digital pressure gauge

[gyuri]

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

 

[BrianG]

I'm not quite clear from your description whether it's just the pressure transducer or the whole instrument including the display which has to work from -40 to +150F, however assuming it does not include the display and converter a good starting point may be the Intersil ICL7106 31/2 digit A/D converter-display device used in battery powered DVMs etc. The link to the data sheet on their site is:

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

 

[gyuri]

Thanks BrianG
The entire unit will have to support that temperature range. Will be built in like a replaceable module.

 

[BrianG]

Hello again gyuri,
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

 

[gyuri]

Thanks again Brian!
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

 

[BrianG]

Hi gyuri,
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!

 

[buzzp]

If you don't want to design something then you could look at a display that accepts the mA output and turn power on to it once in a while (assuming don't need continuous display). Look at Love Controls or other DC powered stuff for a display (some run on lithium batteries-temp may be problem, unless removable device). There are several to choose from. Good luck.

 

[BrianG]

Hi again gyuri,
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.

 

[jbartos]

Suggestion: Check

http://www.electronic-engineering.ch/microchip/projects/alti/docs/SensorSelectorGuide.pdf

for:
MPX5999D
and application notes

 

[gyuri]

Thanks You all!