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How to Change Excitation current NI 9216

Hello,

the NI 9216 has a excitation curren of 1mA. I have to lower it to 0.3 mA to prevent self heating of a very small pt100. Is it possible to lower the current somehow? Maybe a series resistor and make an offset of the temperature measurement?  Or is the only solution to buy a current source like NI 9266?

 

thank you in advance!

kind regards

Robin

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Hi Robin, 
How significant is the self-heating effect? 
The NI9216 should measure temperature correctly with PT100, provided if there is sufficient heat dissipation area. 

Using the serial resistor should also help to solve the problem.

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so its possible to use a serial resisor before the pt100? I has to be like 200 ohm to reduce the current by 2/3.

Do i have to think about anything else?

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Hi Robin, 

First of all, I should mention if you attach a resistor you will start to measure something different. I mean your device will show a temperature change but it will not be a true temperature value. You will need to calibrate the device's indicated number. Your NI9216 is able to measure resistance from 0 to 400 Ohms. So attaching a resistor will decrease the temperature range. 

   If you attach a resistor in series, it will not decrease the current, it might reduce self-heating. But also it will cause a voltage drop so your NI device will measure different resistance. Overall resistance should not exceed 400 Ohms.

If you attach a resistor parallel to PT100, it will reduce current but you will have a voltage drop on your resistor. Take this into account, meaning you need to calibrate your sensor.

 

However, I think there is something strange with your PT100 if it heats from 1mA current. What type of connection are you using? 2,3,4 wire? Can you show the photo of your PT100 and NI9216 connection? 

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ok thank you very much.

I don' think a parallel resiostor is a good solution because the pt100 resistance is changing with temp and so the total resistence that will be measured too. I cant take a photo but it's a 4 wire pt100, dimensions 3,5x0,5x0.5mm (very small) and datasheet says recommended current of 0.3 mA.

i think i have to buy a current source from NI. Like the 9265. If i want to use that instead of the excitation current from the NI9216 do i have to change anything in the labview software?

And is it possible to use one current channel of the 9265 for five pt100? or do i have to buy a 8 channel current source?

 

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Hi Robin, 

indeed your sensor is small.

What does NI9216: it launches current and measures voltage then calculates the resistance. You can read how it is done. It may require either a sensitive Analog to Digital Converter either Wheatstone Bridge. Thus NI9265 wont be the only device that you need.  So I suggest the following:

 

 1. You can estimate the self-heating rate. If it is not happening instantly then you could try to make your measurement in a short time. I mean only during 1 second or shorter but periodically. You don't need to measure all the time. This could be done in Labview by controlling the acquisition rate. 

 

To become sure that self-heating is disturbing your measurement compare the temperature with another sensor like a thermocouple with a sensitive Analog to Digital converter or NI DAQ product that measures voltage. 

 

2. If you have decided to buy another current source, then you need to buy also(if you don't have it already) a device that can measure voltage. 

 

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Somehow I can't get the 9216 specifications 😞

However... from the manual it tells 400S/s aggregate ...  so chance is high that the frontend only applies current during a measurement while swapping the channels....

Mean: If you configure and read 4 channels, chance is high that the mean current for one channel is 250mA or less ...

This and selfheating can be investigated ... wait for thermal equility and start measurements ... do see a ramp up? is it correlated with the number of channels measured?

 

I don't know the internals of that device, however reading RTDs with a 24bit DAQ is usually made by a ratiometric measurement. An internal reference resistor is added in the current path and either you do two measurements with the same DAQ using the same (shorttime stable) reference voltage, or you use the voltagedrop on your ref resistor as a reference voltage for the DAQ...

So using an external current source (like a REF200) is no solution with a 9216. While you can do it (4W config, dummi resistor for 1mA exitation , 200µA source , you loose all specs in your measurement and read ~20 Ohm instead of ~100.

 

 

 

Greetings from Germany
Henrik

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