RH sensor

[Bill in Phoenix]

I need to get a Elektronic EE06 Series humidity sensor to work on my system. ( http://www.epulse.at/ )

I have a NI SCXI-1520 & SCXI-1314 Terminal Block. The wiring is follows:

   P-  = GND

   P+ = V+

   S+ = RH

   TB GND = cable shield

    not connected = Temperature

Measurement & Automation is setup for Voltage with Excitation as follows:

Max = 1 volt  ( EE06 output = 0 - 1V, with a range of 0 - 100% RH )

Min = 0  volt

Ex Source = Internal

Bridge Type = Half Bridge

Ex Value = 5

Terminal Configuration = Differential

Use Ex for Scaling = NOT checked

Custom Scaling = < no scale >

Auto Zero Mode = None

Low Pass Filter Cutoff... = Disable Filter

Aquisition Mode = Continuous

Samples to Read = 100

Rate (Hz) = 1000

Clock Type = Internal

Start Trigger Type = none

Reference Trigger Type = none

I have verified that the device is getting the excitation by using a volt meter to

measure P+ and P- lines for 5 volts. I changed the excitation to 10 volts

and verified that P+ and P- changed to 10 volts. I measured the voltage on S+

and verified it was between 0 and 1. I blew on the sensor and saw a change in

voltage on S+. I believe the sensor is working correctly.

When I run the test function in Measurement & Automation, I always get a reading

of -1.05 volts.

By the way, the LabVIEW version is 7.1. The Measurement & Automation version

is 3.1.0.3021

Do you have any idea of what I am doing incorrectly?

Thank you for any help you can give me !!!

William

[Logan_K]

William,

Your sensor's output looks like it needs to be referenced to it's ground line, so try this:

1) Connect S- and P- together. This will make sure that the reading on S+ is referenced to the ground of your sensor (which of course is referenced to P-).
2) Configure your Custom Voltage with Excitation channel with a bridge type of "Full Bridge". This is the only appropriate bridge type for sensors that aren't bridge-based - like yours.

I would also recommend enabling Auto-Zero mode to get rid of the small offset that will be created by the nulling circuitry. For maximum accuracy you should calibrate your sensor by setting it to output 0V, and then run the offset nulling function, but with this sensor that might be difficult.

Logan

[Bill in Phoenix]

Logan,

Thank you for the response !

I had tried that before. However, I did go back and try it again to verify that I had done that correctly. This did not change anything. I did check the RH voltage on S+ and S- again. The device is outputting 0.33 and 0.34 volts. This should correspond to 33 - 34% RH. I did hold the sensor in my hand and blow really hard on it. I did see the voltage jump upwards to .49 volts. So, I believe the sensor is functioning. I checked the online local weather and they claim that the RH for the area is 36%. I do not have another weather gauge in the lab to confirm the RH in the lab. Since it is close to the outside RH, it is probably correct or at least in the ball park. I do not think the company has any special air conditioning equipment to keep the RH in the lab at a fixed value, which might be too much of a difference from the outside air.

I might also mention that I have tried to move the terminal block to another 1520 and have used another port on the terminal block to try and eliminate the possibility the problem is with the 1520 or port. I do not believe it is the NI equipment. This is new equipment we just purchased.

My guess is that I have not properly terminated or connected the other signals in the terminal block ( QTR, RS+, RS-, SCA, SCB ) or that I have not setup Measurement & Automation correctly. A couple of other engineers at our location can not come up with an answer to the problem.

I am hoping that someone can educate us on how to solve this issue.

Thank you for any help.

[Logan_K]

This should work, so I will try to be more explicit.

I have attached a picture of how the sensor should be connected to the SCXI-1314 terminal block. No other lines should be connected on the terminal block, because that could distrupt the input reading on the channel. He is a list of the connections as well:

P-  = GND

P+ = V+

S+ = RH
S-  = GND

TB GND = cable shield (Do Not Make this Connection)

not connected = Temperature

The only change that you need to make to your software setup from the setup that you listed in your first post is:

Bridge Type = Full Bridge

Finally, if you use a 5V excitation, that will create a common mode voltage on the input of -2.5V (I don't think there is any way around this with the sensor your are using). I have computed that this will cause an error of around -2.5mV - based on the -60dB common mode rejection of the module. This shouldn't be enough to cause your signal to rail like it is, which is why I think if you get the hardware and software configuration setup correctly it should work.

My theory right now is that the cable shield of the sensor is probably connected internally to the ground of the humidity sensor, so when you connect the TB GND to the shield, you are referencing the ground line of the sensor the the chassis ground, which is also tied to the P- line, which disrupts the readings.

Let me know if this works. If it doesn't I would like to see some screenshots of your setup and the readings that you are getting.

Logan

[Bill in Phoenix]

Logan,

Thank you for all of your help !

I had already done and/or tried everything you recommended. I have been trying to determine exactly where the problem is. I eventually got this working on another SCXI chassis. The wiring that you indicated does work on the other chassis. My debug session is long and involved, so I will submit another request for help dealing with what I believe is the real problem that I am seeing.

Again, thank you for your help.

Bill

[Ryan_V[DE]]

Bill,

I saw your post on the PXI forum. If no one is able to help you there, I do have a couple questions about your configuration. Are your 1520s configured for multiplexed or parallel mode? Also, What happens if the both PXI systems are configured, but only one SCXI setup? Does it work then? The behavior you are describing is very bizzare - I'm not really sure what could be causing it. Please let us know.

[Bill in Phoenix]

On the Wednesday morning before Thanksgiving, the system would not recognize the last SCXI. I wiggled the cable used to daisy chain the 2 SCXIs together. After this, everything worked as expected. Since we have multiple labs at our facility, I was not sure if some other group might have shut off some equipment for the Thanksgiving holiday that was causing problems for our equipment ( I'm old enough to have experienced such things ). Anyway, I waited until today to see if that was a possibility. Since it was still working this morning, I wiggled the cable again and lost connection to the SCXI. My assumption is that there is a problem with the cable which manifest itself in the problems I was seeing.

I want to thank everyone who took a few moments to give me some advice on possible solutions.