Accurate Pressure Sensors - Arduino Forum

So I have a little project I want to do. It has been done on a smaller scale :

For more information, please visit our website.

I want to do something similar but on a 15x15 inch surface and measure the pressure in lbs for each sensor. I have never used arduino but I do have some programming skills in java and visual basic which I am hoping transfer over well to the libraries in arduino and python. I am about to purchase the arduino kit as well to get started on my project.

What I was hoping the community help me with along my journey is on what approach is best to take for measuring pressure in this large of a surface area. Basically I am going to load the 15x15 grid with about 225 pieces of velostat pressure sensors and see the readouts on the screen. Not sure what velostat produces output wise but if I take a weight and compare it to the velostat readings then I could convert my output to lbs.

So does anyone know how accurate velostat is and if its not that accurate then what are the alternatives? I need to keep this as thing as possible so velostat seemed great but not sure what the maximum pressure is on this type of sensor either.

Any guidance would be great because this arduino concept has just opened up so many doors for my imagination its incredible!!!
Thanks!

Hi, welcome to the forum.

Velostat is not very accurate.
The Force Sensitive Resistors (FSR) are better, but still not good enough to measure weight.

This question has been asked a number of times, and for myself I would like to measure the weight of each square for example for a mattress. But there is no cheap solution.
Some manufacturers of mattresses have such a grid-like mat, which measures the weight. I'm not sure what they use. Perhaps a lot calibrated FSRs.

Thanks for the tip. I have read about FSRs and as you said their supposively very innaccurate. However, when you indicate that the mattress ones could be calibrated it raises a new question.

Say I have a 5 pound weight on a FSR and calibrate my readings to specifically that sensor meaning this many pounds. Well if I sit that weight on the fsr sensor ten minutes later should my readings be exactly the same as before or is that what is meant by the inaccuracy, that the results change over time for the same sensor?

I have found:

which reads a 3% error percentage which isnt bad but would prefer a cheaper alternative.

In that Sparkfun page is a link to the datasheet.
According to the datasheet, a single FSR has a Single Part Force Repeatability of 2% to 5% of established nominal resistance.
And the Part-to-Part Force Repeatability is 15% to 25%.

So after calibration of each FSR, the repeatability is 2% to 5%. That's not so bad.
The mechanical way how the weight is applied to the FSR is also important. Will it put force on the FSR every time in exactly the same way ?

Chagrin:
Your sensor says it has a 3.8% drift per logarithmic time scale (whatever log that might be). It'll continue to be less accurate the longer the item sits on it.

Interesting. I did not realize there is a drift in error. I got my masters in financial mathematics but sure wish it was physic right about now lol. I can remember in grade schools calc based physics there is a graph related to resistance over time but I could just be thinking of something else. Is there some theory behind this less accuracy over time such as a curve which I can put back into the calculations/measurements to recorrect or a consistene curve which I should be able to predict from?

Peter the repeatability concept you just mentioned really helped me more than you can think. I need to read the specs more to these sensors. yea 2 to 5% is not bad but that site also seems to have this with less than 3% maximum error:

but its 20 bucks a pop. What makes one sensor more accurate than another? Is it the protective barrier tor reduce interferance/outside variables or a completely different material inside? The reason I ask is maybe there is a cheaper way to manufacture this DIY?

I am purchasing my arnuido now so im taking the plunge!!

I didn't pay 20 dollars for them.
7 dollars : Round Force-Sensitive Resistor (FSR) [Interlink 402] : ID 166 : $7.00 : Adafruit Industries, Unique & fun DIY electronics and kits
These are 3.50 dollars : http://erthenvar.com/store/force-sensor-round (Super weird website. The company does not show an address or number on the website. Well, at least there is some information on Linkedin).

I think the material is about the same. Those large differences in prices is normal, even for the same component from the same manufacturer. I'm sure that Adafruit selected good quality components.
The 3.50 dollar FSR look different, they may be from a cheaper manufacturer, but they could be just as accurate.

Peter_n:
I didn't pay 20 dollars for them.
7 dollars : Round Force-Sensitive Resistor (FSR) - 0.3 ~ 10 Newton Force [Alpha MF01A-N-221-A01] : ID 166 : Adafruit Industries, Unique & fun DIY electronics and kits
These are 3.50 dollars : http://erthenvar.com/store/force-sensor-round (Super weird website. The company does not show an address or number on the website. Well, at least there is some information on Linkedin).

I think the material is about the same. Those large differences in prices is normal, even for the same component from the same manufacturer. I'm sure that Adafruit selected good quality components.
The 3.50 dollar FSR look different, they may be from a cheaper manufacturer, but they could be just as accurate.

Well I ordered my arduino uno yesterday night. i am going to get these sensors now. I may buy the 7 dollar one and the 20 dollar one to test. The 3.50 sensor has a weight limit of 2 pounds basically which is much less than I need. The 7 dollar one may be too low as well. I should be expecting around 50lbs per square inch of my grid thus I need to keep that in mind.

I also will be heating these up to about 400 degrees so hopefully that is not a problem lol???

Basically my buddy bought some cheapo heat platen which doesnt have a pressure sensor on it like most of them so we are just trying to make a grid of sensors which will measure the pressure distribution across it when clamped down. The FSR sensors are thin enough to imitate the thickness and feel of the cloth fabric we would be pressing so they are perfect; however we do want to be able to do this with the heat press on since I may throw in some temperature sensors as well. We are going to be using that matrix concept in the first link I gave to get a visual. The flexicare sensor:

FlexiForce HT201 Sensor

Seems to be perfect for temperature and pressure but is insanely expensive. I could do the temperature sensors on top of some temperature barrier and the pressure sensors on the bottom. Is there some sort of thin material that acts as a temperature barrier to protect the sensors?

Neither the pressure sensor or the thermocouple are flat.

A thermocouple up to 400 degrees Fahrenheit is no problem. I think almost all thermocouples can do that, also the normal J-type or K-type.

It seems there are flat thermocouple : Temperature Surface Sensors | Temperature Measurement

The FlexiForce HT201 Sensor seems to be the one for you. Yes, it is very expensive, but you want to do something special.

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A heat barrier, hmmm, that might be possible. Look at clothes for fireman and for people handling melted metals. You need a combination of shiny metal aluminium heat reflective material and isolation that can withstand high temperatures.
Combine that with remote contactless temperature sensing, and you have something that might work.

Peter_n:
I don't know a cheaper solution, beside the clothes for fireman.

When I start with a project, I buy a number of things to learn what it does and to get some feeling with the project. The resulting project could cost only 1/20 of everything I bought to test.

Well to test I bought 4 of the square pressure sensors you mentioned previously from sparkfun. I am going to see if I can calibrate them and predict the drift error to help smooth out my estimates. I honestly think with some statistics or some sort of modeling technique I can narrow down that 2-5% error. Also, I read there is some stuff you can dip the sensor into that will remove other variables as well which fluctuate output. Some sort of rubber dip or spray?

Also, just curious about the surface area. How does surface area effect these FSR sensors? I was debating the smaller circle one from sparkfun vs the square one but did not know if the smaller surface area produce greater accuracy?

Peter_n:
I have a few FSRs, but I never tested it (a project waiting for me), so I don't know how the surface area has influence.
Is there some kind of dip for the FSR sensors ? I didn't know that. Perhaps some latex or silicone caulk.

The numbers in the datasheet are for a nice smooth equal pressure of course.

I guess I kind of figured that the surface area would pay a key roll given the distribution of pressure effects the resistance, thus the smaller the surface area the less likely of unequal distirbution of pressure for repeated cycles hence lower repeatability error. Unfortunately I already ordered the larger surface area so now I am kicking myself lol. After I do some testing I will see if its worth buying more sensors.

What is your project on? The reason I ask is because maybe we can share results or improvements of using these things. When my sensors come in I plan to go full throttle on getting this done!

Pressure sensors come in many different types and seemingly endless configurations. To select the right pressure transducer or pressure transmitter, users should consider where and how it will be used, the application’s demands on the instrument, and whether certain features are necessary.

Some customers want the least expensive pressure sensors that will get the job done. Others prefer ones with the most features – all the bells and whistles that the market has to offer. The reality is that there is no overall best pressure sensor. There is only the smartest choice for a particular application.

Factors to Consider When Choosing a Pressure Sensor

When customers ask me to help them decide which pressure sensor to get and how to configure it, I start by “interviewing” them. My role is to prevent them from spending extra money for unnecessary features and functions. On the other hand, underspending on pressure sensors, or choosing the wrong type, could risk the safety and efficiency of the entire process or facility.

These are some of the things I ask customers about before giving them my recommendation:

1. What is the application?

This is the most important question, and the more details a customer can give me, the better. I want to fully understand the application in order to figure out the specific demands on the instrument.

2. What is the media?

This question is related to details about the application, as special media require special versions of pressure sensors. For example, when working with food and beverages, sensors should have a higher cleanliness level, specific process connections, and food-compatible transmission fluid – or no transmission fluid at all. If a customers were building a new plant, I might recommend the DMSU22SA in-line process transmitter, while the S-20 superior pressure transmitter is a good retrofit solution.

For viscous fluids or liquids that contain solids, the S-11 flush pressure transmitter is optimally designed for media that would clog the pressure channel of conventional process connections.

Pressure sensors also come in different oil- and grease-free versions for oxygen or hydrogen service. For example, the MG-1 pressure sensor was designed specifically for the storage and distribution of oxygen and other medical gases, as only materials that are suitable for oxygen applications are used. And due to the risk of permeation and embrittlement, hydrogen applications require that sensors’ wetted parts be made of special materials like 316L stainless steel and Elgiloy®.

3. What is the operational environment?

I want to know what temperatures – both ambient temperatures and process temperatures – the pressure transducer or transmitter will be exposed to. This is because temperature has a large effect on pressure sensor accuracy. It is also important to know if the sensor will experience pressure spikes/pulsation, what process connection is desired, along with any necessary sealing materials.

• Outdoors or in washdown environments: IP67
• Submersible applications: IP68
• High-pressure steam: IP6K9K

Industrial vehicles and machine are subject to harsh working conditions – dust, precipitation, vibration, shock, and extreme temperatures – that are not applicable in most other applications. That’s why WIKA manufactures two OEM pressure sensors specifically for the extreme conditions control of mobile machines: the MH-4 pressure sensor and MH-3-HY pressure sensor for hydrogen-powered vehicles like forklifts and order pickers.

Hazardous areas call for an intrinsically safe or explosion-proof pressure sensor. These two types are similar but not interchangeable. An intrinsically safe pressure transmitter, like the IS-3, uses a low level of power so that it would not cause an ignition. On the other hand, an explosion-proof pressure transmitter like the E-10/E-11 is designed to withstand explosions and to contain any flames, sparks, and hot gases that are produced, thanks to its flameproof enclosure.

4. What is the desired output signal?

Another important consideration when selecting and configuring a pressure sensor is the desired output signal. WIKA pressure sensors are available in various analog signals, from 4 … 20 mA and 20 … 4 mA to battery-powered (low-power) signals like DC 1 … 5 V.

Industrial IoT call for wireless digital signals, sometimes over long distances. The A- pressure sensor with IO-Link communication and PNP or NPN switching output is ideal for use in smart factories. Other digital options include the CAN-based protocols CANopen and J, as well as USB outputs.

5. What is the desired accuracy?

Different applications call for different accuracy specifications. For refrigeration and HVAC applications, a non-linearity using the BFSL (best fit straight line) method of ≥ ±0.6% is sufficient. On the other end of the spectrum is a non-linearity of ≤ ±0.04% for the precision measurements required for test benches, calibration, laboratories, and certain machine building applications.

6. Pressure considerations

The first consideration when configuring a pressure sensor is the type of pressure to be measured. There’s gauge pressure (working pressure), absolute pressure, and vacuum/low pressure.

The second pressure consideration is the measuring unit: psi, bar, mPa, kPa, etc.

Finally, what is the desired pressure range? This depends on the application’s operating range as well as a comfortable buffer to account for possible exposure to pulsation and pressure spikes. WIKA pressure sensors cover an extremely wide range, from −30 inHg … 0 psi to 0 … 20,000 psi. For the highest pressure applications, like waterjet cutting, we offer the HP-2 pressure sensor with a measuring range as high as 0 … 217,500 psi. Very few pressure sensors in the world can reliably measure pressures of this magnitude.

Features vs. Odds of Failures: Compromises When Choosing a Pressure Sensor

There’s a balancing act when it comes to selecting a pressure sensor. Extra features and functionalities are great and often necessary, but with them comes the greater possibility of sensor failure.

In general, the more complex the sensor design, the less adaptable it is. For example, a display for onsite pressure readings is a very useful feature. On the other hand, having a display:

• Increases the instrument’s size – a consideration if space is at a premium
• Increases its power consumption – a consideration if you want it to run on batteries
• Reduces the sensor’s operating temperature range
• Makes the instrument more susceptible to mechanical damage, shock, and vibration

Similarly, additional software features enhance functionality and add convenience, but they also increase the probability of user error and premature failure due to the presence of processors, memory chips, and other delicate components. Analog sensors and transmitters, on the other hand, have fairly simple and very robust circuitry, and their function and accuracy rely on a few passive components.

So, when deciding on which features you want in a pressure sensor, ask yourself these questions:

• How often would these features be used?
• Do these features need to be present in all the sensors used in the field?

WIKA USA, Smart in Pressure Sensing

WIKA is a global leader in the design, manufacturing, testing, and calibration of high-quality electronic pressure instruments. We understand that with such a comprehensive portfolio of products, it can be difficult to select the right pressure sensor with the right accuracy, pressure range, materials, output signal, process and electrical connection, approvals, and features. That’s why we encourage customers to reach out to one of the product specialists at WIKA USA for expert advice. We won’t steer you toward products and features you don’t need. We will, however, help you find the best pressure sensor for your particular application. Contact us for more information.

Products mentioned in this article:
•  A-10 pressure transmitter
•  DMSU22SA in-line process transmitter
•  S-20 superior pressure transmitter
•  S-11 flush pressure transmitter
•  MG-1 pressure sensor for medical gases
•  MH-4 OEM pressure sensor
•  MH-3-HY OEM pressure sensor for mobile hydrogen applications
•  IS-3 intrinsically safe pressure transmitter
•  E-10/E-11 pressure transmitter with flameproof enclosure
•  A- pressure sensor with IO-Link communication
•  HP-2 pressure sensor for highest pressure applications

Want more information on high accuracy pressure sensor(tl,kk,mn)? Feel free to contact us.