HART Gas Sensor notes

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I have a GTD2000-Tx fixed gas sensor with an output of 4-20mA DC. How do I connect it directly to a raspberry pi 3? The power supply is 24 volts and is supplied externally.

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  • Could you explain what you have tried so far? – joan 4 hours ago
  • Ah let me see. So me, the fake engineer casually read the friendly manual and made a quick and dirty Rpi connection suggestion. WARNING: NO GUARANTEE NO NOTHING WON’T MELT DOWN OR BLOW UP. (1) Introduction: the standard output signal is DC 4~20mA current (not voltage output for noise free operation). It is usually connected to a fixed value resistor and can use a Hart meter to read manually. (2) To use Rpi to connect to this sensor to a ADC (Analog to Digital Converter) such as 10/12 bit SPI interface MCP3008/MCP3208. Rpi can then read the ADC output results and alert or output alarm signals. – tlfong01 4 hours ago
  • Don’t ask me what is a “Hart Meter”, because I never heard of it until 10 minute ago. Reference: (1) GTD2000-Tx Instruction Manual – GasTech gastech.com/sites/default/files/GTD2000TX-Manual.pdf (2) Notes: (a) Output format – DC 4~20mA sink/source current, (b) Section 5.3 Wiring Diagram Fig 7 Hart Meter. Good Luck, Good Health, and God Bless You! – tlfong01 4 hours ago
  • ERRATA and APOLOGIES – After my fake answer earlier, I six sensed that there is something wrong, or more precisely, I have done something very wrong. I did not know what it was, but I did worry that, as I used to say, there is something I don’t know that I don’t know. Long story short, I google HART protocol:(1) What is HART Protocol? youtube.com/watch?v=pXkun-PEiY0. I watched the YouTube, and I have decided to watch the 17 minute video again at least twice. – tlfong01 3 hours ago
  • This tutorial explains (1) What I did not know that I did not know, and (2) Why I was very totally wrong in suggesting something I did not know that I did not know. – tlfong01 3 hours ago
  • Now let me first point out why my idea and suggestion was all wrong in my above answer: (1) My out dated knowledge is based on Bell 103 analog FSK type analog signal, but the OP’s sensor is the new Bell 202 based HART protocol. (2) Bell 103 FSK is analog and one way, but HAR is (a) ANALOG + DIGITAL, (b) two way master slave, (c) point to point or multi-drop. For ignorant newbies like me, I would recommend to read the following introductory article:(1) Beginner’s guide to HART Communicators-HART Protocol drurylandetheatre.com/…. – tlfong01 20 mins ago
  • I was very wrong and foolish to suggest the OP to use the hobbyist’s toy 10 bit ADC which is rubbish. I was also very wrong to think that the HART communicator is just a powerful multi-meter. Now I know HART is digital and analog, master slave, multi-drop so I browsed and found one device a poor hobbyist like me can start learning HART to talk to the OP’s HART sensor is the following US$90 HART modem: / to continue, … – tlfong01 15 mins ago
  • (2) AliExpress USB Hart Modem WS232UP Hart Protocol Transmitter Hart Communicator 475 375 With 24VDC & Built-in Loop Resistor – US $90 aliexpress.com/item/… – tlfong01 15 mins ago
  • This cheapy USB HART modem comes with free HART application software to config the device. I guess it should not be too difficult to use Rpi UART serial to talk to the modem. There are many more articles for newbies to read to get started, including the following: (3) Highway Addressable Remote Transducer Protocol – Wiki en.wikipedia.org/wiki/… (4) What is the HART protocol? – Fluke eu.flukecal.com/blog/what-hart-protocol (5) How HART Works fieldcommgroup.org/technologies/hart/hart-technology-detail. – tlfong01 10 mins ago
  • My lesson learnt: As I said, there are too many things I don’t know that I don’t know. So next time if I see any question with technical terms I never heard of, I MUST first google to make sure I my knowledge is up to date, or understand the basics before I attempt to answer. After reading the HART articles for newbies, I now understand HART is much related to IoT. I am glad that the OP’s question has led me to another new horizon of IoT. Cheers. – tlfong01 just now   Edit

Short answer is “you can’t” as the 4-20mA current loop is an analogue signal and the Pi does not have any direct analogue inputs. You can, however, add a device called an Analogue to Digital Converter (ADC) to your Pi that will let you then take analogue measurements.

Typically you’d “drop” the current loop output of you sensor across a 100 or 120 Ohm resistor and measure the voltage generated (V=I×R) with the ADC.


simulate this circuit – Schematic created using CircuitLab

Sometimes the sensor you’re using is “loop powered”, in which case you’d need to supply voltage in to one end of the loop and measure the output on the return path using your small resistor.


simulate this circuit


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