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I want to collect pressure readings using 5 analog pressure sensors powered by a battery and collected with a DAQ. My sensors require 10 - 36 VDC and draw 38 mA max with 4 - 20 mA output. My battery source must have at least 5x the current draw if my research is correct. So 5 x 38 mA = 190 mA. I'm thinking a 24 VDC battery to power all five sensors. I'll wire them in parallel so voltage can be constant. The manufacturer states that the sensors have a loop resistance of 0 - 1000 ohms and an Rmax = 50(Vps - 10) ohms. I don't know what "loop resistance" is. Would the max R of each sensor be 50(24V - 10V)? Which of these do I use to find the Rtotal?

The DAQ I'm looking at has a 3.6 V battery, a load resistance of 12 ohms, and will read 4 - 20 mA coming from the sensors. How will the DAQ's load resistance impact the breadboard wiring? Does the sum of the sensor currents have to be less than the operating current of the DAQ? Will this setup work properly without frying my DAQ/Sensors?

https://www.dwyer-inst.com/Product/Pressure/SinglePressure/Transmitters/Series626-628#specs

https://www.dataloggerinc.com/product/vl-dcc-hr-current-data-logger/

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  • \$\begingroup\$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. \$\endgroup\$
    – Community Bot
    Commented Oct 22, 2021 at 17:24
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    \$\begingroup\$ Edit the question to link to the sensor datasheet. That way we can verify your understanding is correct. \$\endgroup\$
    – Transistor
    Commented Oct 22, 2021 at 17:32

2 Answers 2

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Your DAC measures voltage, so you have to convert the 4-20mA using a resistor. You can wire each sensor to a dedicated input on your DAC and probably sample all at the same time or if you can sample them consecutively you can enable one at a time to help out your power requirements. Loop resistance is what the sensor uses to control the current going through the loop, which is 4 to 20 mA, so it's really the current you need to worry about.

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  • \$\begingroup\$ I'm pretty sure the DAC measures the current inputed by the sensors? They all have to sample at simultaneously. As long as all the sensors are powered with the excitation voltage they should still send the 4-20mA signal correct? or does the DAC's load resistance affect my wiring? \$\endgroup\$
    – Ryan W.
    Commented Oct 22, 2021 at 19:56
  • \$\begingroup\$ Actually we must be talking about the analog-to-digital converter, so ADC not DAC. \$\endgroup\$
    – pgibbons
    Commented Oct 28, 2021 at 17:15
  • \$\begingroup\$ Something supplies the current, and the sensor changes resistance to vary the current between 4 to 20 mA. Where you want to measure the current, you have to insert a small resistor into the loop. The sensor will compensate for this resistor because it only regulates the current but a voltage will be across the resistor for you to measure. \$\endgroup\$
    – pgibbons
    Commented Oct 28, 2021 at 17:17
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The assumption is your sensors are the -01 option (4-20mA) and this means they are 'loop powered'. For the sensor to operate correctly there needs to be a minimum voltage across it. According to the data sheet, the min voltage is 13V.

Assuming your data logger has its 12Ohm sense resistors going to 0V/common, then is it simply a matter of combining the 5 + wires of your sensors to +24V, the - of each sensor to the respective input of the datalogger and join the 0V of the 24V power supply to the 0V/common of your datalogger.

As for your 'breadboard' - what breadboard??

The datalogger's load resistance is low enough to ensure that the sensor will have enough voltage to operate.

Some fundamentals: current flows in a loop. Ohms law applies.

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  • \$\begingroup\$ I'm thinking I need a breadboard to power the sensors to the +24V since I'll be using a battery pack. I assumed they would need to be wired in parallel to have the same operating voltage. \$\endgroup\$
    – Ryan W.
    Commented Oct 25, 2021 at 11:50
  • \$\begingroup\$ your example refers to a 2-wire sensor. Would a 3-wire configuration work by having the +- wires going to the power supply and then a third wire carrying the 4-20mA signal to the datalogger inputs? Thank you for your response \$\endgroup\$
    – Ryan W.
    Commented Oct 25, 2021 at 12:01
  • \$\begingroup\$ What option are your sensors? The sensor manufacturer have apps engineer to assist you, i’d suggest you take advantage of that. I still fail to understand what a ‘breadboard’ would offer in this instance. Maybe its a language issue? \$\endgroup\$
    – Kartman
    Commented Oct 25, 2021 at 12:22
  • \$\begingroup\$ The breadboard would ensure that each sensor is getting the correct operating voltage by wiring them in parallel. Then the output to the datalogger would simply be the current signal from each one. This way I wouldn't use the common. Prior to your response I had no idea what a common even was, I'm an ME trying to absorb all of this, but you've been a great help thanks again. \$\endgroup\$
    – Ryan W.
    Commented Oct 25, 2021 at 13:15

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