Selecting the right pressure transmitter for ADC project - Volts or amps?

Thread Starter

StrongPenguin

Joined Jun 9, 2018
307
Hi everybody, guess the electronic bug has bit me again and would like to embark on a group project with my engine crew:)

Sometimes we need a pressure transmitter that can show pressures up to 400 bar (5800 Psi) These (the displays./readers.) are very expensive and rarely used. So I was thinking it be fun to make one. Precision just has to be close enough, without the project becoming a moon rocket. The intent is to make a handheld display with 9V battery(s). The battery time does not have to be several hours.

I found a few parts, and jumped the gun on buying a Schnider XMEP400BD21F Pressure transmitter 400 bar G1/4A 4-20mA M12 (12-24 VDC supply)
But later I found out, that this will not be an easy task to feed to an ADC, since most of them run on volts. And it has to be supplied with at least 12VDC.

One that would fit perfect is the XMEP400BD11F Pressure transmitter 400 bar G1/4A 0.5-4.5V M12 (5V supply), given it's low voltage. But the reason for buying the 20 mA one was precision, and that 4-20 mA signals are widely used on ships with great success. A range of 4V to cover 400 bars sounds like a difficult task for the transducer element.

And there is also the XMEP400BD71F Pressure transmitter 400 bar G1/4A 0-10V M12 (24 VDC supply)

Going with the 4-20 mA version, I added this to my shopping cart:

- MCP3301-BI/P (13 bit ADC)
- MCP1541-I/TO (Voltage ref)
- PIC16F1783-I/SP (Compatible with my Microchip HPC board. Also have a few others laying around)
- 2 line LCD
- LM7805CT (5V regulator)
- RCV420JP (Have not added this yet, but read that this could be used for converting 4-20 mA signal to volts. Can see the output is 0-5V...hmm...)
- 2 meter M12 plug wire
- Plugs, op-amps, resistors, caps...

Now the questions:

With all this stuff, do I have the parts need to build this pressure reader?
Which is the best route, the 4-20 mA or the 0.5-4.5V transmitter build? Guess the 4.5V build would be helluva lot easier.
Is a range of 4-5 volts enough to cover such a large range as 400bar/5800 Psi?

Datasheets for transmitters (in Danish..)

https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD11F&scp_id=DK_da
https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD21F&scp_id=DK_da
https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD71F&scp_id=DK_da
 

MisterBill2

Joined Jan 23, 2018
17,814
I suggest investigating "RED LION" as a source of the correct display. And while a "precision" display is not high on the list, you come across as knowing more if you are asking for accuracy and resolution. Accuracy being the reduction of uncertainty of the measurement and resolution generally being the amount of digits or bits in the measurement.
 

Thread Starter

StrongPenguin

Joined Jun 9, 2018
307
@MisterBill2 Found a Red Lion display that does all this and then some. But I would like this to be a learning experience for us all. I may be way over my head, and even it it fails, we will have learned something, at least.

Can you say if I have the parts needed to complete this project? Maybe I should get the 5V sensor for ease of life. It's just that most sensors are 4-20 mA, due to reliability, I guess.
 

Pyrex

Joined Feb 16, 2022
237
6LR61 9V battery has capacity of 400mAh only. Imho, it's much better to use 6x AA batteries in series. The 4.5V transmitter is a better choice, if the device is powered from a battery. It consumes 7 mA @ 5V only
 

MisterBill2

Joined Jan 23, 2018
17,814
The pressure transducer will probably need more than the five volts to function, And while the ADC will operate on 5 volts, the output of the transducer, if you use the 250 ohm resistor, will be one to five volts. , and the output of the A/D will probably be putting out pure binary data. (how many bits?) That binary will need to be scaled to indicate the actual pressure.
Given that the transducer output is analog, not digital, it can cover the entire sensor range from zero to max, the limitation is in the display realm, with the resolution being the limitation.
I suggest asking for a download of the Red Lion pressure display in order to understand what is involved with displaying pressure from the current loop transducer. THAT will provide a much better explanation than I can provide on this thread.
 

atferrari

Joined Jan 6, 2004
4,748
Hi everybody, guess the electronic bug has bit me again and would like to embark on a group project with my engine crew:)

Sometimes we need a pressure transmitter that can show pressures up to 400 bar (5800 Psi) These (the displays./readers.) are very expensive and rarely used. So I was thinking it be fun to make one. Precision just has to be close enough, without the project becoming a moon rocket. The intent is to make a handheld display with 9V battery(s). The battery time does not have to be several hours.

I found a few parts, and jumped the gun on buying a Schnider XMEP400BD21F Pressure transmitter 400 bar G1/4A 4-20mA M12 (12-24 VDC supply)
But later I found out, that this will not be an easy task to feed to an ADC, since most of them run on volts. And it has to be supplied with at least 12VDC.

One that would fit perfect is the XMEP400BD11F Pressure transmitter 400 bar G1/4A 0.5-4.5V M12 (5V supply), given it's low voltage. But the reason for buying the 20 mA one was precision, and that 4-20 mA signals are widely used on ships with great success. A range of 4V to cover 400 bars sounds like a difficult task for the transducer element.

And there is also the XMEP400BD71F Pressure transmitter 400 bar G1/4A 0-10V M12 (24 VDC supply)

Going with the 4-20 mA version, I added this to my shopping cart:

- MCP3301-BI/P (13 bit ADC)
- MCP1541-I/TO (Voltage ref)
- PIC16F1783-I/SP (Compatible with my Microchip HPC board. Also have a few others laying around)
- 2 line LCD
- LM7805CT (5V regulator)
- RCV420JP (Have not added this yet, but read that this could be used for converting 4-20 mA signal to volts. Can see the output is 0-5V...hmm...)
- 2 meter M12 plug wire
- Plugs, op-amps, resistors, caps...

Now the questions:

With all this stuff, do I have the parts need to build this pressure reader?
Which is the best route, the 4-20 mA or the 0.5-4.5V transmitter build? Guess the 4.5V build would be helluva lot easier.
Is a range of 4-5 volts enough to cover such a large range as 400bar/5800 Psi?

Datasheets for transmitters (in Danish..)

https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD11F&scp_id=DK_da
https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD21F&scp_id=DK_da
https://www.ops-ecat.schneider-elec...ist_Id=scope&prd_id=XMEP400BD71F&scp_id=DK_da
Hola @StrongPenguin

Maybe, exploring the dedicated ICs by Burr Brown could spark some ideas. They exist since a long time.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
17,814
The voltage developed across the series resistor will not go to zero, so usually an op-amp stage with some offset is required. And then you can utilize the whole A/D span, which is useful because the pressure display will be 0 to either 400 BAR or 5800 PSI. Scaling 0 to 400 will be easier but the resolution of bars may not be satisfactory, although you might go to tenths of bars.But you will wind up with a fair amount of electronics to package. I was very happy withbthe Red Lion display device I bought but that was several years ago. in 1999, I think. What you get is a package that allows you to set the displayed value for the 4 mA and for the 20 mA, and then you are done calibrating. And it is a nice package easy to use and only a single voltage supply required.
 

Reloadron

Joined Jan 15, 2015
7,452
I found a few parts, and jumped the gun on buying a Schnider XMEP400BD21F Pressure transmitter 400 bar G1/4A 4-20mA M12 (12-24 VDC supply)
But later I found out, that this will not be an easy task to feed to an ADC, since most of them run on volts. And it has to be supplied with at least 12VDC.
Yes, most are 12 to 24 VDC supply voltage but the 4 to 20 mA is easily scaled and overcome. Your pressure sensor outputs 4 to 20 mA proportional to applied pressure. So what happens if I place a 250 Ohm or 500 Ohm resistor (1% resistor) in my 4 to 20 mA current loop? I get either 1.0 to 5.0 VDC dropped across my resistor or 2 to 10 VDC dropped across my resistor, depending on the resistance value. Now we have a voltage directly proportional to the applied pressure. Your pressure sensor is 0 to 400 bar so 0 to 400 becomes 4 to 20 mA. Just for example let's say I use a 250 Ohm resistor in my current loop so now 0 to 400 bar becomes 1 to 5 volts. Easily measured with about any ADC. What you have will resemble this image:

4 - 20 mA Sensor.png

That is all there is to this sort of sensor. So now I have 1 to 5 volts proportional to your pressure. The voltage span is 5 volts minus 1 volt = 4 volts span. 4 volts / 400 bar = .010 volt/bar or 10 mV per bar. The math would be done in your code for any simple inexpensive ADC just as an example an Arduino UNO or similar. Using an Arduino Uno Rev3 board as an example you can add a simple display like this one and you are done. That gives you a 10 bit ADC. If you want greater than 10 bit ADC you can add an ADS1115 to the scheme. This is merely a simple example there are plenty of very inexpensive micro-controllers out there to do what you want to do. The involved code is also very simple and about anyone here can give you code samples and examples.

This or just go buy a cheap off the boat process controller. This is all a matter of budget and the uncertainty you can live with in your measurement plane. I have used Omega Engineering, Red Lion (mentioned) and a host of others. Most are designed around a 4 to 20 mA input or voltage input and you scale them to read the engineering units of your choosing. You define the uncertainty (accuracy) and resolution.

Ron
 
Last edited:

MisterBill2

Joined Jan 23, 2018
17,814
Really, another option could be an op-amp circuit to adjust the offset so the outputwas zero to four volts, and then use a cheap digital voltmeter, 3 1/2 digits to display zero to 400 bar.
 

Thread Starter

StrongPenguin

Joined Jun 9, 2018
307
Hola @StrongPenguin

Maybe, exploring the dedicated ICs by Burr Brown could spark some ideas. They exist since a long time.
Hola sailor :)
I've just looked for TI/Burr Brown ADC's, but it's pretty slim pickings with the DIP version. Other than that, I can't find anything from our dealer.

The voltage developed across the series resistor will not go to zero, so usually an op-amp stage with some offset is required. And then you can utilize the whole A/D span, which is useful because the pressure display will be 0 to either 400 BAR or 5800 PSI. Scaling 0 to 400 will be easier but the resolution of bars may not be satisfactory, although you might go to tenths of bars.But you will wind up with a fair amount of electronics to package. I was very happy withbthe Red Lion display device I bought but that was several years ago. in 1999, I think. What you get is a package that allows you to set the displayed value for the 4 mA and for the 20 mA, and then you are done calibrating. And it is a nice package easy to use and only a single voltage supply required.
The main goal is to learn something. If we can get this thing to say just something that resembles Bars, then I am happy. If at the end of the day we get something that shows 0-400 bars with 2 bar jumps, I will crack open the alcohol free champagne. The most important thing is that we did it, and we learned something :)
I have added the 5V version of the transmitter to the cart. The transmitters are fairly cheap, so I doubt the quality of them. Usually when buying precision high pressure transmitters, they cost an arm and a leg. It's also uncommon to have one laying around onboard, since we never use them. It was just that we had a breakdown recently, where one would have been pretty handy.
A rail to rail opamp to get 0-4V would also be an interesting add-on, although It does not seem necessary, since this could be solved in code..?

@Reloadron Yeah I read a couple of topics, where a resistor was used to "convert" to voltages. This could be viable too.

It looks like I have everything I need to get started. The guys are eager to get started building this display and learn something about electronics. We have all had some minor fiddling with electronics in school, but would like to expand :D
 

MisterBill2

Joined Jan 23, 2018
17,814
Usually, at least in the equipment that we built, the current loop signal (4 - 20 mA) was always converted to a voltage because the sensors all fed an A/D board in a computer. And ALWAYS the analog signal was picked off across the resistor via a shielded pair, over to the computer section. This scheme avoided adding a lot of noise via any shared current paths.
The one instance where a sales guy who thought he was able to simplify and cut costs changed things for his project, it had to be rewired because of the common wire coupling. Sometimes a small bit of knowledge coupled with zero understanding will lead to problems.
One major benefit of using current loops is that they do tend to resist some common sources of noise voltage.
 
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