Help converting pressure sensor readings to PSI

Projects General Guidance
1

Hello,

Recently I have been toying around with DF 9-16 thin film pressure sensors:

Amazon Link Pressure Sensors

The sensor doesn't measure pressure directly and I have been trying to find the best way to convert it's readings to PSI. Right now I am using the following code:

  const int pressurePin1 = A1;
  int sensorValue1 = analogRead(pressurePin1);
  float voltage1 = sensorValue1 * (5.0 / 1023.0);
  float force_g1 = pow((float(22.988)*float(voltage1) - float(11.4437)),2);
  float force_lb1 = float(force_g1) * float(0.00220462);
  float pressure_psi1 = float(force_lb1) / float(0.0775);

I integrated the DF 9 -16 into my circuit using the schematic shown at the bottom of this document: DF9-16 Overview

I got "force_g1" from the code using the following figure. I went to an online curve fitting tool and entered a few of the points shown on the figure, and then solved for F(g).

Vout vs Force
Vout vs Force848×622 107 KB

I guess a few of my questions are:

  1. Is this the appropriate method for determining PSI from pressure sensor? Is there any way to confirm my method is correct?
  2. The PSI's I am expecting for my project range from 0 to 5. I don't think the force's in that range are included in the plot I used to convert my readings. Is that an issue? Do you have any advice on how to get a more accurate equation for converting pressure readings?

Thank you.

2

PSI is pounds/sq inch.
Unless the force is applied uniformly over the sensor area you will never be able to measure PSI with any kind of accuracy.

3

Thank you. Let's say I wanted to tape the sensor to my arm and then wrap my hand around to see how much pressure is exerted. Would you consider that to be uniform? Do you know what type of pressure sensor you might recommend for that application?

4

It could be but your sensor only covers a small portion of the total area.

Do you know what type of pressure sensor you might recommend for that application?

You wnat to measure the pressure your hand exerts on you arm when you wrap you hand around your arm?

Quite an unusual application and I don't have any recommendations.

5

Thanks. As a side question -- would you mind taking a look at the attachment that shows how to wire the DF 9 -16? It looks like one terminal of the sensor goes directly to ground and the resistor goes to VCC (5 volts). Do you know if that is typical for pressure sensors? Does it conflict with the F vs Vout plot I sent? I feel like the schematic implies that the Vout would be 5V when no pressure is applied to the sensor, but the figures shows Vout approaching 0 when no pressure is applied. Did that make sense?

6

I have been trying to find the best way to convert it's readings to PSI

You can't, with much accuracy or repeatability.

That is a force sensitive resistor and the description is misleading. It isn't designed to measure pressure, but it could be could be used for rough applied force scales like "none", "light", "medium" and "high".

If you are serious about measuring pressure, use a pressure sensor.

how to wire the DF 9 -16

What you describe is a voltage divider, and is used in this case to estimate the sensor resistance, which changes as a non-linear function of the force applied.

7

For an FSR it the simplist way to do it

Does it conflict with the F vs Vout plot I sent?

Yes that type of arrangement does not produce a linear output

Did that make sense?

yes

8

The first step is to compute the size of the sensor in square inches.

Edit:
The second step is put the sensor on a flat table and apply a known weight. Measure the sensor output and compute the PSI based on the known weight you are using. That result will be the PSI for that weight. Repeat with other weights.

9

Thank you. Do you happen to know of any sensors that actually measure pressure? I would like it to be flat like the DF9-16. I tried looking around online, but all I could find were FSRs.

10

Not if you are looking for something like that FSR.

What is it that you actually want to measure?

Please describe your project, keeping in mind that pressure is defined as force per unit area.

To learn more about FSRs and how they respond to force, this tutorial should help. Applying weights to the sensor results in resistance change that is roughly linear over a certain range, when plotted on a logarithmic scale.

960×693 189 KB

11

Thank you. To give you a general sense of what I am trying to do -- I am trying to measure the pressure exerted by a cuff that restricts around an arm (think a blood pressure measuring cuff). My plan is to put around 5 of the DF9-16 sensors on an arm and measure the pressure exerted on the arm from the cuff as it restricts. I also have a datalogger set up to save the pressure from each individual sensor and the overall average pressure. I though a thin film sensor was the most convenient way to collect the data I am after.

I am familiar with the plot you posted -- I think the doc I attached in original post included similar force vs resistance plots.

12

Unfortunately, it is almost impossible to calibrate FSRs so that they could be useful to accurately measure pressure.

You can salvage a cuff and the basic pressure measurement sensor from a blood pressure monitor intended for home use (find them in thrift shops for the price of a cup of espresso).

A couple that I have examined use a pressure-sensitive capacitor to control the frequency of an oscillator. Count oscillator pulses for a reading proportional to the cuff pressure.

A number of articles can be found on line for hacking as above, or using differential pressure sensors with Arduino, like this one.

13

Hi, @parrylens
Welcome to the forum.

Why don't you measure the pressure in the cuff, with a gas pressure sensor?
When the cuff is firmly wrapped around the arm the pressure on the arm should be close to the cuff pressure.

Tom.... :smiley: :+1: :coffee: :australia:

14

Nice idea -- this is something I am pursuing, but I am having trouble finding the appropriate gauge for the cuff's valve. I wanted to collect measurements with the pressure sensors as well as the internal pressure with a gauge.

15

Have you ever takes someone's blood pressure readings? What do you think the pressure gauge is reading if it is not the cuff pressure? And yes, there are automated reading cuffs. Use one of them for your Arduino measurements.

16

Previously you said you were trying to measure the pressure created by your hand when grabbing your arm. (See post #3)
So now if you don't realize, your are asking for something completely different.

Exactly what are you trying to do?

17

If you are still talking about the FSRs, I assume that you have not made any serious effort to calibrate one, let alone actually measure "pressure".