Ardunio and labview

[sanvish]

Hi,

I am trying for real time monitoring of temperature inside chamber, using Arudino IDE and marker hub link in Lab view 2014. Using the analog Read block.The Analog read block gives the out put in terms of voltage(0-5V) , but is there any option in marker hub to read the output in terms of current(4-20Ma) in Lab view latest version.

[GerdW]

Hi sanvish,

as the Arduino can only measure voltage signals you cannot "read" current signals.

But you can easily convert that 0-5V signal into your 4-20mA range once you know the specs of the sensor you use!

[sanvish]

Hi GredW,

Thanks for the reply,

In aurdino i am facing the problem of voltage drop in the output , which is in turn affecting my temperature output to 2 deg C variation.

Can you  please suggest ,is there any hardware with 4-20Ma output that can read in Lab view program and accessible to IOT platform.

Awaiting for your views.

[rolfk]

So you have an analog signal from your sensor that is 4-20mA, and an input that can measure 0-5V. The standard method to measure that is to let the 4-20mA signal flow through a 250 Ohm resistor and measure the voltage over this resistor.

A few things to be careful about:

- the power dissipated in the resistor can be up to 20mA * 5V = 100mW, so make sure the resistor is capable to dissipate at least double of that to avoid fast deterioration of the resistor.

- the accuracy of the resistor is directly responsible for the accuracy of your measurement. Sou your resistor should be a precision resistor with less than 1% tolerance, which means usually a metal film resistor or a calibrated wire wound resistor. Carbon resistors, while cheaper, are neither accurate enough nor have enough long term stability to be meaningful as a measurement resistor.

Also you haven't told us about the type of sensor you have and its scale. But 2 degree accuracy when measuring temperature with low cost ADCs as used in the Arduino aren't really considered very bad. There is a reason why high accuracy temperature measurement is typically a lot more expensive for the pure analog measurement part than what the entire Arduino costs.

[sanvish]

Hi Rolf , 

yes, its right the sensor output i have is 4-20ma . which i converted to 0-5V as its acceptable to the Arduino board. but this conversion is creating a drop in the voltage so , when I access the Arduino read in Lab view the voltage is converted to temperature which is my final output ,due to the voltage drop is affecting the final output by 2-3 deg C. As its for  continues control we need analog only and digital is not preferred in this case.

[rolfk]

Your language is very ambiguous. What voltage drop are you talking about? Either your sensor is capable of providing the 20mA signal or it is not. There is a simple Ohms law that translates the current into a voltage through the "shunt" resistor. If the fact that you load the sensor output with a 250 Ohm resistor changes the actual current of the sensor, then the sensor is faulty. It should be able to drive a 250 Ohm load without any significant change in its value, if it claims to be a 20mA output.

If your inaccuracy is however caused by the fact that you use an inaccurate resistor than that is an entirely different matter. You need of course to use the exact resistance value to calculate the current that corresponds to the measured voltage and from there calculate the temperature according to the characteristics of your sensor. That is basic measurement logic.

So if you used a 249 Ohm resistor, which is more common as it is a standard value in the E48 range of resistors, you should of course account for this difference by first calculating back to the original current according to the formula I = U / R, then from there calculate the temperature according to the sensor datasheet. If you need to be even more accurate you will need to measure the actual resistance with a high precision measurement device (your kitchen sink DMM is NOT accurate enough for that) and use that value in your calculation.

So what did you use to convert the 4 - 20mA signal into a 0-5V signal? A high accuracy signal converter or a simple resistor? And if a resistor, what specifications and accuracy?

Or what specification does your sensor have? What minimum (and maximum, as it of course has an internal voltage limit) load does it support?