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Permanent Downhole Gauges: Analog vs. Digital

October 30, 2013 at 2:05 PM

Downhole Piezo-Resistive Pressure Tools

Written by: Jeff Scott

I work for a technology company that manufactures downhole instrumentation pressure sensors which are sold in several markets across the world.  We sell both analog 4-20 mA downhole pressure tools and several types of digital tools.  Both analog tools and digital tools utilize the same type of piezo-resistive transducer, which is often called sapphire or crystal type of sensor.

The only difference is the form of telemetry communication, which sends bottomhole data up to the surface. Our analog tool sends a 4-20 mA current signal to the surface, while the digital tool transmit back a digital modulation that is decoded by a surface receiver. 

I am often asked by clients, what are the pros and cons of each type of tool. The answers are surprising to many.

Differentiators

Analog 4-20 mA Tool

 

Digital Downhole Tool

Outputs

Analog tools typically only measure one thing¹. In this case, our downhole tool only measures pressure.

 

Digital tools are capable of transmitting allot more information. Our standard digital tools measure pressure, temperature, vibration, and diagnostic information.

Multiple digital tools can be connected to one single conductor downhole cable.

ADVANTAGE DIGITAL

Price

Analog tools are typically more cost effective.

ADVANTAGE ANALOG

 

Digital pressure tools are more expensive to manufacture, sold in lesser quantities and command a higher price in the market.

Surface Electronic Requirements

The 4-20 mA standard is universal. Our analog tool is ready to connect to almost any VFD, PLC, SCADA Input or DCS System directly.

ADVANTAGE ANALOG

 

Digital tools typically require a surface interface to decode the telemetry, which adds to the cost.

 

Accuracy

Analog tools can be calibrated to be as accurate as digital tools. Often they are not, in order to keep costs down.

TIE – The customer must decide what accuracy is needed for the test.

 

Digital tools are usually very accurate.

Pressure accuracy is typically +/- 0.05% full scale.

TIE – The customer must decide what accuracy is needed for the test.

Resolution

The resolution of the analog tool is solely determined by the input of the surface device it is being connected to.  This is known as A to D (analog to digital conversion).

The quality of an A-D is measured in bits. High quality inputs are 14-18 bit, whereas lower quality ones are 10-12 bits.

 

The beauty of digital is that once a digital tool has calculated a value it remains in digital quality, which is usually 24 bit resolution.

The optimum pressure resolution of an analog gauge calibrated to 1,000 psi being read by a 10 bit A-D input is about +/- 0.97 psi, whereas a 16 bit A-D input is about 0.02 psi whereas 24 bit would be 0.00006 psi, which far exceeds the physical limitations of the diaphragm that protects the transducer.

ADVANTAGE DIGITAL

Troubleshooting & Reliability

If an analog tool fails for any reason, it can generate false readings. For example if fluid invades the downhole cable, the resistance will lower, causing an increase to the current output.

TIE

 

Digital tools either work or they don’t. Binary data is either received or it isn’t. 

The offset is that there is more that go wrong with a downhole digital tool. 

TIE

Conclusion

There is no right or wrong. The important factors are to understand the test objectives and balance the requirements against your budget. I recommend considering both technologies for your next downhole instrumentation project.

¹Many modern analog tools are equipped with the HART protocol, which is a frequency based telemetry that overrides the 4-20 mA current and provides additional reading and diagnostic information.



Tags: Oil & Gas Digital vs. Analog Downhole Pressure Tools Piezo-Resistive Tools Oil Production Tools Oil Production Techniques
Category: Technology

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