Anyone with pressure sensor experience - Page 1
Forum section: electronics / beginners

author: rocketfire
author_rank: Contributor
timestamp: March 15, 2016, 10:02:40 am
content: Hi,I bought a cheap pressure sensor with a voltage output of 0.5 - 4.5v fromhttp://www.aliexpress.comsku258771. The problem is it wanders about more than I expected. Graph herehttp://i482.photobucket.com/albums/rr185/house132/millivolts.pngIt has a thermister glued to the sensor to check temperature and a pressure sensor nearby. There's a vague correlation with temperature but not much. I bought two and both do the same thing although I've not tested both at the same time as I've only one jig.Is this what's expected of $30 cheapie vs $150 on Digikey? Ive checked the ADC that's doing the measurements and it' spot on, it's the voltage output of the sensor that's wandering.Thanks for any insights.

author: Rerouter
author_rank: Super Contributor
timestamp: March 15, 2016, 11:03:47 am
content: Ok first off, your going to have to work out what your temperature coefficient curve is, grab a sensor at ambient pressure and heat it up, say with a hairdryer, log the reported pressure vs temperature and see if there is a strong correlation, if so then good you have removed that uncertainty,Next up buffer your signal line, I recognize this type if sensor its a piston and a resistance card inside, so don't go chasing tiny errors, the spec is +-1.5% Full scale, in other words 0.225 PSI,and you will be lucky to get any kind of repeatability in that spec, as this type has hysteresis on the way up and way down.What you really want to do this right is a half bridge or full bridge sensor, but yeah they don't come in a nice housing at $30, but there design removes the temperature and voltage influence, and tend to have a much greater repeatability being based on a strain gauge then a piston and wiper.

author: danadak
author_rank: Super Contributor
timestamp: March 15, 2016, 11:43:33 am
content: There is quite a few Honeywell sensors in the $ 30 range -http://www.digikey.com/en/product-highlight/h/honeywell-sensing-and-control/pressure-sensors?WT.srch=1&gclid=CPnZgojOwssCFdgGgQodCgoACQRegards, Dana.

author: rocketfire
author_rank: Contributor
timestamp: March 15, 2016, 12:32:34 pm
content: Rerouter, this graphs shows a temperature swing down to 6 degrees then up to 48 degrees with little change in the pressure readings. The sensor is surprising sensitive and can easily and quickly detect a change in water head of 1cm which is just 1/1000 of an atmosphere, the problem is it wanders for reasons I can't fathom or correct for in software, maybe it's detecting neutrinos or something.Dana, it's intended to be connected to a water pipe at the bottom of a tank so the sensor needs to be rated for sensing water pressure. I'm pretty sure those Honeywell sensor are air rated. I've considered pulling apart a cheap sphygmomanometer and using it as an air pump/bubbler down an air hose to the bottom of the tank then measuring the residual air pressure, that could use an air pressure sensor but the downside is moving parts in the pump.I think I'll have to splurge and buy the expensive suggested by Rerouter.

author: Rerouter
author_rank: Super Contributor
timestamp: March 15, 2016, 01:10:07 pm
content: Ah sorry, I should probably ask this, but i assume your measuring a tank level, how are you subtracting the ambient air pressure?

author: rocketfire
author_rank: Contributor
timestamp: March 15, 2016, 02:01:55 pm
content: Yep, the goal is to measure the tank volume. I believe the sensor is vented. The brown line on the graph is the raw pressure sensor output voltage (as measured by an awesome MCP3424), the blue line is from a separate barometric pressure sensor (HTU21D) and the red line is from a thermister glued to the side of the pressure sensor. If needed software could adjust for the ambient air pressure changes but so far the sensor readings drift more than the surrounding air pressure changes.

author: ade
author_rank: Unknown
timestamp: March 15, 2016, 03:52:54 pm
content: Can you measure from the top of the tank? If you have a partially submerged vertical pipe, with its top sealed off, then the air pressure on the top half of this pipe will vary with the water level. You can use air pressure sensors this way without any moving parts from a bubbler, and compare with with ambient pressure to detect water level changes.QuoteSlightly off-topic, but I'm sure you're using a different sensor? (HTU21D only measures temperature & humidity, not pressure). Just curious which pressure sensor you're using...

author: danadak
author_rank: Super Contributor
timestamp: March 16, 2016, 02:59:48 am
content: Honeywell offers a number of wet/dry transducers, some with SS diaphragms.Give them a call.They also absorbed Sensym which used to have grease/rubber filled sensors with siliconstrain bridge elements in them, also useful for water sensing.Regards, Dana.

author: jeremy
author_rank: Super Contributor
timestamp: March 16, 2016, 03:28:55 am
content: According to this:http://www.aliexpress.com/item/15psi-Pressure-Transducer-Or-Sender-For-Oil-Fuel-Oil-Gas-Air-Water/32489595593.htmlIt isn't rated for water. I don't suppose you have a datasheetI agree with Dana. I have used a bunch of honeywell sensors without issue.

author: rocketfire
author_rank: Contributor
timestamp: March 16, 2016, 08:10:39 am
content: Ade, you are right the HTU21D is humidity, I'm using a BMP085 for barometric pressure.Dana, I'll take the plunge and buy a name brand sensor. Just for fun I had a look at pressure sensors with RS485 interfaces; whoa starting price $1,500.Jeremy, the snippet of "not water rated" escaped me when I ordered the sensor but it was all over the place from the start.

author: electronz
author_rank: Newbie
timestamp: March 16, 2016, 06:26:45 pm
content: This may not be relevant at this point, but I'm currently working with these:http://store.openrov.com/products/openrov-imu-depth-module. They can be potted in epoxy to be waterproof, and that's what I intend to do with them.I'm still in the early stages of figuring out what these are capable of, but should know a lot more over the next week or 2. Let me know if you're interested in what I find.

author: joe 90
author_rank: Newbie
timestamp: March 19, 2016, 10:42:04 am
content: Why not use a MAP sensor off a dead car?Or a water level sensor off a washing machine.

author: gildasd
author_rank: Frequent Contributor
timestamp: March 19, 2016, 07:03:44 pm
content: Do you have a reference sensor and amplify the difference or are measuring absolute values?

author: w3amd
author_rank: Regular Contributor
timestamp: March 19, 2016, 11:15:50 pm
content: Quote from: rocketfire on March 15, 2016, 12:32:34 pm

author: ChunkyPastaSauce
author_rank: Unknown
timestamp: March 19, 2016, 11:32:17 pm
content: The output is likely ratiometric to the supply, so another thing to check for is voltage drift in the power regulation.

author: rocketfire
author_rank: Contributor
timestamp: March 21, 2016, 10:51:48 am
content: The power supply is very stable at 5.005 volts.The sensor has a 0.5v to 4.5v ratiometric metric output via a resister divider 30k/10k with the MCU ADC on the midpoint.My conclusion is the sensors are unreliable.

author: gildasd
author_rank: Frequent Contributor
timestamp: March 23, 2016, 07:35:47 pm
content: I think you are doing it wrong.When I was troubleshooting force sensors, I traced my problem to the reference.Before I used an unloaded sensor as a ref, I had the same wild swings as you simply by a 1°C heat change...The problem that you have is isolating the ref but letting it receive the same heat/thermal latency as your loaded sensor.