dynamic - acquire real values at fluctuating part??

[oceansky]

Dear all: I have a question interpreting the data results. I am using PCB Pressure transducer Model 113A26 which gives the Dynamic Pressure values from Steady State waterline pressure.

Please refer to the attached figure 1 that was generated from NI DAQ system. Initial steady state is 45 psi; so it is showing zero (no changes in pressure) until 7 seconds. When I suddenly change the flow rate (by upstream valve), then pressure transducers are experiencing high and low fluctuations and goes back to steady state again (final steady state is 0 psig by observation from other pressure gage) and it is showing 0 in below figure (no change in pressure).  We can see that Box A is 45 psig in real pressure (initial steady state) but shows zero as there is no pressure change. In Box C, the real water pressure is zero (final steady state) and it is showing zero in figure 1 as there is no pressure fluctuations. In Box B, it is showing the relative pressure from original water line. But at time 12 or 13 seconds, it is approaching zero which means real pressure is 45 psig; however, in reality, inside pressure is zero (final steady state is zero).

I have difficult time interpreting the results at fluctuation part (Box B). Would you let me know if there are some ways to extract real pressure values (not relative to water line pressure) from figure 1?  I am asking this as I want to compare experimental results and numerical results. Numerical results are giving only real pressure values (not relative pressure from steady state).

I would greatly appreciate if you kindly address this problem.

Regards,

 

Message Edited by oceansky on 02-07-2008 04:28 PM

Message Edited by oceansky on 02-07-2008 04:30 PM

[Chris_K_]

Hello oceansky,  

The way I interpret the graph that you attached, is that in BOX A there are no fluctuations in pressure so your sensor reading will be zero as is shown, but in an absolute sense there is 45 PSI in the “tank”.  When you open the valve  and the real pressure goes from 45psi to the final state of 0psi (BOX B), the  sensor reading will show the fluctuations in pressure and these fluctuations will slowly diminish and reach a steady state reading of zero (BOX C).  This zero is not the same as the original zero reading meaning 45psi but rather the steady state of 0psi as there are no longer any fluctuations in pressure. 

If my understanding of your test setup is incorrect then maybe if you explain your test setup to me, I may gain a better understanding of what you are trying to accomplish.

As for interpreting the fluctuations in pressure, check the specifications of your pressure sensor, and determine which signal levels correspond to what pressure levels.

If you require further help, can you let me know what type of DAQ card are you using to acquire this data, and what acquisition software you are using?

Have a good weekend

Chris_K 

[oceansky]

Dear Chris_K_: thank you for kindly replying. Currently, I am creating waterhammer shock experiment. When I suddenly close a solenoid valve (located at upstream), I wanted to observe pressure variation at downstream (where I installed dynamic pressure transducers). As mentioned, the steady state in piping system is 45 psig (see Box A). When I suddenly close upstream solenoid valve, the downstream dynamic pressure transducers are simultaneously experiencing very low pressure followed by high and low pressures (Box B); then finally the pressure goes to zero psig (Box C: the real pressure is 0; and there is no fluctuation, so zero in BoxC).

My question is how I get real pressure values at fluctuating part (part B). For example, the first low pressure can be obtained by (45 steady state - 52 psig = -7 psig in the first lowest peak; please refer to the attached Figure). But in the second high peak, the graph is showing 14 psig; in this case, do I get it by "45 (initial steady state) +14 ???" I am not sure this is correct. This also applies to third low peak, fourth high peak. As this pressure is showing only relative values from steady state, the baseline values (steady state values) for the second peak onwards are not very clear.

I have been using Signal Express to obtain the data results (Analog -5 Volts to +5 Volts: this corresponds to -500 psi to + 500 psi; please refer to the below specification). As the sampling rate was so high, I subset and resamplied the data set at every 0.01 seconds.

Below are the specification of NI Chassis, Module, and Dynamic Pressure transducer.

1. Chassis:

cDAQ-9172 8-slot USB 2.0 Chassis for CompactDAQ, US (120 VAC)

2. Module:

NI 9233 4-Channel, 24-Bit, +/-5 V, 50 kS/s per channel, IEPE Analog Input Module

3. Dynamic Pressure Transducer

High frequency ICP® pressure probe, 500 psi, 10 mV/psi, 0.218" dia. diaphragm, accel. comp, with stainless steel diaphragm, off ground in a 1/4" NPT threaded adapter.

[dsb@NI]

All channels on the 9233 are always AC coupled. This means that DC components (aka steady-state values) will be attenuated. The response that you attached to your first post may be more representative of the AC filter response than the actual dynamic pressure response.

[Ian_Ren]

Not only 9233 is AC coupled, it also has an anti-aliasing filter built-in. Have you tried to acquire data at a higher sample rate?

Ian