Absolute pressure sensor - Page 1
Forum section: electronics / projects

author: IconicPCB
author_rank: Super Contributor
timestamp: February 02, 2016, 04:30:31 am
content: I have been asked to look into a pressure sensing issue.Consider a rigid fluid container exposed to the atmosphere with an absolute pressure sensor connected to the bottom level of tank fluid via a side tube.As the fluid level changes so the pressure inside the connecting tube changes and as the the sensor senses absolute pressure it will register level changes.But as the sensor IS an absolute pressure sensor i will also be affected by changes in atmospheric pressure. In case of water one atmosphere is approximately 10.33 metre high column of water.Note atmospheric pressure variations are in the order of 1037.4hPa and 986.8 hPa ( 1057.8cm of water and 1006.2cm of water ).How will this variation in atmospheric pressure be reflected in the output of the absolute pressure sensor?

author: joeqsmith
author_rank: Super Contributor
timestamp: February 02, 2016, 04:53:17 am
content: It must be late as I am not sure what you are asking.There's going to be an offset in the sensor's output based on atmospheric pressure but I don't think that's what you are asking.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 02, 2016, 04:53:59 am
content: At the bottom of the tank, the absolute pressure is equal to the atmospheric pressure plus the pressure from the height of the liquid.If you want to measure the height of the liquid, use a differential pressure measurement. Then the atmospheric pressure won't matter.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: February 02, 2016, 05:38:57 am
content: Yes,You need two pressure sensors . One in the bottom of the tank, the other outside for atmospheric pressure.You get the level of water with the differencePbottom - PoutFor a precise measure, you need first to calibrate your sensors so when they are bouth outside the diff is zero. Just record the diff at zero liquid level.

author: mikerj
author_rank: Super Contributor
timestamp: February 02, 2016, 09:24:04 am
content: Quote from: IconicPCB on February 02, 2016, 04:30:31 amNot quite sure what you are asking here, but without correcting for atmospheric pressure you will have an error of up to +-51.6cm in the measured fluid height, depending on the atmospheric pressure when you calibrated the system. A differential pressure sensor would be the best solution to minimise the error, otherwise a second sensor for atmospheric pressure could be used.

author: IconicPCB
author_rank: Super Contributor
timestamp: February 02, 2016, 12:59:44 pm
content: Thank You for confirming the error due to atmospheric variations.It has been my contention there would be a need for a differential measurement in a discussion with a potential client.This may call for a rethink.

author: dwpatter53
author_rank: Contributor
timestamp: March 02, 2016, 01:06:35 am
content: To measure accurately, level in a closed tank you need a differential pressure sensor.Connect the atmospheric port on the absolute pressure sesor to the top of the tank.LP connected to top of tank and HP connected to bottom.You therefore are only measuring the head from the liquid.You may have to fill the HP leg with liquid and zero it's head pressure out.

author: IconicPCB
author_rank: Super Contributor
timestamp: March 02, 2016, 03:21:16 am
content: Thanks Davo.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2016, 03:56:46 am
content: It seems odd to be "reinventing the wheel" like this. If the application is to sense the level in a tank full of liquid, the reliable and logical solution is to obtain a purpose designed level sensor from a qualified instrumentation vendor.Does the client know all the technical details about how to design and calibrate such a sensor for long term and reliable operation? Instrumentation vendors have the experience and application knowledge to do the job right.Another thread somewhere mentioned the Dunning-Kruger effect. That rather comes to mind here.

author: dwpatter53
author_rank: Contributor
timestamp: March 05, 2016, 01:57:13 am
content: It is re-inventing the wheel; but what ever.Your standard level transmitters available from any vendor like Rosemount (Emerson) or E&H would be;-ultrasonic-microwave-displacement-differential pressure-float-conductivityFor a closed tank or vessel under pressure dp is the most common.Microwave works if it can handle the temperature and pressure; versions that will almost handle 900# steam are available.http://www2.emersonprocess.com/en-us/brands/rosemount/pressure/pressure-transmitters/3051-pressure-transmitters/pages/index.aspxhttp://www.ca.endress.com/enGood luck

author: TheDirty
author_rank: Frequent Contributor
timestamp: March 05, 2016, 04:23:17 am
content: Getting on the liquid level sensing train here as I'd like some help with this too. I'd like to measure the level in my water tank in the trailer. I was planning on using this diff sensor:http://www.digikey.com/product-search/en?keywords=MP3V5010DPI know nothing about these pressure sensors and am not an EE. Just making sure this doesn't have any gotcha's that I'm not seeing. There will not be more than 1 meter of water in the tank so I'm assuming the 1.45psi working pressure is okay. The tank is open vented at the top so will not be pressurized. It should be as easy as plugging it into the drain hose at the bottom and measure the output. Is there anything I'm not seeing, do I need a higher working pressure?

author: retrolefty
author_rank: Super Contributor
timestamp: March 05, 2016, 07:39:02 am
content: Quote from: TheDirty on March 05, 2016, 04:23:17 amMany inexpensive pressure sensors are only rated for dry air service, not liquid service. Matching a pressure sensor to a given service/application is most a matter of chemical media compatibility and rated min/max temperature service. Industrial pressure sensors will have a solid metal barrier diaphragm, usually stainless steel, or other material for acid service, etc.

author: TheDirty
author_rank: Frequent Contributor
timestamp: March 06, 2016, 03:48:41 am
content: Quote from: retrolefty on March 05, 2016, 07:39:02 amThanks. In this case it is not rated for liquid use. Had to go deep into the datasheet to even see a mention of it.Looked for something else and I think the MPX5010DP looks okay. The datasheet specifies liquid use and has a specific application sheet for water level measurement in washing machines. I certainly want as cheap as I can get, this is not a precision measurement and I have time to calibrate it myself.

author: Brent C
author_rank: Newbie
timestamp: March 06, 2016, 04:19:29 am
content: There are several manufacturers of level transmitters similar to the ones shown in the links below.These transmitters use a ventilation tube that is ran within the cabling between the device and the atmosphere. This allows for a differential pressure measurement to be achieved.http://www.wika.ca/il_10_en_co.WIKASpec Sheet:http://www.wika.ca/upload/DS_PEIL10_en_us_16678.pdf

author: JimRemington
author_rank: Regular Contributor
timestamp: March 06, 2016, 05:04:49 am
content: For a small water tank, an external tube and a couple of strips of foil make a simple but accurate, DIY capacitive level sensor. It was described in Nuts-n-Volts a long time ago, and I marveled at its simplicity. Reprint herehttp://engineering.nyu.edu/mechatronics/Control_Lab/ME3484/Readings/NV27-Measuring%20Water%20Level.pdf

author: SeanB
author_rank: Super Contributor
timestamp: March 06, 2016, 07:08:00 am
content: If you want a cheap reasonably easy to use sensor that will do a 1m or so head then try a washing machine sensor. The modern ones are a 2 wire LC circuit that uses a CD4069 as an oscillator, giving a frequency proportional to level, and are mounted with a thin tube leading to the bottom of the tank so the air pressure in the tube is proportional to water depth. Gives a frequency output for feeding into a MCU pin which is then going to be used internally in a counter.

author: John Heath
author_rank: Frequent Contributor
timestamp: March 06, 2016, 10:22:58 am
content: Quote from: SeanB on March 06, 2016, 07:08:00 amI remember having a similar problem in a chemical plant trying to computerize storage tanks to prevent overfill. The problem was an inconsistency of the liquid weight so pressure sensors could not be relied on for 50% or 95 % full alarm. It ended up being brute force with 3 pressure senors at 50% , 90% and 100% full by drilling holes in the side of the tank. Not proud of it but hey it worked. The pressure difference caused by liquid is a hundred times greater than a weeny atmosphere difference so it was not given much thought. Point 0f interest is the computer was a 100 buss Z80 with a smoken 1 MHz clock rate and 16 K byte ram , you heard me right "K" as in 16 1000 bytes of memory with a CPM operating system no less. We could conncor the world with this kind of computer power.

author: retrolefty
author_rank: Super Contributor
timestamp: March 06, 2016, 05:16:21 pm
content: Quote from: John Heath on March 06, 2016, 10:22:58 amGood story. Takes more then simple pressure measurements to gauge liquid high level if the specific gravity of the contents is not known or changing. Most such facilities also required independent height measurement and high alarm level sensing."conncor the world" Is that something you all got from the French?

author: TheDirty
author_rank: Frequent Contributor
timestamp: March 06, 2016, 07:38:00 pm
content: Quote from: JimRemington on March 06, 2016, 05:04:49 amThis is interesting and I'll give this a try. I can just T off my drain hose, and run a vertical hose up beside the tank.

author: dwpatter53
author_rank: Contributor
timestamp: March 07, 2016, 04:10:03 am
content: Quote from: TheDirty on March 05, 2016, 04:23:17 amThe datasheet shows a silicone seal on the pressure sensor so even though its mostly for gas service clean water is probably OK.If you are running at full scale with the tank full, doesn't matter.You want to know when it's getting empty.Max pressure is 75 kPa (10.9 psi) well above your working pressure.Looks goodGood luck

author: TheDirty
author_rank: Frequent Contributor
timestamp: March 07, 2016, 08:24:20 pm
content: Quote from: dwpatter53 on March 07, 2016, 04:10:03 amThanks for checking this. With a sealed tube and a u-bend I could make an air pocket and measure that air pressure. Not certain how much better that is because of the humidity, but as you said, there is a silicon seal. I have put the MPX5010 on my next Digikey order.I am going to try out the capacitive measurement method as well. An unsealed tube with copper tape up the sides.

author: meeder
author_rank: Regular Contributor
timestamp: March 07, 2016, 08:45:52 pm
content: Why would you use a absolute cell? A relative cell isn't affected by the atmospheric pressure and it is very easy to measure the level with it.There is however one thing to take into account and that is the density of the fluid. The pressure can be calculated by P=rho*g*h.You could also mount a guided radar probe in the tube.Differential pressure isn't necessary since the tank is open to the atmosphere, otherwise you would need a delta-p measurement.A decent pressure transmitter will have options to enter a linearization table so you can calculate the volume directly and scale that to the output.In my job as a service engineer I deal with applications like this almost on a daily basis.What kind of liquid is there in the tank and how big is the tank?

author: IconicPCB
author_rank: Super Contributor
timestamp: March 08, 2016, 01:48:12 am
content: Meeder,Can you suggest a sensor ?By the by a sensor which is exposed to the atmosphere will provide the differential pressure reading( between atmosphere and say water column plus atmosphere.

author: meeder
author_rank: Regular Contributor
timestamp: March 08, 2016, 10:49:25 pm
content: Quote from: IconicPCB on March 08, 2016, 01:48:12 amFor the exact sensor type you would need more information. I think it is wise to contact a suplier to get the exact type you need.What you call a differential sensor is what we call a relative pressure sensor.Basically there are three types of sensor.Relative, the side opposite to the process connection is open to the atmosphere.Absolute, the opposite side is closed off.Differential, this sensor typically has two process connections and is either connected through impuls lines connected directly to the process or capillaries with secondary membranes connected to the process.https://youtu.be/QrT8VWxdxwk

author: Circlotron
author_rank: Super Contributor
timestamp: March 11, 2016, 04:44:07 am
content: Quote from: SeanB on March 06, 2016, 07:08:00 amAnd don't remove and replace the tube to the sensor while there is water partway up the tank because this partway will then become the new zero. Result = Mrs Circlotron not happy (again) and bans all further experiments on washing machine.

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: March 11, 2016, 05:02:58 pm
content: I've got two 5000 gallon water tanks I pump well water into and initially considered using some kind of pressure sensor to sense the level of water in the tanks. I looked into all the options and finally settled on something completely different.I'm using an ultrasonic distance sensor mounted at the top of the tank to range the distance to the top of the water surface. I then use this distance measurement to calculate the amount of water in the tank using simple geometry.The sensor I'm using is this onehttp://www.maxbotix.com/Ultrasonic_Sensors/MB7589.htm, which is designed for harsh environments like the inside of these black plastic tanks in the summer (150F and 100% humidity).

author: meeder
author_rank: Regular Contributor
timestamp: March 11, 2016, 06:49:01 pm
content: Ultrasonic is indeed a suitable method for level measurement.Condensation could however be a issue at some times but that also aplies to radar measurements.There is so much to choose from when looking for level measurement: pressure, ultrasonic, radar, capacitive and even electromechanical (search for Silopilot for example).

author: babysitter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 07:24:57 pm
content: Differential pressure sensor won't work on a _closed_ tank system. Taking liquid out of it without refilling the air bubble on top kills the relation to the ambient atmosphere.

author: meeder
author_rank: Regular Contributor
timestamp: March 12, 2016, 07:43:31 pm
content: Quote from: babysitter on March 12, 2016, 07:24:57 pmEmptying the tank without venting will kill the tankI have seen a tank implosion first hand, a few millibars is enough to implode a tank.But for level measurement in a closed tank you need a differential pressure sensor, a single sensor in the bottom of the tank will not work in that case.If there is also a gas blanket on top of the liquid then the top mounted sensor (or impulse line or secondary membrane) is also used to compensate for the additional pressure.

author: babysitter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 07:50:00 pm
content: You can get vessels which are made to hold a vacuum. Its just no connection from top of tank to ambient - differential pressure from tank bottom to ambient not working.

author: IanB
author_rank: Super Contributor
timestamp: March 12, 2016, 08:21:00 pm
content: Quote from: babysitter on March 12, 2016, 07:50:00 pmYou can indeed get vessels that are made to hold a vacuum. But usually those vessels are not also used to hold a liquid. If the vessel is used to hold a liquid it will typically have some form of venting arrangement at the top to equalize the pressure (otherwise it is really hard to get the liquid in and out).

author: meeder
author_rank: Regular Contributor
timestamp: March 12, 2016, 08:23:43 pm
content: Quote from: babysitter on March 12, 2016, 07:50:00 pmAh, I get it now... You mean a relative pressure sensor which is indeed the differential to the ambient pressure but when you work with these sensors a differential pressure sensor has two process connections and a relative sensor has only one. Visually relative and absolute sensors are almost the same and a differential sensor looks distinctly different.But for a closed tank you would indeed need a absolute sensor.Some examples:Absolute and Relative/GaugeDifferential sensor:

author: dwpatter53
author_rank: Contributor
timestamp: March 13, 2016, 12:42:01 am
content: Quote from: TheDirty on March 07, 2016, 08:24:20 pmWhile industrial ultrasonic and radar level gauges are great and economical; they are $500-1500.There is an Arduino ultrasonic distance sensor that is only $C3.34http://www.banggood.com/HY-SRF05-Ultrasonic-Distance-Sensor-Module-Measuring-Sensor-Module-p-91444.htmlEven adding a PS and Arduino nano it will be an inexpensive level gauge.Cheers

author: retrolefty
author_rank: Super Contributor
timestamp: March 13, 2016, 12:58:23 am
content: QuoteI think you got that wrong.A 'closed tank' would be one that has some unknown liquid level and some unknown tank pressure. Such a tank could only use a differential pressure gauge with the taps located at bottom and top of tank. Or use two pressure sensors located top and bottom and calculate the difference.An absolute pressure sensor would only measure the total system + liquid level pressure but not either individual value.

author: dwpatter53
author_rank: Contributor
timestamp: March 13, 2016, 03:42:12 am
content: Quote from: babysitter on March 12, 2016, 07:24:57 pmOf course they will; you are talking system design where a closed tank will require a pressure blanket and/or PRV or vacuum relief valve and/or outflow matches inflow.The level measurement method does not determine the system design.dp level measurement is the most common method for level measurement in pressurized or closed vessels.

author: meeder
author_rank: Regular Contributor
timestamp: March 13, 2016, 11:21:09 am
content: Quote from: dwpatter53 on March 13, 2016, 03:42:12 amYes exactly.

author: meeder
author_rank: Regular Contributor
timestamp: March 13, 2016, 11:26:11 am
content: Quote from: retrolefty on March 13, 2016, 12:58:23 amYes absolutely. I forgot to mention that. When working with stuff like that on a daily basis it is easy to forget to mention the obvious things

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: March 14, 2016, 06:17:40 pm
content: Quote from: dwpatter53 on March 13, 2016, 12:42:01 amThe problem with that $3.34 sensor is that it won't last ten minutes in a harsh environment. The air about the water surface in my water tanks in the summer can get up to 150F (65C) and 100% relative humidity with water literally dripping off the sensor. You need something designed to handle that kind of environment, like the one I linked in my post above. It's considerably more expensive than that $3.34 sensor, but it's much less than $500-1500.