Manuelles Auslesen von Thermoelementen

[1Johanna1]

Hallo zusammen,

ich beschäftige mich erst seit kurzem mit Labview. Ich arbeite mit einen veralteten Thermoelementtyp L, der nicht von DAQmx unterstützt wird. Ich versuche deshalb die Spannung der Thermoelemente und die der Kaltstelle manuell einzulesen und in die Temperatur umzurechnen. Mein Problem dabei ist, dass ich bei allen Thermoelementen eine Spannung von Null erhalte. Meine Frage ist, wie ich die Spannung der Thermoelemente abgreifen kann. Bisher habe ich es mit einem einfachen Spannungstask versucht. Wenn ich den automatischen Nullabgleich aktiviere erhalte ich einen zu großen Wert.

Mein Messaufbau ist folgender:

SCXI 1300 Terminal Blocks

SCXI 1100 Module

SCXI 1001 Chassis

PCI-MIO-16-E4 DAQ-Board

Danke für eure Hilfe!

[GerdW]

Hi Johanna,

you should ask in English in this English LabVIEW board…

Your DAQ hardware is also quite old (as is your thermocouple).

The SCXI1100 is meant for generic voltage measurements, whereas the SCXI1102 should be used for thermocouples. Does your SCXI1300 block contain a CJC sensor?

Mein Problem dabei ist, dass ich bei allen Thermoelementen eine Spannung von Null erhalte.

You are measuring just zero? Really? No noise?

Which measurement range do you use? Which settings?

Meine Frage ist, wie ich die Spannung der Thermoelemente abgreifen kann.

Buy a standard Type-K thermocouple, connect it on a AI channel and place it near the CJC sensor. Then measure the CJC temperature using a DAQmx task configured to read a TC-K sensor…

To read your type-L sensor setup a DAQmx task reading voltages at the smallest measurement range…

[1Johanna1]

Hello Gerd,

thanks for your reply!

Yes, it's a quite old plant which I have to put into operation. My SCXI-1300 blocks contain CJC sensors and I get good values for the CJ temperature by adressing the _ctemp.

I want to measure temperatures between 0°C and 80°C which refer to a voltage range from 0mV to 4,1mV at reference temperature 0°C. I set the range from -5V to 5V.

Thermocouple type L is really similar to typ J. So I tried to read the temperature by assuming type J which is supported by DAQmx. With the thermocouple task for type J my measurement is working correctly that got me confused. For the voltage I measure just zero without any noise. Did I forget any settings?

[GerdW · Accepted solution]

Hi Johanna,

why do you use a ±5V range when you want to measure voltages below 5mV?

Your SCXI allows much smaller ranges due to its PGIA! Use the smallest range possible!

According to the manual you should be able to set a range of 50mV or 100mV…

So I tried to read the temperature by assuming type J which is supported by DAQmx. With the thermocouple task for type J my measurement is working correctly that got me confused.

Why don't you use this measurement and apply a correctional polynomial to convert from Type-J to Type-L?

it's a quite old plant which I have to put into operation.

I hope you send all your (old!) DAQ devices for a calibration!?

Otherwise all your work might be quite senseless…

[1Johanna1]

Hello Gerd,

thank you for your help! With the samll range it's working!!

To convert from Type-J to Type-L is our contingency plan in case this idea does not work.

The calibration will be our next step. We first wanted to check if the measurement still work.

Nevertheless, I have one more question you may know the answer: Do I have to switch on the auto zero for my device (see picture below)?

[GerdW]

Hi Johanna,

I don't know.

I guess the answer is in the manual of the SCXI system/cards…

[1Johanna1]

Hi Gerd,

thanks anyway!

Best regards,

Johanna