by David Owens david@greennet.net
by David Owens david@greennet.net
If you are using enriched air on any of your dives you know the importance of analyzing your gas before diving it. You may have also considered purchasing your own analyzer to perform analysis away from the blending location (such as on the boat immediately before a dive), to analyze tanks not picked up by you, or to analyze tanks after blending them yourself. The cost, however, of commercially available analyzers prohibits most of us from purchasing one. There are cheaper alternatives to the commercial analyzer. One is using a standard multimeter set up on an O2 sensor to read mV output, and then calculating by hand a conversion to FO2, and the other is a simple oxygen sensor reader you can make yourself that converts the electrical output of the sensor to a FO2 reading for you.
I have made several of these readers, spent much time comparing and testing them, and have found them to be as accurate and easy to use as the commercially available product. There are several versions of similar plans floating around various web sites, and although my plan does not differ much from these readily available plans, the following step by step procedure and accompany photos and schematics will show you how to make and use a reader quickly and efficiently.
All the parts you will need are readily available from a number of suppliers, including mail-order, Internet, and the local Radio Shack (See below for sensor order information). Remember, the cheapest parts are not always the best, and for this project it is worth spending a dollar or two more to get something better. That said, however, you can easily put together the reader from Radio Shack in an afternoon and it will work fine.
I get most of my parts from :
All Electronics Corp 800-826-5432 www.allcorp.com
Another source is: Parts Express
Note: Make sure you get the PM128 not the PM128A! The A model uses subminiature parts and cannot be modified as in these directions.
Parts-
Prices and page number from All Electronics catalogue except pot from RS:
Tools-
If you don‚t already have them. tools will set you back about ten to fifteen dollars or so.
Vance Harlow in his Oxygen Hacker‚s Companion has good information on modifying the panel meter for an analyzer. I have basically ripped off his method here. I like to connect everything temporarily outside of the housing to make sure it all works, and then mount it into the housing permanently.
The Heart of the Box:
The panel meter should be handled with care so as not to damage connections, get it covered with finger oil and grease, or to damage the readout face. Look at the back of the meter so the printing is right way up. On the bottom left you will see where to connect your battery leads, and above this, vin/gd is where the lead for the O2 sensor will be connected. Don‚t solder anything yet! See diagram.
The resistor (blue cylindrical thing with colored bands) immediately to the left of the larger blue trimming potentiometer needs to be removed. The open connections are where you will place the leads from your larger calibration pot. Cut the resistor out with very sharp, fine snips as close to the ceramic as possible to leave you some tabs upon which you can solder your pot leads, rather than desoldering and removing the resistor entirely. You must solder one 100ohm resistor to the pot lead between the board and the pot and another 470 ohm resistor across the pot leads themselves to modify the pot so that it can calibrate the sensor output.
Make sure that you solder a lead across the terminals labeled P3 to shift the decimal place of your readout to FO2.
Check that the panel meter works by hooking up a sensor at this point (or not depending on how lucky you feel).
The Housing:
Once you are ready to mount the meter, scribe around the face onto your housing, drill the corners of the housing and cut out a frame with a jig saw with a fine blade (the jig saw is for lazy people like me, but you have to pay attention not to cut too much etc. The hand blade is more accurate). File down the edges and make minor adjustments with sandpaper. Mount the meter.
Connect the battery leads, but make sure you run them through the battery compartment first. Your switch is in line with one of the battery leads before it gets to the panel. Make sure there is room for the base of the calibrating pot and the switch between the compartment and the panel meter. You will also need to leave room for the style of jack, or strain relief, you will use going to the sensor. Drill and sand holes for each component, set them in place and make all your connections.
The Sensor:
By far the best deal on sensors is the Ceramatec Max-250 (formerly Cag-250). This is a two year galvanic sensor and comes with a choice of connectors. I have mostly the rca jacks and have purchased ready made cords from radio stores as well as fashioned my own from parts. The reader, however, can be used with any sensor.
Ceramatec Max-250 $50-$65. 1-800-748-5355 www.ceramatec.com
Use your analyzer as you would a commercially available model (i.e. you should already know how to use it!).
It is a good idea to test your analyzer against other models whenever possible. I always take mine to the shop and compare readings on several different tanks in addition to my own. It gives me peace of mind to know that it performs as well as the commercially bought models, and makes me chuckle to see the faces of divers who have shelled out $250.00 more than me for their analyzers. Be careful when you use your analyzer in an "unknown" shop, as the staff may take a disliking to you threatening their sales market. I have seen and heard some disparaging remarks about my analyzer, but hey, I don't care.
A good test technique is to hook up a multi meter to your sensor on occasion and measure the milivolts for a known quantity (air 21%O2), divide mV by 21 and you have mV per percent of O2. Now measure a tank of EANx, record the mV, divide by mV per percent and you should get something very close to your analyzer.
Calibrate on air:
11.5mV/21= 0.548mV/percent O2
Measure EANX :
20.8mV/0.548= 37.96%O2
I have also read the manuals to several other analyzers and taken information on their use and care and applied it to my analyzer. Keep your ears open and your eyes skinned for any information, techniques, and developments concerning sensors and oxygen as information in this area changes without a lot of fanfare.
Remember, you should be trained and certified in the use of enriched air (nitrox), in handling oxygen, and thoroughly familiar with the risks of using oxygen in diving applications. You should also know how to analyze and be familiar with different blending techniques before you attempt to build your analyzer. Your analyzer is your own responsibility. Making sure that it works, gives correct readings, recognizing erroneous readings, maintaining your reader and sensor, is your responsibility and yours alone.
Good Luck!