Pressure sensor Readings

[JohnS (STUDENT)]

Hi

I'm currently measuring pressure readings but the program I've created is not giving me readings im expecting and I cannot find the problem so im thinking it may be the problem. I tested the initial open system reading and found an accurate reading for atmospheric pressure (pressure sensor is 0.5v to 4.5v). However when i run the program it goes to the maximum allowed pressure before the Max pressure is found. The readings is around 960kPa. I am wondering if I need a different amplification factor or scalar factor?

also if someone could let me know how to make an array with the write to spreadsheet program that includes a time (seconds from 0seconds ongoing) that would be very much appreciated.

Thanks

John

[James_McN]

Hi John,

I wonder if you could explain the readings you are getting a little more.  When you start the program is it going to the maximum scale of the sensor or the DAQ card (it would be 10V if it is the DAQ card)?  Am I right in understanding that it goes full scale and then settles to the correct pressure?

In terms of writing to the spreadsheet I believe if you use the Write to Measurement File express VI (express>>output) and wire directly to it rather than converting to an array.  This should then embed a start time and an offset time from this for each reading.

Regards,

[aeastet]

It sounds like to me that you are either not reading the correct channel or you did not connect your pressure sensor properly. What output do you have for your sensor (i.e. 0-20mV, 4-20mv, 3 mM/v) . It would be very unusual to get a sensor that has a 0.5 to 4.5 Volt output. I am suspectiung that you have a 4-20mV sensor and you are going to need to do the voltage conversion. I would also check your connections to your DAQ card. I would reccomend that you connect your sensor differentially. Make sure that you have the correct channel selected in your data acquisition system.

[johnsold]

Some amplified strain gauge sensors have 0.5 to 4.5 V outputs.  That is 2.5 V common mode with +/-2V of signal.  These devices typically run from a 5 V power supply.

I agree that a connection or channel setting problem is likely.

Lynn

[JohnS (STUDENT)]

It goes straight to the 0.5V reading. I attached the PDF weblink of the general information and guide as well to show you what pressor sensor I am using.

Thanks

John.

http://sensing.honeywell.com/index.cfm/ci_id/140450/la_id/1/document/1/re_id/0

also the pins I have connected to are as follows incase I have connected them incorrectly, pin 1 of the pressor sensor is connected to the pin 10 ( +5v of the NI USB 6218 ) and pin 2 of the pressor sensor into pin 14 (ground of the NI USB 6218) and pin 3 of the pressor sensor into pin 32 (of the NI USB 6218).

[James_McN]

Hi John,

Thanks for that info.  It would be worth checking a couple of things.

1. The configuration you have it in is Referenced Single Ended (RSE).  It might be worth checking you are using this setting in software.  If you are using test panels in MAX or the DAQ assistant it is the dropdown box called terminal configuration or input configuration.  If you are using DAQmx VIs then it is an input to the create virtual channel VI.  I suspect it is not this however as you are getting a sensible reading (if incorrect) for your sensor.

2. One possibility is that it is an issue with powering the device from the 5V of the DAQ card.  That said the DAQ card should be able to supply 50mA (shared with digital outputs) and your device only needs 10mA but could be worth checking.  There are two ways of trying this:

a. Disconnect the output pin from the DAQ card and take a reading using a DMM to confirm that the DAQ card is reading the correct voltage.

b. Supply the device from an external power supply and see if this effects the DAQ card reading.

Also what range sensor do you have?  Would be worth knowing just to check what we would expect at ambient pressure.

Regards,

[JohnS (STUDENT)]

According to the box that the pressure transducer was located in it it 15PSI.

I'll tell you a bit more how I started recording with the pressure sensor.

For initial readings I used the atmospheric pressure at the base reading and so from the initial 0.5V that I was reading I used 202 as a amplification factor so I got around 100kPa as the open system reading (atmospheric pressure). However if this is so then the initial reading when I connected up the system was still around 100kPa. I was going to subtract the 100kPa from my final reading but if the pressure sensor is truly a 15PSI then the reading of 101kPa when left open to the atm pressure is incorrect would it not be?

And if this was so then would it not stand to reason that 0.5V/101kPa -->4.5 V /909kPa was the Max pressure reading?

Thanks

John

[johnsold]

John,

That device is specified as a "gage" type (see Table on second page of datasheet).  That means that the pressure readings are relative to atmospheric pressure, not absolute pressure.  Gage pressure is what you see with a tire gage: It reads zero when connected to atmosphere.

So with the pressure port open to atmosphere (15 psi absolute, 0 psi gage), you will get a reading or 0.5 V which corresponds to a reading of zero gage pressure.  If you apply 15 psi of additional pressure (above atmosphere), you will get a reading of 4.5 V.  The pressure span is 15 psi and the voltage change for that span is 4 V.

Pressure (psi gage) 0 15

Output voltage 0.5 4.5

Scale your readings accordingly.

Lynn