Calculating compression ratio is of little value to the service technician on the job. The compression ratio changes very little with changes in pressures that occur in the field. You will have major problems long before the compression ratio indicates there is a problem.
Volumetric efficiency is a better indicator of a problem but that is not possible to calculate. You need the capacity charts for your particular compressor to determine the volumetric efficiency.
We actually do use compression ratio as a troubleshooting tool on ultra low cascade compresors. The compression ratio should be at least 11:1. If not, the compressor capacity is diminished enough that we won't make temperature. This is on a Copeland 1 HP Hermetic.
When a system won't quite make -85 deg.C, you are looking at a partial restriction (usualy oil logging), refrigerant leak, or weak compresor (leaking valves). this is just one more tool to help determine which one is the problem. The refrigerant charge is quite small by volume and very critical.
To calculate, you need to take your absolute Discharge pressure and divide by your absolute suction pressure. The answer is expressed as a ratio to "1". PSIA ( absolute pressure) is PSIG from your gauges plus atmospheric pressure. In this area, we would use 14.7 added to our gauge reading.
Hope this helps!
The most useful application of compression ratio on cascade systems is when you are designing one and you need to know how many cascade stages it will take to divide up the total compression ratio. And, to determine how much of the total ratio each stage will take.
You get far more useful information if you obtain your saturated suction temperature and pressure and your saturated discharge temperature and pressure. Then obtain a capacity chart for your particular compressor and then you can read the following off the chart.
1. Actual compressor capacity in btuh for those conditions
2. Actual total heat of rejection out the compressor discharge and into the condenser
3. Actual operating wattage for those conditions
Using the chart you can move around on the chart and see exactly what the results will be if you change the high side or low side pressure. You can see how many btuh of compressor capacity you will gain or loose with different suction and discharge pressures.
This is far, far more useful than messing with compression ratio. I have used both and compared them to each other. Small changes in compression ratio actually amount to great changes in compressor capacity and power consumption. Compression ratio is just not accurate enough to be meaningful compared to the compressor charts.
The hardcopy compressor performance data is a thing of the past. Go to Copeland's website: http://www.copeland-corp.com/ and download their Compressor Performance Calculator program. (You'll have to register first if you're not already in the system)
I've got it on my laptop in the truck. I can print out a performance sheet with all the data I need.......pretty cool.