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Differential Pressure Sensor to Absolute Pressure Sensor

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adaminc

New Member
Hello All!

A few years ago I was working on a weather station as a school project, I was going to add a pressure sensor as a bonus but never got around to it, but I did order a sample from Freescale before hand.

The sensors is the MPX5999D

Datasheet http://www.freescale.com/files/sensors/doc/data_sheet/MPX5999D.pdf?pspll=1

Anyways, turns out I ordered the wrong free sample, I ordered a differential pressure sensor. Now I want to use it as an absolute pressure sensor to measure water pressure for a backup pump system, so if the pressure drops below a certain level, the primary pump will be turned off and the backup pump will kick in, if the backup can't get the pressure back up, it will set off an alarm.

So my question is this, if I seal the back hole with epoxy (the reference port), put an oring around where the front sensor hole is to have an area that will flex, then fit the sensor inside some heat shrink tubing, shrink the tube around it, then fill the top and bottom with epoxy so it is sealed. Do you think this will work?

Here is what I am thinking of doing.

 

SPDCHK

Member
Using a ΔP transmitter for level measurement requires the Low Pressure (LP) end to be OPEN to atmosphere and the High Pressure (LP) end connected to the bottom of the OPEN ended tank.

If the tank is sealed you will need to use both LP and HP legs, but this time the HP leg is connected to a POD (filled with the same liquid as in the tank) at the same height as the MAX level, and the LP leg connected to the bottom of the tank. The reading will be inverse but that's simple enough to translate back to 0..100%
 

adaminc

New Member
Using a ΔP transmitter for level measurement requires the Low Pressure (LP) end to be OPEN to atmosphere and the High Pressure (LP) end connected to the bottom of the OPEN ended tank.

If the tank is sealed you will need to use both LP and HP legs, but this time the HP leg is connected to a POD (filled with the same liquid as in the tank) at the same height as the MAX level, and the LP leg connected to the bottom of the tank. The reading will be inverse but that's simple enough to translate back to 0..100%
It isn't for level measurement, it is for pressure measurement. If one of the pumps fail (completely or partially) the pressure will drop. So my circuit will detect that, turn off the failed pump, and turn on the backup pump.

The pumps are connected to piping which, at the end, has spray nozzles (it's for aeroponics) so there should be a pretty constant pressure when the pump is running. I am going to have a small dead end section with this sensor sitting in it.

I am wondering if my mod of the pressure sensor will work to give me absolute pressure (which will be offset by 1 atm). Since the reference side is sealed off, I am hoping it will always be at 1atm. Then the other side will change with water pressure.

Here is a diagram.

 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
Whats the working pressure on the 'high' side of the sensor.?:)
 

Boncuk

New Member
As I understood your pressure sensor is a single pressure port type. The single port types are precharged to standard atmosheric pressure of 1,013.2mb, 1,013.2HPa or 29.92 inch/Hg.

A pump weren't a pump delivering not more than the atmospheric pressure. (at max a whirling machine :D)

Don't care for the sealed pressure port. As long as the pump pressure is above atmospheric pressure your circuit should signal a "healthy pump". If the pressure drops to the sensor internal pressure it's a bad pump for sure. :)

Boncuk
 
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marcbarker

New Member
If you break open a discarded washing machine you can find a water fill sensor switch, which is adjustable. Follow the plastic pipe from the drum to find this round thing with wires attached to it. :)
 

crutschow

Well-Known Member
Most Helpful Member
Actually, differential pressure is better for measuring pump pressure than absolute since it automatically compensates for any change in atmospheric pressure. Thus if you set the trip point at 5psi (for example) then it would always trip at 5psi pump pressure independent of atmospheric pressure.
 

adaminc

New Member
@ericgibbs

I would say it is at most 100psi.

@boncuk

It has 2 ports, one on the front, one on the back. The sensor is going to be in heatshrink totally surrounded by water. There will be no reference unless I seal the reference port. The datasheet has a diagram on page 3 to see what I mean.

@marcbaker

I don't need a fill sensor, I need a pressure sensor.

@crutschow

The sensor is a button type. I know it isn't made to be submerged in water. That is why it is going to be put inside some heat shrink tubing. Since it won't have access to atmospheric readings for the reference port, I am going to seal the reference port so it is always at atmospheric pressure. Then I will seal off both ends so I can put it into the water.
 

Boncuk

New Member
Hi,

according to the datasheet you mentioned in your initial post that sensor isn't a differential pressure sensor, since it has one pressure port only. Further that pressure port is "open" meaning no way to connect a hose to it. You might use that one to construct an altimeter.

Pressure sensors are designed to measure air pressure. Under no circumstances dunk a pressure sensor into water. It will be destroyed the moment it submerges.

It should be mounted well above the highest expected water level.

You'd be better off using an MPX5500DP. Maxim pressure 500KPa. "DP" stands for dual port, meaning it is fitted with two plastic pressure ports to connect hoses to them.

To measure the actual water pressure it requires to sense the water pressure on the positive pressure port and atmoshperic pressure on the negative pressure port, resulting in true water height regardless of loacal air pressure variations, which might fall to 850HPa in a low weather system and rise to 1,100HPa in a high.

Additionally, using a single port pressure sensor the measured pressure will always be difference between water pressure and standard atmospheric pressure which is 1,013.2HPa.

For better understanding I attached a small drawing showing water level detection. It works the same way detecting water pump pressure.

Pic2 shows a dual port differential pressure sensor.

Boncuk
 

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adaminc

New Member
Hi,

according to the datasheet you mentioned in your initial post that sensor isn't a differential pressure sensor, since it has one pressure port only. Further that pressure port is "open" meaning no way to connect a hose to it. You might use that one to construct an altimeter.

Pressure sensors are designed to measure air pressure. Under no circumstances dunk a pressure sensor into water. It will be destroyed the moment it submerges.

It should be mounted well above the highest expected water level.

You'd be better off using an MPX5500DP. Maxim pressure 500KPa. "DP" stands for dual port, meaning it is fitted with two plastic pressure ports to connect hoses to them.

To measure the actual water pressure it requires to sense the water pressure on the positive pressure port and atmoshperic pressure on the negative pressure port, resulting in true water height regardless of loacal air pressure variations, which might fall to 850HPa in a low weather system and rise to 1,100HPa in a high.

Additionally, using a single port pressure sensor the measured pressure will always be difference between water pressure and standard atmospheric pressure which is 1,013.2HPa.

For better understanding I attached a small drawing showing water level detection. It works the same way detecting water pump pressure.

Pic2 shows a dual port differential pressure sensor.

Boncuk

I appreciated the help, but I am not trying to measure water level.
I am trying to measure pressure, and yes this is a differential pressure sensor. It says it right on the first page of the datasheet, it only comes as a differential pressure sensor. On the 3rd page it shows TWO PORTS, P1 and P2.

If you missed my diagrams somehow, you will see that I am sealing the sensor in heatshrink tubing so that it won't get wet.
 

crutschow

Well-Known Member
Most Helpful Member
The problem is that if it is sealed in a small volume of heatshrink it won't be able to readily detect the water pressure. It needs to have the water directly access the pressure port.

Or it would need to be sealed in a large volume of air in a flexible bag so that when when the water compresses the air it would transfer the pressure to the transducer. The volume should be large enough so that the volume of air remaining when compressed to 100 psi would be about equal to the transducer volume.
 

adaminc

New Member
The problem is that if it is sealed in a small volume of heatshrink it won't be able to readily detect the water pressure. It needs to have the water directly access the pressure port.

Or it would need to be sealed in a large volume of air in a flexible bag so that when when the water compresses the air it would transfer the pressure to the transducer. The volume should be large enough so that the volume of air remaining when compressed to 100 psi would be about equal to the transducer volume.
Yeah, that makes sense. I was thinking that if I placed an oring in there, it would have enough space for the heatshrink to stretch into and effect the pressure inside, but since the air inside is so easily compressed, the volume wouldn't be enough to equal the outside pressure. Maybe I can keep the oring there, but put a nonconductive fluid in there, like mineral oil, that should work. I will have to test it out to see I guess.
 

crutschow

Well-Known Member
Most Helpful Member
The air volume would need to be at least 100 times the transducer volume so that the air volume is no smaller than the transducer volume at 100 psi.

Certainly mineral oil (or some other inert fluid) in the cavity should work since it will transfer the water pressure to the transducer without compression.
 

Boncuk

New Member
I appreciated the help, but I am not trying to measure water level
I wrote, the same applies to sensing pump pressure.

Why over complicate things?

If you connect a hose to the pump outlet the air in it will be compressed with increasing water pressure. The principle remains unchanged. All you measure is air pressure, no matter if it's caused by a water column or a pump.

Nothing to seal watertight if you connect the hose to the positive pressure port of the sensor and the other end to the pump outlet leaving the negative pressure port open to the atmosphere.

All my household water pumps work the same way with a pressure reservoir to avoiding frequent and rapid switching of the pumps while water is being taken off the pressure pipe. The pressure switch is mounted directly at the pump outlet and never gets wet because it only contains air.

Remark: 100psi correspond to 680KPa. You might check the sensor's data sheet not to exceed the maximum allowable pressure.

Actually you won't require a pressure sensor (which has to have an amplifier circuit for a useful output signal (MPX series pressure sensors have a maximum output voltage of 40mV)). All you need is a pressure switch which is readily available at pump stores - and they are adjustable to any desired pressure.

Boncuk
 

Boncuk

New Member
The air volume would need to be at least 100 times the transducer volume so that the air volume is no smaller than the transducer volume at 100 psi.

Certainly mineral oil (or some other inert fluid) in the cavity should work since it will transfer the water pressure to the transducer without compression.
Hi Carl,

any fluid getting in contact with the diaphragm will destroy a pressure sensor.

Regards

Hans
 

crutschow

Well-Known Member
Most Helpful Member
Hi Carl,

any fluid getting in contact with the diaphragm will destroy a pressure sensor.
How does it destroy the diaphragm?
 

Boncuk

New Member
How does it destroy the diaphragm?
I didn't say it destroys the diaphragm, but it destroys the sensor. The hole between pressure port and diaphragm is extremely small (about 0.1mm) and any liquid penetrating that hole toward the diaphragm will stay at it without changing capacitancy of the sensor with changing liquid pressure because of capillar force.
 
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