newbie trouble setting up pressure sensor

[PezDeSpencer]

I am no expert on your pressure transducer, but did give a quick read through the manual. If everything seems to be functional at lower excitation voltages, I'm wondering if this note from the manual might apply...

"Significantly higher thermal transient errors occur if the excitation voltage exceeds 10 Vdc. For sensitive phase change studies, many users reduce the excitation to 5 Vdc or even 1 Vdc"

Perhaps because you are working with such low pressures currently the excitation voltage is too high, and the changes are not being registered due to some internal thermal transients. This is just a speculation, perhaps try to increase the pressure applied to test this theory?

Spencer R | NI

[boletus]

Thanks, I missed that note in the spec sheet but that could be it. Was thinking something along those lines too but figured since I can verify about +15mV with a handheld meter when the pressures is applied that the software should be seeing it too. Will check with the sensor manufacturer to see what they say, thanks.  

[THERMAN]

Hi boletus,
I am having a very similar issue with the same pressure transducer. Have you had any success in dealing with it?

Thanks

[boletus]

Hi Therman,

After working on this with National's tech support we did get to the bottom of it. They determined that my sensor must be exceeding the 'common mode voltage range' of the 9237 when operating at 10v excitation. It was explained that probably the voltage between my AI-/EX+ or AI+/EX- was too high. The solution offered was therefore either use at lower excitation voltage, or use a different device such as the NI-9218. Not exactly what I had wanted since this was not made clear when we bought the 9237, but for now we've just moved forward with using lower excitation voltage. Hope that helps. 

[THERMAN]

Thanks, it was indeed the same issue exactly. Lowered the excitation voltage and now it works.

[boletus]

I would like to open this discussion up again as we are looking to buy more equipment and would like to try to solve this issue before committing to more 9237s.

One thing that I have figured out is that if I connect the excitation wires of the sensor directly to the power supply, essentially bypassing the 9237 and chassis altogether for excitation, I am then able to read pressure response at 10v. Can anyone help me understand what this means, and whether or not we risk or sacrifice anything by operating this way? I should note that I can verify that when the power supply is connected to the 9237 as intended I do get 10v at the sensor, I just don't get a signal. So I don't think it's simply a bad connection somewhere.

[Minions]

Most pressure sensors work on a strain gauge principle with one of the legs variable with respect to pressure [Yes, some are capacitive].  As long as the bridge is balanced at a known condition along with the voltage-to-pressure conversion then the pressure can be determined.  Additionally, some sensors are in environments with wide temperature variations and require temperature compensation as well to determine any changes to the variable resistive element.  The formula for temperature compensation should be provided by the sensor manufacturer.

[boletus]

Could you clarify? It's not clear to me how those points relate to my particular question but maybe I am misunderstanding.

What I am trying to determine is if it is okay for me to power the sensors directly from the power supply, rather than powering them through the module (by connecting the power supply to the external excitation connector of the 9237)? When I use external excitation through the module I'm not able to get a reading from the sensor at 10V, while if I power them directly, bypassing the module, I can get a reading.

Is there any problem I could have because the module does not have access to the excitation voltage? Is there some error this will introduce, or some function I won't be able to perform? Or some damage that I risk causing?

Or does the module only need the signal, and does not need the excitation at all? When I run the excitation through the module is it simply passing through, or is it used in some way to process the resulting signal? 

[Minions · Accepted solution]

The information was presented for a purpose.  You need to understand your equipment and what you are interfacing to that equipment.  You provided no information as to the sensor, so I was filling in that gap with generic information. 

You mentioned that the NI rep said it was an issue with the "Common-mode Voltage" yet the rating for this device is for 60V with respect to ground.  It sounds like you are using an external source to supply the excitation voltage.  It is possible for these supplies to exceed that 60Vdc Common-mode Voltage limit.  Most DC Supplies are floating with respect to ground and will attempt to drive both the + and - outputs equally.  That is why it will not work reading the excitation voltage.

However, if you tie the negative output of the supply to the ground (many older supplies have the three banana plugs on the front) causing it to drive the + output only, then the 10Vdc excitation will be within the Common-mode Voltage range. 

This is not an equipment issue, it is an implementation issue for not taking into account the specified limitations of the interfacing equipment.

[boletus]

The sensor information was already given earlier in the thread. The power supply has 3 plugs in the front, it is a Tekpower TP3005T and was approved by National before we got it. Here is the link. Are you saying that it is the power supply, or how we are using it, that is causing us not to be able to read when powering the the module?

http://tekpower.us/power-supply/tp3005t.html