Today I won’t be going too much into what subcooling and superheat is and how it works, but more of a ‘tool review‘ for an easy way to check your refrigerant charge. Also notice like many other posts on this site, this one is not intended for the homeowner and only intended to help those that have the proper Hvac training and certification to be able to check refrigerant levels.
Most air conditioning gauges have P/T charts on them, among the most common for residential applications are R-22 and 410-A. When checking your subcooling with gauges you will need to have your air conditioning gauges and hopefully they have the right P/T chart on them. On top of that you are going to need to have a meter and a thermal couple device that is capable of taking the line temperature, and with a little bit of subtraction you get your subcooling and superheat.
The reason I like the SSX34 is that it pretty much takes the calculations for you with out the need to do any subtracting in your head or looking at any P/T charts. For around $200 you can get a tool that is one of the easiest out there to take your subcooling and superheat readings.
There are other test instruments that are out there such as the Fieldpiece Sman3 and the Testo 550 that will actually 
calculate your subcooling and superheat at the same time, unlike the SSX34 which you can only test one at a time. But there have been many problems with the Testos and Sman3 in reliability and malfunctions. When they are working right but they are good but I have worked with many Hvac technicians that have had multiple problems with the both of them.
Reasons I choose the SSX34 over the Sman3
Portability – much easier to carry around, less hoses, less headache when you are trying to climb up a ladder to get on the roof. I have a tool suitcase where I keep a couple service wrenches, a meter, megohmeter and some other hand tools, and the SSX34 fit’s perfectly in there. So if I am diagnosing and outside unit I can do so with everything in my tool suitcase.
Reliability and Durability – I have owned my Fieldpiece SSX34 for over 2 years and I use it almost everyday all day checking superheat and subcooling readings. Not sure how long the lifespan is for the other 2 as they have not even been out for 2 years.(?)
Price – It’s a couple hundred dollars cheaper. Sure it can’t do as much as the Testo or the Sman, it doesn’t have a micron gauge and it can be a little tougher to actually charge a system using the SSx34, but for me it’s not that big of a deal. It would be nice to have both in your Hvac Tools arsenal but not all of us get paid that much money to afford all these nifty tools.
Just remember tools are designed to make the Hvac Technicians job easier and more efficient. You can have all the Hvac tools in the world but without the proper Hvac training and knowledge to know what you are looking out, these tools are pretty much useless. I would not recommend the SSX34 for the technician that is just getting out of school.
This would be more the the technician that has been in the field for a couple of years and has the experience and really understands what superheat and subcooling is within the refrigeration cycle.