Airline and government regulations require you to empty the cylinder and remove the valve.

Filling the SMACO S400 from A Full Size Scuba Tank

There are three methods available to fill the cylinder. The first is a high-pressure hand pump. This is a specially designed pump that will allow you to increase the pressure inside the tank to the 3,000 psi goal. Please note, that a bicycle pump will not work. It can not increase the air pressure to the pressure needed. Also, this is a slow process and a great deal of muscle power is required. You can also purchase a small electric compressor. This is a convenient way to charge the tank and can be powered by a car battery or household current. However, these devices can be on the pricey side. The third method is to charge the mini-scuba tank from a full-size one. Since my planned use of the SMACO S400 is as an alternate air source, this is the best method for me, as well as the most affordable. The video shows me filling my pony bottle from a full-size AL80 scuba tank using an adapter. The process is very easy. The adapter is attached to the AL80 in the same manner as a 1st stage regulator would be. Verify the “O” ring in place on the adapter and place the adapter over the value of the scuba tank, aligning the ports and tightening the adapter finger tight. The adapter also has a quick-connect connector on it. This works in the same manner as the quick connect does on the BCD inflator. The cylinder of the S400 has a port to accept the quick-connect connector. Connect the cylinder and the adapter. The adapter has a pressure release mechanism. Make sure that is closed. You are now ready to fill. Slowly open the valve on the full-size tank slowly, and the mini tank will start to fill.

Please read the full review on my website.

The post SMACO S400, An Alternate Air Source? appeared first on The Scuba Diving Nomad. SMACO S400