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Permanent Downhole Gauges: Analog vs. Digital

Downhole Piezo-Resistive Pressure Tools

At GEO PSI we manufacture downhole instrumentation sensors which are sold in many different 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 a 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 transmits 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.

Outputs

Analog 4-20mA Gauge

Digital Downhole Tool

Analog tools typically only measure one thing, often pressure. In this case, we have an analog tool which is able to measure both pressure and temperature. Even still, the analog gauges are limited in the amount of data they are able to transmit.

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 4-20mA Gauge

Digital Downhole Tool

Analog tools are less expensive to produce, and are typically more cost effective.
ADVANTAGE ANALOG

Digital pressure tools are more expensive to manufacture, and command a higher price in the market. While they used to be sold in lower volumes, their many advantages in accuracy have made them more popular in the market, and economies of scale is starting to bring the price down.

Surface Electronics Requirements

Analog 4-20mA Gauge

Digital Downhole Tool

The 4-20 mA standard is universal. Our analog tools are 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 complexity and cost of the complete system.

Accuracy

Analog 4-20mA Gauge

Digital Downhole Tool

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

Analog 4-20mA Gauge

Digital Downhole Tool

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

Analog 4-20mA Gauge

Digital Downhole Tool

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 – Both our analog and digital tools are built to provide industry leading reliability and durability

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 – Both our analog and digital tools are built to provide industry leading reliability and durability

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. To learn more about which tools may be right for your next project, feel free to contact one of our regional sales staff and they would be happy to help you find the perfect tool to suit your needs.

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