digital pressure gauge
digital pressure gauge
(OP)
Hello All!
I am a mechanical engineer, and i have a project, which requires to "implant" a digital pressure gauge in it. Standard gauges can not be used because of the size, and can not be protected. The gauge not need to be highly accurate (+/- 1% full scale 0-150 psi (0-10 bar)), but need to work from -40 to +150 deg F. I have tried to contact gauge manufacturers, but the quantity is too low for them to get involved. All the mechanical part of the gauge i can make it, only with the electronics will need some advise. I found pressure sensors for this temp. range, i need 3 or 3 1/2 digit,3/8 to 1/2 inch high character lcd modules, also how to drive it with the sensor's output (can be mA or mV). The unit is in a remote place, no power supply is available, so must use battery. If anybody can help with advice, schematics, links will be greatly appreciated.
Thanks in advance
I am a mechanical engineer, and i have a project, which requires to "implant" a digital pressure gauge in it. Standard gauges can not be used because of the size, and can not be protected. The gauge not need to be highly accurate (+/- 1% full scale 0-150 psi (0-10 bar)), but need to work from -40 to +150 deg F. I have tried to contact gauge manufacturers, but the quantity is too low for them to get involved. All the mechanical part of the gauge i can make it, only with the electronics will need some advise. I found pressure sensors for this temp. range, i need 3 or 3 1/2 digit,3/8 to 1/2 inch high character lcd modules, also how to drive it with the sensor's output (can be mA or mV). The unit is in a remote place, no power supply is available, so must use battery. If anybody can help with advice, schematics, links will be greatly appreciated.
Thanks in advance





RE: digital pressure gauge
http://www.intersil.com/design/parametric/partsearch.asp?txtprodnr=ICL7106
This is powered from a 9V battery and will drive LCDs directly but it's only rated at 0-70 deg C.
Hope that's of some help
RE: digital pressure gauge
The entire unit will have to support that temperature range. Will be built in like a replaceable module.
RE: digital pressure gauge
OK so you need the whole thing to work over the full temperature range. The +150F is not so bad as that's about +65C and is within the "commercial temp" range (0-70C)of the chip I suggested. The -40F is more of a problem as it's outside the range of the chip, at least in theory.
It is possible to run some commercial grade chips lower than 0 deg C but of course the manufacturer won't guarantee at that temperature. You could try it, but the results could be unpredictable.
The other problem at -40F is likely to be the LCDs themselves, as ordinary grade ones don't like very low temperatures - the "liquid" in liquid crystal can freeze!
If you really need this sort of low temperature then you will need at least an "industrial" grade A/D measurement chip (-40 to +85 deg C) or more likely a full "military" temperature range (-55 to +125 deg C). At this temperature LCD readouts are likely to be very difficult to find.
Although LCDs were an ideal first choice for your application - because of the need for battery power and working from a simple 9V battery - can you consider LED readout? These have no problem with low temperature but are very power hungry. How about having the displays unpowered until you press a push-to-read switch? Alternatively you may have to consider bigger battteries. What sort of batteries are feasible for you - i.e. primary cells only, or removable rechargeables ?
I will try and find some industrial or military A/D measurement chip types (and possibly LCDs) for you.
Regards,
Brian
RE: digital pressure gauge
I looked up some lcd manufacturers, and they have the option for a different liquid, (wide temperature range) wich are in my temp. specs. Also searched for A/D measurement chip, with no success. What You think about using an analog to frequency chip + a counter chip? I found some in industrial package.
The space is important, no room for large batteries in the unit, so the led display is out of question.
Gyuri
RE: digital pressure gauge
Well done for finding low temperature rated LCDs. I am not haveing any success at the moment with the A-D converter chip. For simplicity the "3 1/2 digit DVM" type would be the best as it will drive the LCDs directly. Unfortunatley the only wide temperature types I have found so far do not have decoded decimal outputs to drive displays but have parallel outputs for interfacing with computer chips. Not impossible to use, but it makes things more complicated as without a microprocessor chip or computer it will need several other chips to make it work.
To answer you question about the V-F converter and counter, yes you could use this approach, but again, you will need some other logic chips to make the whole thing function. Also, the overall accuarcy is not likely to be as good as with a purpose-built A-D chip with its own reference.
Noted your comment about large batteries, but what about my comment of "press to read" for LEDs to save power? Do you need continuous readout without touching the unit?
I will keep looking!
RE: digital pressure gauge
RE: digital pressure gauge
the best low temperature A-D device so far seems to be the MAX138EPL from Maxim. The E suffix part (various package shape and pinout options)is rated at -40 to +85 deg C and will drive LCDs directly, however note that it works from 5V, not a 9V battery. It's a 3 1/2 digit device with input full scale = 200mV, so maybe your pressure transducer might need an amplifier.
Hope that's useful.
RE: digital pressure gauge
http://www.electronic-engineering.ch/microchip/projects/alti/docs/SensorSelectorGuide.pdf
for:
MPX5999D
and application notes
RE: digital pressure gauge