analogue outputs

[cgrodzinski]

I need your help on setting analogue outputs.
The output is +/-5Vdc. The units are absolute pressure.
If the pressure transducer is calibrated to gauge pressure 0-10bars and output settings are 500mV/bar and offset 1bar what voltage should be read at -5V, 0V, +5V.
I understand that the 0V = 1bar and +5V should read 11bars. What about -5Vdc=??
I appreciate your advice.

 

[danw2]

>If the pressure transducer is calibrated to gauge pressure

When an absolute pressure transmitter is 'calibrated' for 0V = 0barg, then it will evidence a ± shift around 0V as the atmospheric barometric pressure changes. It will only read 0V at the same barometric pressure as was applied when the device was calibrated.

An 'interesting' way to use an absolute pressure transmitter . . .

>I understand that the 0V = 1bar and +5V should read 11bars.

the statement requires some notation indicating absolute pressure:
0V = 1barA and +5V should read 11barA

With calibration to gauge pressure, a 0-10 barg span with a 1 barg offset over -5Vdc to +5Vdc output scale could be:

0 bara = ~-500mV
~1 bar a = 0 barg = 0 V (at sea level)
~2 bar a = 1 barg = 500mV
~3 bar a = 2 barg = 1V
~4 bar a = 3 barg = 1.5V
~5 bar a = 4 barg = 2.0V
~6 bar a = 5 barg = 2.5V
~7 bar a = 6 barg = 3.0
~8 bar a = 7 barg = 3.5V
~9 bar a = 8 barg = 4.0V
~10 bar a = 9 barg = 4.5V
~11 bar a = 10 barg = 5.0V

 

[Compositepro]

Such a sensor calibration makes no logical sense. To use a +/- signal the sensor must be measuring a physical quantity that has a stable zero and +/- values. Absolute pressure has only positive values. Your sensor is probably measuring gage pressure and not absolute pressure. Trying to get it to display in units of absolute pressure will only cause confusion and measurement errors.

Your calibration probably needs to be -5v=-15psig, 0v= ambient pressure, +5v=+15psig

 

[cgrodzinski]

Thanks gentlemen, I admit that this was done in an awkward way. I changed this setting to -5V=1bars and +5V=11bars. This had to be done because the related sensor was gauge sensor and my customer requested reading in absolute pressure. I know that this may cause an error when the pressure drops to 0 and atmospheric pressure is below what was set during commissioning but this will never happen during operation.
Thanks again!!

 

[danw2]

Cover your case.

Make sure that the customer knows that 'absolute' does not mean absolute in this case; that the pressure readings are only approximately (~) absolute, since the transmitter was cal'd to gauge pressure, with gauge pressure's inherent reference to the atmospheric barometric pressure at time of calibration.

It isn't just open-to-atmosphere that's off, all pressure readings are off by the difference between whatever the barometric pressure was when cal took place and standardardized 1 bar absolute.

 

[cgrodzinski]

dawn2, Thanks. I made sure that they know about it.