pressure measurement

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pressure measurement

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inayathn
Contributor I

i have to measure pressure of compressed air around 3- 4 bar, which is heated of 80 degree celsius. I have to fix this sensors on the cylinder, thatswhy i prefer sensors with some projections, I found two pf them,MPX5500DP or MPX5700AP/GP/GP1, which is better?, why MPX5500DP with two projections.? it represents one for vaccum? , am prefering MPX5700AP/GP/GP1, is there any problem for this to my application?

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Martin35804
NXP Employee
NXP Employee

MPX5500DP or MPX5700AP/GP/GP1 both are similar in function with difference in max pressure range for 500 kPa and 700 kPa. The MPX5500DP comes in package CASE 867C with two ports so you may measure the difference between two applied pressures. In general when leaving the port P2 opened you will get the difference between applied pressure to port P1 compared to ambient air pressure left on port P2 and that is similar to the gauge pressure sensor. So when you take any other differential pressure sensor and left the port 2 at atmospheric pressure basically you have gauged pressure sensor.

MPX5700DP comes with the same package as well.

Gauge pressure sensors GP have only one port for inlet pressure the P1 post and then a small vented hole at the back of the sensor for the ambient pressure. Absolute pressure sensors have this hole sealed with precise reference pressure (vacuum) that is not changing. MPX5700GP1 has only the 90 degree lead form compared to GP version. The gauge sensor is sort of differential sensor where the pressure is measured compared to gauged side which is exposed to ambient air pressure without any specific geometry of port.

Absolute = vacuum reference. Gauge = atm. pressure reference.

So it depends on your design. You may also consider for instance MPX6400A that is absolute as well with different package up to 400 kPa with max temp. range of +125degC. The package you may see at following link:

https://community.freescale.com/docs/DOC-98952

Vented Gauge package that we offer is measuring diff. pressure between applied – measured pressure and atm. pressure which is not sealed so the measured output would be difference between applied pressure and current atmospheric pressure at certain location. So form this definition it wouldn’t be the right choice for the application unless the ref. atm. Pressure side would be perfectly sealed. For gauge pressure sensor you can measure the over or under pressure compared to ref. atm. pressure.

Gauge Pressure readings are a special case of differential measurements in which the pressure applied to the “Pressure” side is measured against the ambient atmospheric pressure applied to the “Vacuum” side through the vent hole in the chip of the differential pressure sensor elements.

There is no internal reference, there is simply a hole on the back side of the part, so it is open to the ambient pressure.

In the same respect, absolute pressure can also be considered a differential pressure where the measured pressure is compared to a perfect vacuum.

It is not always straightforward when to use the absolute pressure sensor or when to use the gauge one but generally if you want to measure or control a pressure that is influenced by changes in atmospheric pressure, e.g. the level of liquid in an open tank or the output pressure of an air compressor; you would use a gauge pressure sensor since you are interested in the pressure reading minus the atmospheric pressure component.

If there is a requirement to measure pressures that are not influenced by changes in atmospheric pressure, e.g. leak testing a completely sealed non-flexible container, you would use an absolute pressure sensor. If a gauge pressure sensor was used instead to measure the container pressure and the barometric pressure changed then the sensor’s reading would change despite the fact that the pressure in the container remains the same.

In case of differential packaging the ref. side would need to be maintained on certain desired pressure to get the reading of the difference between the measured inlet and the reference inlet.

In compare with the vacuum - absolute package you get readings directly the difference compared to sealed ref. vacuum side.

An absolute pressure measurement is one that is referred to absolute vacuum. The best example of an absolute referenced pressure is the measurement of barometric pressure.

In order to produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. Therefore if you hold open the process pressure connection of an absolute pressure transmitter to the air it will read the actual barometric pressure.

  • Differential: Difference in pressure between two points is measured as pressure is applied to both sides of the die/sensing element.
  • Gauge: Bottom side is exposed to the atmosphere, while pressure is applied to top of die.
  • Absolute: Pressure is applied to the top of the die while the bottom of the die is a vacuum-sealed reference.

A vented gauge pressure transmitter for example allows the outside air pressure to be exposed to the negative side of the pressure sensing diaphragm via a vented cable or a hole on the side of the device, so that it always measures with reference to the ambient barometric pressure. Thus a vented gauge reference pressure sensor reads zero pressure when the process pressure connection is held open to atmospheric air.

A sealed gauge reference is very similar except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure applications such as measuring hydraulic pressures where atmospheric pressure changes will have negligible effect on the accuracy of the sensor so venting is not necessary.

There is another way of creating a sealed gauge reference and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then by adjusting the electronics, the output signal is offset by 1 bar so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read exactly zero when left open to atmospheric air, because atmospheric pressure is always changing and the reference in this case is fixed at a nominal 1 bar.

You may refer to AN1573, Understanding Pressure and Pressure Measurement for further information.

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