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'[PIC][EE] Pressure sensor ESD protection - HELP'
2010\02\17@100951 by Carl Denk

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I have a pressure sensor application, where I use Integrated pressure
sensors that have: ground, +5VDC, and signal wires. The output signal is
in the 0 - 5 VDC range. I have had several of these of 2 manufacturers
fail, don't know the cause, but thinking ESD might be a factor. The
sensors are Freescale MPX5700GP, and Measurement specialties
MSP-300-100-P-3-N-1. Failures have been both in circuit, and no
connected. The system has been in operation for 4 years with 3 failures.

Both varieties have a plastic weatherproof connector on the end of 2
feet pigtail cable. The pins are recessed so accidentaly touching of the
pins not likely, though a spark could jump the 1/16" recess. The
Measurement Specialties unit comes with a covered 3 conductor cable. The
Freescale unit is meant for board mounting. I solder wires to the
terminals with heat shrink over the pin/solder and the 3 wires which
then got to the connector as above. he sensor is then potted in a
epoxy/milled fiberglass material with a 1/4" brass close nipple for
pressure port and a hole at the ambient port (these are gauge not
absolute pressure units).

There are the recommended capacitors on board with the PIC, I'm thinking
more of protecting the loose sensor.

The usage is natural gas, 50 PDI. max., the gas has a water vapor near
freezing, and the water in the well is salty.

I could easily add some discrete components like MOV, Diode, etc. to
help the situation. What are suggestions.


2010\02\17@104136 by Alan B. Pearce

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>The usage is natural gas, 50 PDI. max., the gas has a water
>vapor near freezing, and the water in the well is salty.

Sounds to me like the gas movement in the pipe is producing high voltages
that then produce ESD type arcs.

The description on your sensor mounting, it sounds like there is no metallic
housing that could be grounded to trap ESD events due to the gas motion, or
weather conditions. I would be tempted to wrap the sensor in an aluminium
foil shroud, and use shielded cable to a suitable grounding point.

2010\02\17@112823 by Carl Denk

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Thanks for the reply
All the pipe in the area is steel, using 130 feet of 6" steel well
casing as the ground point. I would think the gas movement in the area
would not be an issue. After 3 feet of 1.25" pipe which goes
underground, the pipe does transition to PVC pipe. For the Measurement
specialties sensor, it has a stainless steel body that is threaded to
the steel pipe. There is no shield on the cable, just the 3 wires in a
sheath. The Freescale sensor is a plastic body, with pins sticking out.
The brass pipe screws into the steel pipe, but other than that, I have
not shielded the wire. The service area is very low electrical activity,
just a little cabinet heater and 2 heat trace on the pipes in a small
aluminum enclosure. The sensors are mounted to 1/4" steel or brass pipe
at least 6" long with a 1/4  turn valve.

The PIC and everything connected to it is isolated. Power is a wall wart
with a DC power connector at the aluminum box that the  PIC is in. The
box is hard bolted to the pipe. The PIC output is fiber optic 62.5/125  
cable.



Alan B. Pearce wrote:
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2010\02\17@163742 by YES NOPE9

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> I have a pressure sensor application, where I use Integrated pressure
> sensors .....The system has been in operation for 4 years with 3  
> failures.


#1 How expensive is the failure ?  That is .... how much does it cost  
to get the
application working again ?
#2 Are the only failures in the pressure sensors ?

Gus


2010\02\17@173609 by Carl Denk

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The Measurement Specialities sensors are $110, all I have to do is add
the connector. The Freescale sensors are $11 + $2 for brass nipple +
time to make the encapsulation and install the connectors. For the few
pieces, actual cost isn't so much of an issue, than reliability. Yes the
only failures are the sensors. I haven't done anything to the PIC board,
which has a PIC 18F1320. There is a Freescale MPX5010G sensor and a
LM335 temperature sensors also which have not been problems.

On the reliability, the PIC is >> fiberoptic RS-485 >> PLC that manages
a 12.5KW standby generator, switching the fuel supply between the
natural gas well, propane and gasoline depending what is available. If
the generator fails, my basement could fill with water when the sump
pump doesn't have power. That's expensive. Oh, and the PLC does have a
water level switch on the sump, and will send an E-mail to my cell phone
in case of high water level.

YES NOPE9 wrote:
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2010\02\17@181355 by Carl Denk

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I should have mentioned, I have at least 2 spares for each item, and I
am finishing documentation that should provide almost anyone with the
facilities

YES NOPE9 wrote:
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2010\02\19@155053 by YES NOPE9

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{Quote hidden}

Sorry...... I am not clear on this ..... is the Freescale sensor  
failing ?
Are you substituting for the Freescale ?   Exactly which sensors are
you having problems with ?
I was thinking that the encapsulation might case problems.  But of
course not if your substituted sensor model fails as well.

Gus

2010\02\19@165512 by Carl Denk

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First it was the 2 Freescale sensors failed say a year apart, then
thinking it was a particular model that was the problem, I bought 1
Measurement specialties sensor as a trial, and after a near a year ago,
and bought 2 more after a few months as spares. When one of the newer
sensors failed, I began to question if there was another common issue
causing the problem, and asked for help.

One item I turned up as ordering some new Freescale sensors. Several
years ago, there was only one (at least that's what I thought) model
sensor of this type/pressure available. Now there are 2. There is a
MPX5010G, newer MPX5010G1. The newer is commercial/industrial qualified,
and comes WITH LDFRM (What ever that is, I couldn't find).

As for the encapsulation, I had thought that as a possibility, adding
considerable to the bonded mass of the original case, with thermal
effects the concern. Protection from light physical damage, and moisture
is main concern. The 3 wires are soldered (probably a no lead) to the
terminals, carefully heat shrink individual wires (aircraft quality
teflon), and then heat shrink over the 3 wires to the connector. I have
opened the connector area, and no issues found there.

One possibility is, the ambient pressure port, and pressure ports could
not, both, be pointed downward to drain any liquid, and the application
could have a little moisture exposure. For the new encapsulation, I plan
to provide both ports in the same direction, and mount in a downward
direction to minimize liquid intrusion. I had questioned what the
preferred orientation of the Measurement Specialties sensor, never
received an answer. I mounted it horizontal, to minimize damage to the
cable which comes out the opposite end from the male threaded port. A
2nd issue, was I couldn't find an ambient port, and assumed the cable
sheath provided the port. Again Specialty didn't respond.

YES NOPE9 wrote:
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2010\02\19@173950 by Marcel Duchamp

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It sounds like you are not quite sure as to whether the failures are
caused by ESD or damage from incompatible media.

One type of solution that could make both cases better but is more
complex would be to use a bubbler system.  This prevents contact with
the media and can help to isolate the sensor electrically as well.

Basically, you hang a conduit (tygon tubing, or whatever) down into the
location to be measured.  The top goes to a 'T' fitting that splits out
to your pressure sensor and a pressure source.  The source can be any
supply of air pressure that is slightly higher than your full scale
pressure to be measured.  The air pressure line has a solenoid in it so
you can turn it on to purge the line, shut it off, wait a bit then take
a pressure measurement.  The air pressure in the tubing will bubble out
the bottom (hence the name bubbler).  When it turns off the system will
return to static pressure to be measured.

On 2/19/2010 1:55 PM, Carl Denk wrote:
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2010\02\19@175951 by Carl Denk

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Don't think a bubble is applicable. The natural gas well is a 1000 feet
deep, 6" diameter hole mainly in shale stone. The natural gas seeps out
of fissures and porosity of the shale. In general the hole is dry, but
there could be water near the bottom. The top 160 feet of hole are lined
with steel casing with a coupling and plug 3 feet above ground level. A
1.25" steel pipe is welded to the side of the casing, and there are some
valves, and a tee fitting for the pressure sensor, and a visual gauge.
There is no liquid to bubble. :)

Marcel Duchamp wrote:
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2010\02\19@222556 by YES NOPE9

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Isolating the pressure sensor may be a good idea.  Even if a bubble
won't work, perhaps a long tube that is pressurized at the top with
a inert gas that slowly leaks out the bottom.  There is probably a
way to simplify turning the inert gas reservoir on and off.

What pressure does the gas pressure sensor operate at ?
Gus


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2010\02\20@094743 by Carl Denk

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0 to 50 PSI. working pressure. Other than occasional opening  a valve,
pressure changes are very slow, 5 psi. in  day.

I need the sensor mounted to the pipe, tubing that could fail is not an
option.

Zero maintenance is a priority. I won't be around forever. The idea is
that with the troubleshooting/operations manual, a plain person with a
few wrenches and screwdrivers can maintain the equipment.

YES NOPE9 wrote:
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