New.. and confused about load cells & pressure transmitters ohm's

Hi, I have what I consider a simple project, but am having a bit of trouble understanding the basics. All I want to do to read the signal from a hydraulic pressure transmitter (such as, SETRA C206 Pressure Transmitter 24VDC, 0-7500 PSIG, 4-20mA) and display it on a load cell digital indicator. This is to use it as a scale on an industrial loader.

All the load cell displays I've seen can drive 1 or more 350Ω load cells, but the pressure transducers seem to be 800Ω. Is it possible to 'plug' a pressure transmitter into a load cell display and get it to work? The reason why I want to use a load cell display is because they're cheap on eBay, less than $100(and less than $100 for the pressure transmitter), whereas complete loader scales are $700 and up.

I'm just a dumb farmer and while I can happily plant and grow a couple of thousand tonnes of wheat, lupins, canola, etc. I just get lost when I try to work out electronics! It frustrates me that this would seem to be a simple problem for someone with probably basic knowledge, but it's beyond me.

Any help would be greatly appreciated. Even if someone can just explain the Ohm difference between the 2 and what it affects.

Thanks

ps. Yeh, it's taken me 5 years to make my first post. You can't rush these things you know..... ;-)

I can't grow anything with the exception of some tomatoes, peppers and basic garden veggies and even then it is a challenge.

OK, you seem to have a few apples & oranges mixed up in this crop. The pressure sensor you linked to is a 0 to 7,500 PSIG sensor (Pounds Square Inch Gauge). The output is 4 to 20 mA of current and it uses a 24 VDC power supply.

Typical strain gauge devices like load cells and pressure gauges do work with a bridge. Yes, 350 Ohm is a popular bridge. Their outputs are typically measured in milli Volts based on their excitation voltage. That output is then amplified and scaled. You have a different animal here. The pressure gauge you linked to has the amplifier built in and outputs a linear current proportional to the applied pressure.

So what you have is this. 4 to 20 mA = 0 to 7500 PSIG. That is a span of 16 mA (since we start at 4) equals 0 to 7500 or we could say 16 / 7500 = .00213 mA per PSI.

There are indicators out there that will scale and display your 0 to 7,500 PSIG to give you what you want based on a current or voltage input. You have current but if the transducer output were run through a precision 500 Ohm resistor it would yield 2 to 10 volts = 0 to 7,500 PSIG. thus an 8 volt span.

Forget the bridge resistance in this case since the transducer output has already been amplified and conditioned.

So what do you want? Just a display to display the pressure?

Ron

You need to connect the pressure transmitter to a 1-5V or 0-5V input with a 250Ω resistor.
If you're display/gauge/meter does not have such an input, it's incompatible with the transmitter.

+24V to transmitter positive
transmitter negative to positive display input
250Ω resistor between positive and negative display inputs (converts 4-20mA to 1-5V)
negative display input to ground

Without seeing the specs for the readout, it will be hard to say what needs to be done to mate these two devices. Like Ron said, a strain gauge bridge output is in milivolts. Like cachehiker said, you can put a resistor in the 4-20 loop and get a voltage (E=IR). Your voltage needs to be milivolts, not volts. Just to throw stuff out here, if your readout is 10mv FS and the xdcr is at 20ma, R=E/I = 0.01/0.02=0.5 ohms resistor needed in the 4-20 loop. At the other end, 4ma from the xdcr will put 2mv into the display. You can buy 4 terminal very accurate 0.5 ohm resistors. They are used for current sensing, just like we are doing here. You can uses this same procedure for whatever value you disply is calibrated to.
One reason these load cell readouts are so cheap is that they are probably not certifiably accurate.

Wow, thanks for the replies. Ron, I want to be able to convert and display the pressure from the transmitter as a weight in kg's, just so I know how much fertiliser I've got in the loader bucket(up to 3000kgs) Just want a cheap way of weighing the load( after a drought last year, I'm only doing 'cheap' this year!)

I found some very cheap pressure transmitters here; eBay Australia Store and have found cheap displays here; eBay Australia Store and here; **broken link removed** and thought if the 2 would work together then I'd be able to save $500 or more on the cheapest loader scales I can find.

If there is something else I can use as a cheap display, that will allow me to scale and display the transmitter output as kg's, I'm open to suggestion. Just need to be able to zero the loader when empty easily and display the tare weight.

I posted a reply but it has links and is only my second post so will have to wait for it to be approved

It's appeared now.

paulmac said:

Hi, I have what I consider a simple project, but am having a bit of trouble understanding the basics. All I want to do to read the signal from a hydraulic pressure transmitter (such as, SETRA C206 Pressure Transmitter 24VDC, 0-7500 PSIG, 4-20mA) and display it on a load cell digital indicator. This is to use it as a scale on an industrial loader.

All the load cell displays I've seen can drive 1 or more 350Ω load cells, but the pressure transducers seem to be 800Ω. Is it possible to 'plug' a pressure transmitter into a load cell display and get it to work? The reason why I want to use a load cell display is because they're cheap on eBay, less than $100(and less than $100 for the pressure transmitter), whereas complete loader scales are $700 and up.

I'm just a dumb farmer and while I can happily plant and grow a couple of thousand tonnes of wheat, lupins, canola, etc. I just get lost when I try to work out electronics! It frustrates me that this would seem to be a simple problem for someone with probably basic knowledge, but it's beyond me.

Any help would be greatly appreciated. Even if someone can just explain the Ohm difference between the 2 and what it affects.

Thanks

ps. Yeh, it's taken me 5 years to make my first post. You can't rush these things you know..... ;-)

Click to expand...

Reading your post again and trying to read into the detail I am missing something here. For example:

All I want to do to read the signal from a hydraulic pressure transmitter (such as, SETRA C206 Pressure Transmitter 24VDC, 0-7500 PSIG, 4-20mA) and display it on a load cell digital indicator. This is to use it as a scale on an industrial loader.

Click to expand...

The device you linked to is a pressure transducer, while I did not find a 7,500 PSIG version I found plenty made by Setra along these ranges:

0 to 25, 50, 100, 250, 500, 1000, 3000, 5000, 10000 psig

Click to expand...

Most of that matters not anyway but what matters is when you mention:

This is to use it as a scale on an industrial loader.

Click to expand...

That right there has me questioning things. Here is why. If I want to use pressure to give me the weight of a load there is more to it. Actually, much more to it. If I take a hydraulic cylinder with a piston in it and set it vertical with a large platen and begin to pile wheat on it then gravity will obviously act upon the wheat. The cylinder is filled with oil. If the area of my piston is exactly 1 Sq. In. then things are fine. If I place 5,000 pounds of wheat on the platen the gauge will read 1,000 PSIG. However, what if the piston area is 2 Sq. In.? That same gauge will now read only 500 PSIG.

Now let's say my cylinder has a stroke. With the cylinder empty I place 1,000 pounds of wheat on the platen. My piston has a 1 Sq. In. diameter. I begin to pump oil into my cylinder and using a pressure gauge measure the pressure as I pump. The platen will begin to float when my pressure is 1,000 PSIG. However if my piston diameter is 2 Sq. IN. then I will only need 500 PSIG to float with a 1,000 pound load. When equilibrium is achieved, the pressure value is known, it being a direct result of the sum of the forces from the weights, piston platform and the piston divided by the effective area of the piston and cylinder assembly.

So if you are trying to measure weight as in a scale weight you are not going to easily get there using a pressure gauge transducer. Thus enter the load cell which will get you where you want to go.

This all assumes I am not over reading into your initial post.

Ron

Hey, Ron. Cool Thread. I was just reading this off the net. Have a look.

killivolt said:

Hey, Ron. Cool Thread. I was just reading this off the net. Have a look.

Click to expand...

That was a nice find. Wish the hell I would have thought about using Google before I typed all that stuff!

Years ago I did quite a bit of work with dead weight testing for extremely accurate measurement of pressure. Precision weights, precision pistons, precision cylinders and more precision than I care to remember.

I just happen to get to thinking about the first post and the thread and the wording had me thinking a little more. Hey, Easter Weekend so I have time to think and drink beer. Anyway, after reading the first post I got to thinking about things. Maybe I am missing something or reading too much into it but something just did not make sense to me. The reference to pressure in PSIG and scale just didn't click right and thus my post.

If we want to use a pressure transducer and measure pressure it matters not what it outputs be it mV, volts, or for that matter mA it can be scaled and converted to engineering units of PSIG. The same is true of a load cell using a strain gauge bridge. Just becomes a new animal the way I read things.

Again, nice link.

Ron

I used to work with pressure gauges and such. I also have a fundamental view of how hydraulics work. But, I just couldn't put it together in my head. I didn't even know what a load cell was / transducer is a general term but, just how it functions with a Hydraulic cylinder was beyond my ability to see it in my mind.

So, I went hunting.

Thanks, kv

G'day Guy's,
Yea this is a cool thread Personally I could use a couple of these on my farm equipment and the only way to go would be using a pressure transducer. Say if one put the PT on the lower entry to the lift cylinders there would be some friction with the seals etc. But once the bucket is stationary the load could be measured, I did look at load gauges for the forklift at my old work and the price was beyond sensibility.

It would be a cool project for a DIY load gauge one could fit onto their own machinery without breaking the bank..........

Cheers Bryan

Ok Ron, let me give some more details. I have an 11 Tonne loader with a 3 tonne lift capacity,that uses 2 x 5 inch diameter rams( That's ~39 Inches2 total area and ~ 98,000lb total pushing force). The hydraulic system works at 2500psi. The way that I understand the commercial scales work, using a pressure transmitter is this;

1. the transmitter is plumbed into the hydraulic circuit on the pressure(lifting) side of the circuit and therefore ram. If its a 4-20mA, 5000psi transmitter then 4mA = 0psi and 20mA= 5000psi. This would be entered into the display.

2. to get a accurate weight, you must first calibrate and then weigh the load at the same lift point each time. This is critical as due the the geometry of the loaders rams and arms, the pressure for a given weight will change as you lift through the range of the loaders lift as the geometry will change. My loader has a height indicator on it, so to first calibrate it, I would make a mark on this, then I would need to lift slightly above this mark, lower down to it(this releases any extra lifting pressure, to give the actual weight pressure) then zero the display. This gives the first datum for the linear conversion scale.

3. To get the next point you would pickup the known test weight(which should be near the maximum lift weight), lift the loader slightly above the weighing point then drop down to your marked point. This give you your second datum point. As you know the weight, you enter this into the display to calibrate it.

So, lets say with the loader empty and just the weight of the arms, bucket etc. you get a pressure reading of 500psi. Then with a know weight of 3000kg in the bucket you get a pressure reading of 1500psi. That's a range of 1000psi, so as the range is linear that means with a 1500kg weight you would get 1250psi (500psi starting pressure + 750psi for weight) for 1000kg you would get 833psi(500psi starting pressure + 333psi for weight) and so on. To convert psi to kg's ;

A= 500 ( pressure with no load)
B= 3000/(1500-A)= 3(test weight divided by it's pressure reading less start pressure)
C= 1200psi (Unknown weight's psi reading example)

So,

(C-A)*B = Unknown Weight
(1200-500)*3 = 2100kg

As you are weighing at the same lifting point each time, the relationship between the psi on the rams and weight in the bucket is linear. Therefore these two points give the calibration for the display. This is how all the cheaper commercial loader/forklift scales work, such as here; **broken link removed**

I guess I should just buy the one above, but that's like buying a electronics kit pre built, putting it together and getting it to work it all the fun! (though mind usually don't work )

I hope this explains better what I am trying to do. I know have mixed transmitters and transducers which has probably confused things as transducers are millivolt based and transmitters are mA or 1-5Vdc etc. What is the difference between a pressure transducer and a transmitter

Hi Paulmac,
I did go look on Frugle today and there is NO cheap DIY solution I just missed missed you in chat bugger. get back into chat mate and get your post count up too.....

Cheers Bryan

Hi paulmac & bryan

Well as we can see thus far, including Bryan doing some homework, problems like this do not have an economical or simple solution. I had a feeling this was about a scale configuration to measure weight.

Generally for an application like this a load cell would be the choice of transducer. The signal out would be signal conditioned and scaled using an indicator device. Sometimes the use of two or more load cells is required.

The use of a pressure transducer can be incorporated but as we have seen this get's very complex. Along the lines of keeping it simple:

1. the transmitter is plumbed into the hydraulic circuit on the pressure(lifting) side of the circuit and therefore ram. If its a 4-20mA, 5000psi transmitter then 4mA = 0psi and 20mA= 5000psi. This would be entered into the display.

2. to get a accurate weight, you must first calibrate and then weigh the load at the same lift point each time. This is critical as due the the geometry of the loaders rams and arms, the pressure for a given weight will change as you lift through the range of the loaders lift as the geometry will change. My loader has a height indicator on it, so to first calibrate it, I would make a mark on this, then I would need to lift slightly above this mark, lower down to it(this releases any extra lifting pressure, to give the actual weight pressure) then zero the display. This gives the first datum for the linear conversion scale.

3. To get the next point you would pickup the known test weight(which should be near the maximum lift weight), lift the loader slightly above the weighing point then drop down to your marked point. This give you your second datum point. As you know the weight, you enter this into the display to calibrate it.

So, lets say with the loader empty and just the weight of the arms, bucket etc. you get a pressure reading of 500psi. Then with a know weight of 3000kg in the bucket you get a pressure reading of 1500psi. That's a range of 1000psi, so as the range is linear that means with a 1500kg weight you would get 1250psi (500psi starting pressure + 750psi for weight) for 1000kg you would get 833psi(500psi starting pressure + 333psi for weight) and so on. To convert psi to kg's ;

A= 500 ( pressure with no load)
B= 3000/(1500-A)= 3(test weight divided by it's pressure reading less start pressure)
C= 1200psi (Unknown weight's psi reading example)

So,

(C-A)*B = Unknown Weight
(1200-500)*3 = 2100kg

Click to expand...

That would pretty much be about it. However, I am not a guru with everything involved with a loader, I admire the things.

I would guess that if we placed a standard amount of weight in the bucket and lifted it to a point and stopped we would read a pressure. Then if just sitting there we add more weight the pressure as measured at the cylinder would increase. When all was said and done I am not sure how accurate we would be but the above method would likely work. I am just not sure of the variables.

The link is interesting and a Google of the part number brings up plenty of hits like this one. The price on the linked unit was actually pretty good as it is not unusual for me to spend about $1,000 for a Honeywell Sensotec 10,000 pancake load cell offering tension and compression outputs. I use plenty of these creatures.

Overall I guess using a pressure transducer over a load cell can be done. Calibration could be achieved and it all depends on what someone wants to spend.

Ron

G'day Paul and Ron,
In my research yesterday I did find all the commercial ones used pressure transducers and one simply taps in at the junction point to the lower bucket cylinder lower hoses. Now using one of **broken link removed**
it would be an easy case to hook it upto a pic and have either some 7 segment leds or a glcd to display the data. To make the calculation easier one could incorporate a push button to take the load measurement after the bucket has settled with the load.

As that ebay store has said they are on holidays it will give me a bit of time to sort some $$$ to get some of those transducers so I can start playing with them. All in all this will be a challenging project but the main goal will be a cheap load gauge farmers can use to measure loads.

Regards Bryan

Hi Ya Brian

Not bad at all for under $45 USD. Very little too. For the price I would give it a shot. Nice job of homework too.

Ron

Good one bryan. Add another un-usaual project to your list of accomplishments.

(DIY) lift, loader, Transducer measuring. Well Um, you know - thingy.

Bryan,

Sounds great. I checked on my loader and it has a 1/4" port on the lift side that I can screw the transducer straight into. On the forklift I'll have to put a tee in, but that's easy( Hydraulics I'm good with.. just not electronics) ;-)

The commercial displays have a zero, tare, total, and print buttons, but as we just want to get a 1 off weight, it will just need a ZERO button ( and maybe a HOLD button like you suggested), to zero the empty bucket before picking up the load. I see no reason why this should be any less accurate that the 1-2% accuracy of the commercial systems like this one; **broken link removed** , as we're measuring using the same technique and hardware. If I can get within 30-60kg on a 3 Tonne bucket of fertiliser /grain, I'd be rapt.

From the little I understand, it would seem that a transmitter is less affected by magnetic/electrical interference (and cable length) than a transducer, but if the commercial ones use a transducer, then I guess that's all we need.

Well, I'm EXCITED Bryan. After thinking about this for so long it's great to find someone with the brains to get it to work. Look forward to hearing how you get on.

Keep us upon how this progresses. Should be cool.

Ron

Paulmac! I make fork truck scales **broken link removed**, for a living if you need any information I'd be happy to help you. I use 0-5v output pressure transmitters ( they cost me around £40 ).

The electronics is minimal you just need a 0 to 5v LCD module and move the decimal place so it reads "500 kgs" max (they are auto zeroing as well)

Cheers Ian