help with creating bridge scale

[boletus]

Could someone please explain how to create a bridge scale for the attached pressure transducer specifications? This is the 0-100psi model. Sensor has 225mV full scale output at 10.0Vdc and notes that the minimum is 140mV. I'm a little confused by the mV/V units that LabVIEW is asking for, rather than just mV. Are these the correct values to use? When I enter them my slope preview does not show any line as the electical units displayed only go from -1 to 1.   

First electrical value = 140mV/V

First physical value = 0 psi

Second electrical value = 225mV/V

Second electrical value = 100 psi

Thanks for any help, I am new to all of this

[Henrik_Volkers]

The sensor output bridge voltage 'lineary'* depends on the bridge supply voltage. So the output is defined by mV(output voltage) / V (per supply voltage).

*) Piezoresistive sensors are 'not so linear', say only at the supply voltage specified. That's why the sensitivity of piezoresistive sensors is often not specified by mV/V , instead the supply is assumed to be fixed (10V here) and declaired sensitivity is in mV.

The spec tells that the zero offset should be in the +- 20mV range. Yours is 140mV, why ??

Overloaded in the past??

Did you used a torque-wrench to apply a mounting torque of 1.7 Nm (+-.6)!! ??

How does the offset look like with an unmounted sensor??

However according to the spec I assume you should enter

First electrical value = <your offset voltage>/<supply voltage>   (14.0mV/V)

First physical value = 0 psi

Second electrical value = <your offset voltage +225mV/10V>   (36.5mV/V)

Second electrical value = 100 psi

BUT usually you get a calibration sheet with that sensor. THESE values are the ones you should use.

In case you have to apply more than 1.7Nm torque , you would need a dedicated calibration for the torque applied!

Well, at least if you want to do serious measurements 😄

EDIT: I never used this express scaling thing... but I assumed that if they ask for mV/V they will later ask (or lookup) for the supply voltage applied.

[boletus]

Thank you for the response. I checked and there was indeed a separate calibration report with the sensor, which I have attached. This report shows 'zero pressure output' of -7.10mV and 'full scale output' at 189.8mV. So would these be the values I should use?

First electrical value = -.71 mV/V  (-7.10mV output / 10V excitation)

First physical value = 0 psi

Second electrical value = 18.98 mV/V (189.8mV output / 10V excitation)

Second physical value = 100 psi

The sensor is brand new and has not been installed yet so no torque applied. Sounds like that is something I should be careful about though? I'm guessing I misinterpreted the spec sheet regarding the 140mV minimum. I think it was saying that actual full scale output of the sensor will be somewhere between 140-225mV, mine apparently falling at 189.8mV.

[Henrik_Volkers]

First electrical value = -.71 mV/V  (-7.10mV output / 10V excitation)

First physical value = 0 psi

Second electrical value = 18.98 mV/V (189.8mV output / 10V excitation)

Second physical value = 100 psi

Looks right for my eyes 🙂

And yes, it is always wise to follow the spec if you mount a sensor 😄 😄

(and this one is not a cheap MEMS throwaway 😉 )

[Bob_Schor]

An important point conveyed by the Manufacturer's Calibration Report is the linearity of the Sensor, which looks pretty good.  This means that if you know the Voltages corresponding to two Pressures, you can get the Pressure corresponding to any other Voltage by using a straight-line plot between the two points on your Voltage/Pressure plot.

If you want, you can define a Custom Scale for your sensor using DAQmx, which will give you outputs directly in units of Pressure (the conversions being done inside the Driver, so you don't need to mess with it.  This can be especially useful if you have a Sensor that does not have a Calibration Sheet, but has a fairly easy Calibration Procedure -- Calibrate first (using no Scale Factors), determine the Scale Factors, define Custom Scales, and take you (now-scaled) data.

Bob Schor

[boletus]

Okay, thanks both of you for the input. Just want to run through this one more time to make sure that I am following everything correctly, don't want to get started off on the wrong foot. I was confused by the mV/V units but I think I get it now.

Calibration report shows that at 10v excitation I have -7.1mV at zero and 189.9mV at max (100 psi) and that this is linear. For scaling I divide this by the excitation voltage, which is then cancelled out when I enter 10v as the excitation in LabVIEW (189.8mV/10V * 10V). This is the part I was missing so if that's correct it makes sense to me now.

I do have one further question about the sensitivity though. Report shows 1.90mV as the sensitivity but if I have -7.1mV to 189.8mV that's a total range of 196.9mV, divided by 100 psi which gives 1.97 mV/psi. A little confused about that. Should I be using 0mV as my 0 psi point? This would give me 1.90mV/psi.

[Henrik_Volkers]

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boletus wrote:

I do have one further question about the sensitivity though. Report shows 1.90mV as the sensitivity but if I have -7.1mV to 189.8mV that's a total range of 196.9mV, divided by 100 psi which gives 1.97 mV/psi. A little confused about that. Should I be using 0mV as my 0 psi point? This would give me 1.90mV/psi.

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Good question 🙂 I would ask Meggit 🙂  They work according to standards (9000,17025)* so they should be able to answers that question.

*) What's the difference between ISO 9000 and ISO 17025?

Spoiler

8025 🙂