current output sensors or voltage output sensors and DAQ System selection ?

[Mohammed.Ashraf]

Dear All

i want to buy some pressure transducers , i found some of them returns a voltage signal and others return a current signal

i'm also having problems choosing the right DAQ solution from the point of view of these factors:

1 - Isolation type

2 - Current Input or Voltage input

3 - Sampling Rate

which is more sensitive to noise ? is it the current output or voltage output pressure tansducers ? , since the transducers will travel for meters ,

another question , should i use the differential mode or i can use the Single ended mode and still have good readings ?

what about the isolation type , if it is Channel-to-Earth isolation , will i suffer from cross talks between channels while acquiring from all of them (10 sensors ) ? or i have to go for the Channel-to-Channel isolation ?

 what about choosing a Voltage input DAQ and using a signal conditioning module (current converters and isolators) ? 

i know that i might be asking lots of questions in one thread , but i'm really in a hurry and i have to order the hardware very soon

thanks in advance

[Jason_D]

Mohammed,

I would encourage you to give your NI sales representative a call.  This is something they can help with and at least be another opinon from those that you get here.  If you do not know who this is I can get you in touch with them if you would like.

First the DAQ card (as this could impact the sensor we get) we have multiple options.  One thing you will want to consider is what format you want to use SCXI, PXI, PCI, or a compact option.  The examples linked are merely a couple of the many options.

In specific response to your questions:

1) I don't think noise should decide this as they should be comparable depending on the type / price you are looking for.  What do you mean travel for meters?

2) I would generally recommend differential.  If however your accuracy requirements are not as tight and/or you have a large number of sensors to measure you may consider SE.

3) For isolation what are your accuracy requirements?  Unless they are very tight this is probably going to make minimal difference.

4) I would say the biggest advantage of choosing a regular DAQ card with modules is reusability on other different projects is this something you need?  Also depends on format.

[Mohammed.Ashraf]

Dear jason

Thanks alot for your reply

i found that NI recommends the cDAQ Chassis with NI 9219 as a DAQ solution for measuring pressure

can i use an external analog Multiplexer with the NI 9219 or it is just for the E series ?

what i understood from your post that isolation type is not that much a factor regarding to noise , am i right ? , so what about the gorund loops that i might suffer from as a result from the isolation type ?

 i mean the Ch-to-Ch isolation will remove the ground loops and cross talks between channel. thus minimizing the noise superimposed on the sensor even during simulatenous acquisition from all channels or due to the distance , which might exceed 5 meters for example. and that's what i meant by traveling for several meters 

i'd go for the compact daq option

the regular DAQ that i might choose is the NI 6215 M series USB DAQ , and i'll buy Current converters and isolators for it .. do u think this is a good solution to measure pressure from a number of pressure sensors ? please advice

 Thanks in advance

Message Edited by Mohammed.Ashraf on 07-13-2009 05:29 PM

[Jason_D]

Mohammed,

I see no reason why you cannot use an external multiplexer keeping in mind that you are going to have to control it and account for the time it needs to switch and settle between channels.  You will then have to somehow correlate the timing between when the multiplexer switches and what the device is reading.  I am not exactly sure why you would want to do this as I really don't know any of the details of your program but I see no problem with it.

I am sorry I was not clear.  Isolation is a very important aspect of any setup, I was trying to emphasize that depending on the accuracy you need it may or may not be critical to your application.  I am biased in that I see to often customers make isolation their number 1 priority at the cost of many other attributes you have mentioned that can be equally important.  Either way the 9219 has channel-channel isolation and you will undoubtably be better off with it.

If you are looking for large number of sensors you may consider SCXI options.  The 6215 could work (once again I am not aware of how accurate you need to be) but is not specifically tailored towards this type of application like the 9219 or other cards are. 

In the end good is in the eye of the beholder.  With the information you have provided so far the 9219 sounds like a good fit.