Absolute Pressure vs. Gauge Pressure

In measurement knowing how your sensors are configured is extremely important. In the case of static pressure measurement, you need to know if you are measuring the absolute pressure in the meter run or the gauge pressure of the meter run. When you are installing and configuring new meters knowing which to choose is imperative.

Some terminology to get out of the way first. Look some of this information up online. Some great articles about it.

Pressure – This is a force applied perpendicular to an area of an object, so says Wikipedia. In most of the scenarios I deal with, this is the force pushing outwards on pipes and vessels.

Gauge Pressure – This is the pressure inside a pipe or vessel, but referenced to atmospheric pressure. So if the pipe is open to atmosphere the pressure sensor will read zero.

Absolute Pressure – This is the pressure inside a pipe or vessel, but referenced to a perfect vacuum. This means that the pressure sensor will read atmospheric PLUS the pressure in the pipe.

Atmospheric Pressure – This it the pressure exerted by the air around you. Typically the higher in elevation you are, the less air pressure. In Alberta typical atmospheric pressures range from 85-100 kPa (12.3-14.5 psi).

EFM – Electronic flow meter. This is the brains behind many of the measurement solutions in the oil and gas industry (and others).

Now that we know a little more about pressure we need to know why it is important for measurement. Here it is: Most measurement formulas use Absolute Pressure in their calculations. If you are sensing pressure with a gauge pressure sensor you must add the atmospheric pressure to the reading. This means you need to tell the flow computer the reading is a gauge reading AND you must tell the flow computer what the typical atmospheric pressure is where the meter lives. With both those settings the EFM will correct the pressure reading to the correct value.

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

Instrumentation, measurement and field technicians need to know about this. I did some work for a client last summer and they were having field meter balance issues. Bottom line: their sales gas number was differing widely from their gas meters on the producing sites in the oil field. We found the absolute/gauge setting and the atmospheric pressure in the client’s meters were not set properly. Making a couple minor adjustments to their field meters we reduced their field balance errors by a large margin.

Most new 3-in-1 transmitters (aka multi-variable sensors, MVS) are absolute pressure sensing from factory and have been for some time. This means you could select ‘absolute’ in your EFM configuration. However, this makes life difficult when it comes to calibration.

Most technicians are equipped with hand pumps and a special sensor and display for calibrating meters and sensors. Most often these calibration kits are measuring gauge pressure. If you use your handy gauge pressure hand pump to calibrate an absolute pressure sensor you may have inadvertently introduced a 85-95 kPa error in your sensor readings. Combine this with a mis-configured EFM and you have a recipe for up to 10% error introduced in your measurement solution.

What I have found the best is to calibrate the sensor using your gauge pressure gear. This will re-zero the sensor so 0 = atmospheric pressure. Then you can correct the value back with the EFM configuration.

How do we get past all the potential complexity and chances for error?

  1. Calibrate your meter’s sensors to gauge so it matches your calibration equipment, unless you have a reliable and approved way of calibrating absolute sensors.
  2. Set your EFM to read the pressure as gauge and configure a proper atmospheric pressure based on the meter’s elevation.
  3. Have a third party or the client check your work and make sure the meter configuration meets the client’s standards. Or include a check on your calibration report for the meter.
  4. Get knowledgeable about metering. There are resources for people wanting to get better at their craft.

Feel free to comment below if you think I am missing anything.