PCI-6250 only reads voltage, hmmmm?

[philj@digitalpersona.com]

I was under the impression, after speaking with NI, that the PCI-6250 interfacing with the SCB-68 terminal interface block could take input of 4-20mA from a differential input and be configured in NI-DAQmx to read the output from a humidity meter? 4mA = 0% RH, 20mA = 100% RH.

The Omega HX-15 Relative Humidity and Platinum 1K RTD temperature thermocouple outputs have a biased current power supply loop.
The output is non-referenced to ground and I have verified the output from the psu has no direct/indirect connection to earth ground from outlet.
In affect I will be contacting Omega technical support to clarify functionality.

I have opened NI Measurement & Automation Explorer - I created a new task in Data Neighborhood, configured Ai0 for temperature (thermocouple), configured Ai1 for current, have input my resistance of the total current loop resistance including 22 awg cable = 765 ohms.

#1. Will the PCI-6250 only accept voltage input, whereby I convert to current under new task for Ai0 and Ai1, using a precision resistor of __ ohms?

#2. Do I need to configure the SCB-68 with accessory power off, since the output of hygrometer/RTD is supplying a current loop?

#3. Do I need to configure the SCB-68 with accessory power on, to enable the temp sensor IC output?

#4. Do I source another humidity meter that does not have it's own current loop supply, and setup a bias resistor to read voltages on Ai0 and Ai1?

#5. Please don't call me Wang!

I have read the user manual, the associated readme.txt files, online tutorials, and I am still coming along rather slowly in configuring the testbed.
All replies and assistance is greatly appreciated, this is my first time configuring a NI DAQ card for datalogging.

Sincerely,
Phil Johnson
Hardware Technician
http://www.digitalpersona.com/

[Matt_A]

Hello Phil,

I have tried to answer your questions as best I can below:

#1. Yes, the SCB-68 will only accept voltage inputs. However, it has sockets for precision resistors that can be used for current measurements. You mentioned that you had viewed the documentation online, but I am not sure which documentation you were referring to. If you have not seen that document, I would recommend that you check it out. If you have seen that document, let me know which part is not clear and I will do my best to explain. Additionally, this method is also described on page 5-16 of the SCB-68 68-Pin Shielded Connector Block User Manual (page 61 of the PDF).

#2. & #3. When accessory power is enabled, I/O pin 8 is fused and is intended to be connected to +5V. This enables the CJC on the SCB-68 for use with your external thermocouple. There is more information about how to configure this option on page 4-2 of the SCB-68 68-Pin Shielded Connector Block User Manual (page 44 of the PDF). This should not have an affect on the current measurement as described in #1. So, I guess the answers to these questions are: #2. To my knowledge, it should not matter as long as you are not using ai0. and #3. Yes, if you are planning on using the onboard CJC with your thermocouple.

#4. While the SCB-68 does have some onboard signal conditioning options built in, it is still basically a connector block. It cannot provide excitation current for external devices.

#5. I can't help but ask, is there a story behind this? "Wang" is definitely a long stretch from Phil.

I hope this information is helpful. Please let me know if there's anything else I can do to help.

[Farside1]

Thanks Matt,
first let me say that while searching and reading forum posts on pci-6250 and scb-68,
that there was a NI product user that was having difficulty understanding that the 6250
only accepts voltage. People referred back stating that he kept asking the same ?.

As you can tell it's my first time working with setting up NI hardware and software.
I have read thru the user manuals and have looked at the pages you point to but
as I configure and run the self-test features I get no reading from Ai1.

We are looking at purchasing Labview Base System for Windows. I understand it's
an easier task to setup using TEDS library features for manufacturers hardware.

We have an Omega HX15 humidity/temp probe supplying it's own current loop,

and it's connected to terminal block as "Differential non-referenced".

New task configured in Measurement & Automation "Data Neighborhood".

My setup is SCB-68 connected to PCI-6250 and configured:

Ai0 > Omega RTD 1K platinum probe analog input into channel 0 as "RTD"

 - looks like it's taking test samples, but I am very much a beginner in SW

Ai1 > Omega HX15 Humidity probe analag input channel 1

 - no reading of samples when self test is run

I have tried setting switches on SCB-68 to temperature enabled,

with accessory power on and vice-versa. I will have to jumper

the internal ic temp sensor for my setup will I not?

I have a problem with the math, but could be I have to put a resistor

across the differential pairs and setup Ai0/1 to take 1-5 volt reading?

IF PCI-6250 accepts current readings, please help calculate loop resistance:

Rmax = (24 volt supply - 7 volts)/.02 amps

850 ohms? I have 22 awg wire on loop, should that total 865 ohms Rmax?

When I enter current max "20m" for 20 mA and 865 ohms I get error message,

and it is expecting a different ohm total. Math is my weakest conceptual subject.

Should I have to return the Omega probe and seek a probe that does not

supply it's own current loop. That seems like the answer to this problem.

Regards,

Phil Johnson

[Matt_A]

Hello Phil,

The internal CJC on the SCB-68 is connected by default, so you will not need to set any jumpers to enable it. Enabling the CJC is done through the switches described in the SCB-68 user manual. Specifically:

"For single-ended operation, connect referenced single-ended analog channel 0 to the temperature sensor by switching S5 to the up position. The signal is referenced to AIGND. Set the switches as shown in Figure 4-1 [in the user manual linked above] ... For differential operation, connect differential analog channel 0 to the temperature sensor by switching S5 and S4 to the up position, as shown in Figure 4-2 [also in the user manual linked above]."

To confirm that you have connected you signals correctly, I would recommend that you view the Connection Diagram tab in the Measurement & Automation Explorer (MAX). As described by the MAX help: "Use the Connection Diagram tool to view the required signal connections between your sensor and connector block. To use the tool, you must first specify an accessory for your device. Right-click your device under Devices and Interfaces and select Properties to specify an accessory." You can find the Connection Diagram tab by selecting your DAQmx Task in MAX and looking at the bottom of the configuration window.

The connection diagram should help you with your second task as well. You will see how to configure the precision resistor with your differential channel to read a current. Some considerations from the SCB-68 user manual are:
   
Selecting a Resistor
For best results when measuring current, you should choose a resistor that
has the following characteristics:
    • Low wattage of approximately 1/8 W
    • Precision of at least 5%
    • Temperature stability
    • Tolerance of 5%
    • 232 Ω (suggested)
    • AXL package (suggested)
    • Carbon or metal film (suggested)
If you use the resistor described above, you can convert a 20 mA current to
4.64 V by setting the device range to either (–5 to +5 V) or (0 to 5 V).

I hope this information clarifies things a little for you. Let me know if there are any concepts that need further explanation.