8 July 2018


The VPFlowTerminal enables you to measure flow, pressure, temperature with one single VPFlowScope. In addition, the VPFlowTerminal offers 4 analog input channels, which can be used for logging of any 4..20 mA based sensor. In this example, we will show you how to use the VPFlowTerminal to monitor compressor performance. Please note that this is not intended as an official compressor test, but you can use the data to track changes in performance, or to create a “practical” efficiency curve of the compressor when installed in the field. It is a great tool to get more insight in the real behavior of your system. This application note is also not intended to be a course on how to work with spreadsheet programs, so basic knowledge of a spreadsheet program is recommended.

Compressor efficiency or specific energy

Compressors convert electric power into compressed air and heat. Most of the electric power is converted into heat, only a fraction is converted in compressed air. This makes compressed air a very expensive energy source.

The efficiency = output/input. In this example we look at the ratio between output mass flow and input electric power. For compressors, this efficiency number is expressed as kW/m3/min. Typically, this number varies between 5(very good) and >10 (poor, small systems). The efficiency varies per compressor type. Also, an older compressor might have a poor efficiency due to clogged inlet filters, leaking internal valves. This is a complicated subject, which we rather leave to compressor experts.


If your system has a desiccant dryer, and the flow meter is mounted downstream of this dryer, you need to be very careful: the dryer might have a significant internal air consumption for the re-generation process. This can be up to 10%...20% of the design capacity of the dryer. If your air demand is low, the percentage will increase. As a result, a flow meter downstream of this dryer will only see a part of the compressor output.

If your system has a refrigerant dryer, you can install the flow meter downstream of the dryer, to keep the air conditions as clean and dry as possible.

When the compressor has sufficient built-in after cooling and condensate drainage, you can use a flow meter directly on the discharge pipe. However, water drops will show as spikes in the measurement. Also there might be a chance of flooding. The flow measurement will be useless in those cases.


So what do we need? An ampere meter, to measure the input power and a flow meter to measure the output? No, that is not enough. You also need to measure pressure, to relate the efficiency number to the compressor specification (especially with centrifugal compressors- and in that case you should also measure blow off valve position). A temperature measurement would be nice, to see if the compressor is running stable, with proper after cooling. The VPFlowScope offers flow, pressure and temperature measurement in a single device. This greatly reduces the complexity of your audit.

Image Data logging

You also will need a data logger. The VPFlowTerminal is a multi-channel data logger designed for the VPFlowScope and four additional analog inputs.

The ampere meter will be installed in, or near the compressor, to measure the current on one of the three phases. Please note, this is dangerous work and should be carried out by certified electricians! VPInstruments supplies VPlog-i current sensors, which can be used for indicative power measurement.

After installation of the ampere meter, the VPFlowTerminal needs to know the scaling of the meter, to calculate power consumption.

Configuration of the VPFlowTerminal

You need to configure the VPFlowTerminal via your personal computer, using VPStudio software. There are two things to configure:

  • The pipe diameter for the VPFlowScope
  • The logging interval for each channel
  • The 4..20 mA scaling for the power sensor.
  • The pipe diameter needs to be checked or measured. You will need the inner diameter. In some cases you can read it from the printed text on the pipe. In other cases, you will need a circumference tape and an ultrasonic thickness gage to measure the inner diameter properly.

    Current to power calculation

    In this example we will explain how to calculate the power corresponding to the 4..20 mA output value. Before we can do this we need some more data.

  • Measure the supply Voltage (! remember, this is a task for certified electricians)
  • Check the power factor of your motor. The power factor will be a number between 0.7 and 0.9, and is assumed to be constant.
  • The formula to calculate the power is as follows, for a three phase motor:
    This formula can be simplified:
    Lets assume a cosinus(φ) of 0.7 For a 200 Amp current clamp, the Pmax (maximum power) is:

    This number will correspond to 20 mA and this is the number to fill in for your analog input channel. So now you have configured all input you need. You can start logging now.

    Setting the logging interval

    The logging interval depends on what you would like to see. If you are interested in control system behavior, you can set the logging interval to 1 second, and measure for a couple of days. If you would like to see slow variations, for example the influence of daily production on the performance, you can set the interval to once per minute. For see seasonal influence, a 15 minute interval provides enough data to make the right analysis.

    Data analysis

    Let’s take a look at some data from a logging session. To understand what is going on, a simplified P&ID sketch is very useful to have. It be a sketch, like in this paper, or you can use the official P&ID symbols. For outsiders, a sketch might be easier to understand.


    In this system, we have 250 kW base load compressor and an 125 kW trim.

    When imported in a spreadsheet program, The data of this system on a certain day looks as follows:

    In the spreadsheet program, the data can be combined in a graph. In the following graph, the red line is the 125 kW compressor, and the blue line the 250 kW load/ unload compressor. The green line shows the airflow into the plant. Data is averaged per minute. The efficiency can be defined as: Output/ Input, or Input/ Output.


    A common unit for efficiency is kW/m3/min. Let’s plot the efficiency over a one hour period. See the graph below. The average efficiency is around 11 kW/m3/minute. The short cycling of the compressor results in a rather poor efficiency for this example system. So it’s time to tweak the control system or to advice a different combination of compressors.


    In this example we have seen the convenience of having a multi-channel data logger to measure efficiency of compressed air systems. All signals are logged simultaneously, so it is easy to find the relationship between the signals and to perform complex analysis.