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# Pressure offset across sensor2

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## Pressure offset across sensor

(OP)
I have a question about a pressure transducer reading that doesn't make any sense. I have a Pressure sensor soaked in water at 125c and 11,000 psi for 24 hours (See images). When bleed off, gauge pressure is 1,000 psi less than sensor pressure. Is there any way that atmospheric pressure cold be reduced enough to give a 600-1000 psi offset pressure that is greater than actual reference gauge pressure? Even if atm. chambers bled to full vacuum how could 14.7 psi increase in sensor pressure from say 5,000 psi to 5,014.7 psi yield a 1,000 psi difference with sensor pressure? Outgassing would create opposite problem as pressure in chamber would increase. Pics of Test Hoohup sketch and cross section of gauge with sensor mounted are attached. I am new to working with electrical components and am learning as I go...

There was a Doe Corning Dieleectric gel applied to wire bonds that is in a gelatin state. Don't see what is can do its so non-viscous and shows no signs of curing.

Sensor Span 2.7V mV/V +/- .2
Vsupply : 10V
Wheatstone bridge mounted on diaphragm of transducer.
Sensor Over pressure is 20,000 psi
Sensor Burst pressure is 58,000 psi
Sensor Rating: 10,000 psi (we exceed rating to serve as safety factor check for gauge and sensor)

y=370x
Ouput V P (psi)
1 370.3703704
5 1851.9
10 3703.703704
15 5555.555556
20 7407.407407
25 9259.259259
27 10000

Any thoughts would be much appreciated. Thanks

### RE: Pressure offset across sensor

Was the transducer specifically designed to be fully submerged at 11,000 psi? That would be unusual but possible. If not, the case could be deforming enough to change the stress on the diaphragm.

### RE: Pressure offset across sensor

Filename = Smartlog_11ksi_pressure_qualification_test_sketch.pdf

If it's a Pressure Qualification Test, then maybe that's a Test 'Fail'.

### RE: Pressure offset across sensor

(OP)
The pressure transducer is designed to be sumbmeged at the diaphragm and the flow wetted areas. The rest is sealed in atmposphric chamber.

I want to know how much the output changes at 14.7 psi. It is sealed and the pressure inside would have had to drop to give a output pressure 1,0000 psi greater All reference pressure (same as external pressure and pressure in diaphragm. I don’t think it would be much. I think that even though the proof pressure is 20ksi that going to 11ksi for 24 hours at 250F deformed the element on the diaphragm.
1 psi output is 370 psi. I see no way the atmospheric chamber could change pressure enough to cause the results we are seeing.

We calibrated again and are testing to 10ksi to see if the pressures stay close. That would suggest the diaphragm was plasticallu deformed.

Yes. The test is a failure but we are trying to understand why. Eventually I’ll cut the pressure transducer off and send to manufacturer for analysis.

Thanks for commenting.

Thanks,

War Man

### RE: Pressure offset across sensor

I'm more concerned with the temperature limits - I've never seen, much less used, a sensor that was rated for operation at 125 C. That's getting hot enough to see pretty rapid degradation of silicon microcircuitry. I'd expect to see the output shift with increasing temperature (outside the design compensated temperature limits of the transducer) as well, but it's not clear if that is what you are measuring (what temperature is the sensor when 1000 psi shift is measured?). Do you have a spec. sheet for the device you could share?

### RE: Pressure offset across sensor

#### Quote (warman)

The pressure transducer is designed to be sumbmeged at the diaphragm and the flow wetted areas. The rest is sealed in atmposphric chamber.

From your sketch, it looks like you submerged the entire transducer case, including leadwires. CPro's question was: is the transducer designed for that service? How is the atmospheric side sealed from the 11000 psi service, and how vented to atmospheric pressure?

### RE: Pressure offset across sensor

Strain gages, as in what probably comprises your Wheatstone bridge, are cranky devices.
If you've got an installation that's rated for 10k psi, and you ran it up to 11k psi, you may have induced a permanent offset. ... because the actual signal from a strain gage is very feeble, so it's normally sized to approach its yield point at its 'rated' extension.

You need to talk with your transducer manufacturer about what you have done, and what you intend to do next.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Pressure offset across sensor

(OP)
Here are some specs from the unconditioned element, pack and pack assembly. Uses 4 terminal Wheatstone bridge strain gauge with additional 5 th terminal for temperature output. Vsupply 10V. Electrical, thermal and pressure element ad pack tables at coir below.

Thanks,

War Man

### RE: Pressure offset across sensor

Ok, the long circuit board inside the housing - presumably this does some signal conditioning, or temperature compensation, or both? And the leadwire tube is also the atmoshperic pressure (gage pressure) vent path? This is somehow supported to prevent pressure loads from bearing on the sensor/circuit board assembly? Are the components used in that board made with the temperature design limits in mind, i.e. thermal drift/offset for all resistors and such are known? Is there any way to independently connect to the strain gage sensor element with known good components to check which section took damage, whether from thermal or pressure load effects?

As far as sealing goes - you could connect a vacuum pump to the vent, routing the flow through a LN2 trap prior to the vacuum pump, to collect condensate from the vented space, and analyse that condensate to see if you have any significant leakage.

Presumably the sensor was calibrated after the eb welds were made.

In the end, it looks like you are going to have to cut the device open to see what happened. Ain't engineering fun? ;)

### RE: Pressure offset across sensor

And one last stupid question, but was the pressure in the test chamber controlled/monitored during the heating and cooldown, i.e. there was no possibility that the chamber was locked off under pressure and then heated (allowing thermal expansion to create higher than intended pressure)?

### RE: Pressure offset across sensor

(OP)
See attached with pressure housing shown transparent.

I do not know if it was heated with pressure already applied or cooled before removing pressure. I do know the pressure as ramped up in 1000 psi increments to 10ksi. The there was power outage and stopped recording data. After it came back on the tech noticed the pressure had drifted up to 11,000 psi on sensor. He bled off and tried to re pressure and that is when the problem was discovered. Let me find the actual procedure. I just find it hard to believe a sensor with 20,000 psi proof pressure would deform at 11,000 psi even though it is 1,000 psi over the calibrated pressure.

Thanks,

War Man

### RE: Pressure offset across sensor

War man, it is apparent from the pictures that your pressure sensor is NOT designed to be operated in a high pressure environment. It is designed to be installed in an ambient pressure environment (14.7 psia +/-1)and to measure a pressure that is applied to the diaphragm though sealed tubing to a remote source of pressure. Whyever would you think that you can put the whole transducer into an 11,000 psi chamber without destroying it?

### RE: Pressure offset across sensor

(OP)
Procedure update" Test fixture was full of fluid and heated to 125C before pressure testing. Pressure was bleed back down with heat still applied at 125C. No thermocycles were conducted.

Thanks,

War Man

### RE: Pressure offset across sensor

(OP)
I guess I'm not explaining very good.

1) There was no pressure inside the electrical components.
2. The gauge is welded closed and has a metal to metal seal where the TEC wire comes in from the top and then the kemlon adapter has o-ring and the chassis has an o-ring, both are redundant.
3. The gauge is just full of air at atmospheric.
4. The only parts/surfaces that see pressure are the external tubes and the ID entry into the diaphragm where the SAE 7/16" Thread is located.

Refer to eDwg. It is .exe file yo can run without the software. You can rotate and spin the part and section it too. This should be clear when you open the sketch I drew with the gauge hanging off the TEC line in the test fixture located in my first thread.

Thanks,

War Man

### RE: Pressure offset across sensor

And the gauge was clearly not designed for that use. The diaphragm on an 11,000 psi transducer will be a tiny area because of the forces involved. The transducer casing has a surface area hundreds of times larger. The casing will deflect and deform the diaphragm due to mechanical loads for which it was not designed. Contact the manufacturer.

### RE: Pressure offset across sensor

(OP)
You need to explain why my design won’t handle 10,000 PSI. I need to see a visual representation of what you’re talking about. There are no moving parts. It’s just a wire connector with seal and pCBA board and the sensor packaged in a chassis with cylinder to separate the is of tool from external or internal pressure.

This tool is been qualified and has 70 successful installations and three failures. That occurred several months after the install. Of course the pressures are much lower.

The problem is that when we go to 11,000 PSI the sensor output pressure is erroneous once the pressure is held at 11,000 psi for an hour or more. The pressure drifts up to 1000 PSI higher than the reference gauge pressure. When pressure is bled off the sensor pressure also drops maintaining around 1,000’psi higher pressure. There are no mechanical failures, leaks or visible damage to the to the gauge. The problem is with the sensor strain gauge wire element. The psi enters inside the diaphragm and acts on a surface with the strain gage and wire bonds There is a compressive force due to hydraulic load
Induced by end areas of plugged cylinder. The location of the diaphragm prevents it from being loaded by anything other than pressure. It seems the question is why would the sensor read 1000 PSI more then the reference gauge and calibration pressure when the proof pressure is 20,000 PSI and burst is 52,000 psi. it seems the question is why would the sensor read 1000 PSI more then the reference gauge. Does the charactertics change when we exceed max output pressure? If yes, then the proof pressure is too high I have contaacted the manufacturer and they don’t believe the fault is with their tool. They need proof so I have to cut the EB welds off and desolder the sensor pins from the PCBA board and try and pull it out without damaging.

Before this I would also like to recalibrate the tool and then run external 24 hour soak test but at 10,0000 psi max to see if the drift and offset pressure disappear. Im tired of explaining this on my eye phone where I can hardly type or see.

Thanks,

War Man

### RE: Pressure offset across sensor

(OP)
btrueblood (Mechanical)13 Apr 18 17:54
Ok, the long circuit board inside the housing - presumably this does some signal conditioning, or temperature compensation, or both? And the leadwire tube is also the atmoshperic pressure (gage pressure) vent path? (Its TEC wire. Essentially control line with wire inside) nested This is somehow supported to prevent pressure loads from bearing on the sensor/circuit board assembly? Are the components used in that board made with the temperature design limits in mind, i.e. thermal drift/offset for all resistors and such are known? (Yes)Is there any way to independently connect to the strain gage sensor element with known good components to check which section took damage, whether from thermal or pressure load effects?(yes. Good idea.)

As far as sealing goes - you could connect a vacuum pump to the vent, routing the flow through a LN2 trap prior to the vacuum pump, to collect condensate from the vented space, and analyse that condensate to see if you have any significant leakage.

Presumably the sensor was calibrated after the eb welds were made. Yes

In the end, it looks like you are going to have to cut the device open to see what happened. Ain't
TRUE. That’s our job

Thanks,

War Man

### RE: Pressure offset across sensor

#### Quote:

There are no moving parts.

At 10,000 psi, _everything_ moves.

The transducer in the spec provided is intended to have pressure inside the lumen and against the distal face of the male threaded end, and of course against whatever diaphragm is present. It is clearly not intended to have pressure on the larger face adjacent the pipe threads, nor on the wrenching hex, or basically anywhere proximal to the pipe threads.

Is that actually the transducer you are using, or are you using something more specialized, not yet revealed in detail?

Just curious about this: the data sheet provided as photographs has an inked arrow pointing to the 6000psi transducer, not the 10,000psi transducer.

The transducer documented with a drawing could be used as intended within a machined capsule having a pipe thread through the blind end the 'wrong' way (provide wrenching clearance around the transducer and a long thin socket wrench for assembly), with the other end closed by screwing it into the end bulkhead or an assortment of couplings and nipples to do the same thing.

Most of us would probably just put a gland in the end bulkhead, stick a tube through it, and leave the transducer completely outside the pressure containment, unless there is some unrevealed reason why you want the sensor and your electronics all within the pressure containment.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Pressure offset across sensor

(OP)
Look at cross section of Pressure transducer. The Top Hat looking device is EB welded to end of hole. There is a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. Its based on piezoelectric technology .Uses piezoresistive effect of bonded strain gauges to detect strain due to applied pressure, resistance increasing as pressure deforms the material. The strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and to reduce sensitivity to errors.

The pipe thread is there for two reasons
1) to thread gauge into mandrel that is ported to ID so when it is made up to tubing string it measures tubing pressure and not casing pressure (produced fluid). The pressure across the thread for our test was balanced so there was 0 differential. the threads have nothing to do with this test. Only real life where it mst keep tubing pressure from leaking into casing.
2) To connect SAE 7/16" UNF w/37 degree flare fitting into gauge for pressurizing diaphragm during bench test with a hand pump (enerpro hand pump to 10ksi) and to thread into mandrel on tubing string
3. The device is run in on a short specialized piece of tubing with and offset port welded to OD so the gage is attached. The port diverts to the tubing ID while the TEC control line is attached and spooled out back to surface to data aq. The short tubing is threaded with OTCG pipe threads and RIH to depth to Gage will be set. So the outside of gage will see casing pressure. Generally conditioned fluid to prevent corrosion and weighted to prevent blowout should lick occur they can reverse the fluid. The pressure most likely tells the pumping unit when to come on and shut off based on fluid level in tubing.

copied bits from web page to explain...
[u]Explanation of Wheatstone bridge mounted piezo-resistors and diaphragm[/
nearly identical piezoresistors buried in the surface of a thin silicon diaphragm.
A pressure or force causes the thin diaphragm to flex, inducing a stress or strain in the diaphragm and also in the buried resistors. The resistor values will change depending on the amount of strain they undergo, which depends on the amount of pressure or force applied to the diaphragm. Therefore, a change in pressure (mechanical input) is converted to a change in resistance (electrical output). The sensing element converts (transduces) energy from one form to another.
All four resistors will change by approximately the same value. Note that two resistors increase and two decrease depending on their orientation with respect to the crystalline direction of the silicon material.
The signal voltage generated by the full bridge arrangement is proportional to the amount of supply voltage (Vcc) and the amount of pressure or force applied which generates the resistance change. The diagram is EB welded to end of Pressure Transducer Body on CL with hole so pressure can be measured. This is the only location fluids enter the tool.

I don't know the application for 10ksi very well but it mst be deeper secondary recovery wells or something completely different. A large order was placed and we have these on hold until we figure out what happened and correct.

Thanks,

War Man

### RE: Pressure offset across sensor

5,000 psi to 5,014.7 psi yield a 1,000 psi difference with sensor pressure?
Could be a calibration error, where your reading instrument is not set to the sensor's correct sensitivity in mV/V.
Could also mean a damaged diaphragm due to over pressure.

PS. Let me know if you want a unit where pressure is unaffected by temperature drift.

### RE: Pressure offset across sensor

People are pointing out that the application of the sensor element appears to be wrong. You keep saying it can't be wrong and that it's properly designed. But, if that was the case then it'd be working correctly, wouldn't it?

I tend to agree that the sensor is designed to have pressure applied to the port only. If not that, then investigate heat soaking of some component next. Test at ambient temperature to see what happens.

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