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Fasttrack
02-01-2010, 04:12 PM
Does anyone know of a hydrualic pressure transducer on the cheap? I've found plenty of gas pressure transducers, but I need one that can plug directly into a hydrualic system without any nitrogen buffer, etc and measure the pressure directly.

Range would only have to go to 1000 psi, max. (Normally in the 100-500 psi range).

macona
02-01-2010, 04:19 PM
You might look at a capacitance manometer. MKS has ones that will go way past where you want to read and I can think why they wouldnt measure liquid as well as gas. Its just pressure deflecting a diaphragm. Maybe mount it upside down to get the air out.

beanbag
02-01-2010, 05:18 PM
I think just the regular cheapo pressure gauge that has a round dial will work. It is the bourbon type pressure gauge. just gotta get the air out. You might want to pay a touch more and get one from a reputable company like wika, where they have a datasheet for the gauge that explicitly says it can measure fluids, and says what the insides are coated with in case there is a possibility of corrosion. U can also google for hydraulic pressure gauge.

Unless you really mean transducer - what do you want to convert the pressure into?

edit: never mind, I'm stupid

Fasttrack
02-01-2010, 05:30 PM
I think just the regular cheapo pressure gauge that has a round dial will work. It is the bourbon type pressure gauge. just gotta get the air out. You might want to pay a touch more and get one from a reputable company like wika, where they have a datasheet for the gauge that explicitly says it can measure fluids, and says what the insides are coated with in case there is a possibility of corrosion. U can also google for hydraulic pressure gauge.

Unless you really mean transducer - what do you want to convert the pressure into?


I really want a transducer :) I would like to convert the variable hydraulic pressure from a brake system into a voltage signal. I've been looking at some transducers on ebay.

Evan
02-01-2010, 05:38 PM
Make a pressure divider and then you can use an ordinary oil pressure transducer from a car.


Rough drawing, add O-rings on the pistons as required. Pressure at the transducer is of course proportional to the areas of the pistons.

http://ixian.ca/pics7/pressure.jpg

EddyCurr
02-01-2010, 05:38 PM
Does anyone know of a hydraulic pressure transducer on the cheap?What kind of budget is there, what pressure capacity is required and
what signal voltage is desired?

I have engine oil and brake fluid pressure transducers I use for a data
logging application - their output is 0-5VDC. At $250, they may not fit
the criteria for being inexpensive. However, there may be suitable
OEM units that can be adapted for your purpose - if so they will be
available from an auto wrecker ...

Another source may be Omega (http://www.omega.com/).

Edit: After looking at prices on the Omega site, they may not be a
suitable resource.

.

EddyCurr
02-01-2010, 05:42 PM
I would like to convert the variable hydraulic pressure from a brake
system into a voltage signal.Saw this after posting. Looks to me like moderately high pressures in
a critical application.

.

Weston Bye
02-01-2010, 05:57 PM
Evan's suggestion has some merit, except that the piston o-rings will have some "stiction" and will introduce some response lag, as will the added inertial mass of the piston. Care would have to be taken to eliminate any air between the low pressure side and the low pressure transducer, as the compresability of any air would also slow response.

Still not a bad idea, I may make use of it in a low speed application.

beanbag
02-01-2010, 05:58 PM
I really want a transducer :) I would like to convert the variable hydraulic pressure from a brake system into a voltage signal. I've been looking at some transducers on ebay.

after my initial post, I was bothered coz it seemed like too easy of a question. Anyway, for approx $100 or less, you can buy various (brake) hydraulic sensors from honeywell, bosch, etc. Digikey sells a bunch, see here for example

http://search.digikey.com/scripts/dksearch/dksus.dll?pname&site=us&lang=en&wt.mc_id=Dxn_US_US2010_Catlink&name=734-1044-ND

Browse their catalog plus minus a few pages to get an idea.
Just pay the damn $68 and be done with it - don't build additional crap. It's research money, right?

If you are super cheap, use your "pulls needles off of a dial" contraption on the bourbon gauge and rig a pot or some kind of rotary sensor to it. Ya cheapskate.

Evan
02-01-2010, 06:00 PM
A return spring on the low pressure side should alleviate any stiction problems or lag.

John Stevenson
02-01-2010, 06:09 PM
What about this ?

http://cgi.ebay.com/CEC-0-1000-PSI-Pressure-Transducer-28V_W0QQitemZ300390931531

Or is it too expensive ?

.

beanbag
02-01-2010, 06:10 PM
P.S. For brake pressure datalogging, I heard that people like to use sensors up to 3000 psi

Weston Bye
02-01-2010, 06:26 PM
A return spring on the low pressure side should alleviate any stiction problems or lag.

Stiction is a fact of life in any o-ring application, as the o-ring at rest works to displace any lubricating film between the o-ring and the wall of the cylinder, if the o-ring remains at rest long enough. The o-ring in motion rides up to coast along on a very thin film of oil and presents less resistance to motion. This is also true of hydraulic cylinder seals, and many designs have been evolved to minimize the phenomonon. The pressures in hydraulic actuators tend to relegate stiction to an inconsequential element. Pneumatic cylinders are notorious for erratic motion due to stiction.

To be sure, there are many piston-type high pressure switches and transducers, but diaphragm type sensors provide better response. Adding a return spring has no effect on stiction other than helping overcome it. The spring will also introduce a pressure measurement offset that must be taken into account.

This may all be moot as Beanbag has come up with a reasonable solution.

Weston Bye
02-01-2010, 06:29 PM
What about this ?

http://cgi.ebay.com/CEC-0-1000-PSI-Pressure-Transducer-28V_W0QQitemZ300390931531

Or is it too expensive ?

.

It says used. Used or used up?

strider
02-01-2010, 07:17 PM
Fasttrack,
A couple of things. First, define cheap. What accuracy do you need, cheap and accurate are mutually exclusive. As for materials compatability, most good xdcrs have stainless wetted materials, no problem with your hydraulics. Very low pressure xdcrs/xmtrs are typically the devices that can't handle liquids. However, if it's not a welded diaphragm (data sheets don't always tell you that) then the manufacturer is using a seal of some sort, typically an O-Ring in the cheap units, may not be compatible. Will the pressure be pulsating or suddenly increasing or decreasing? That will really mess with some units. You'll need to install a snubber or dampener also. As for a little air entrained in the cavity of the sensor, at 1K psi it will introduce some error, but that's something that can also be handled, but most users don't worry about it . Don't forget you can take a current loop unit, i.e. 4-20 mADC, and measure the voltage drop across a resistor. From your dearth of detail, almost any unit in your pressure range would work for you, for a moment or two.

Fasttrack
02-01-2010, 07:23 PM
LOL Beanbag - this one is coming out of my own pocket. As a broke college student, I am a cheapskate :D

I asked because I honestly didn't know where to look for a transducer. Evan's idea also crossed my mind but since this is for a brake system, I felt uncomfortable making any extra devices that weren't absolutely neccessary. I'm not sure what happens if motor oil contaminates brake fluid (probably not much of anything) but I didn't want to risk it (if I don't have to).

John - I actually bid on a similar one from the same seller. I believe them to be in good working condition, but its hard to say for sure. I figured I'd test them out before I get to far along in the project.


For those who are interested, I'm making a brake controller for a trailer. I've got all of my electrical components ordered from Digikey, actually. I didn't think about getting the transducer there, also. The controller will output a pulsed signal at "full lock" so it is almost like having anti-lock brakes on the trailer. It will also have a manual over-ride toggle in case any of the electronics or p/u brake failure. The transducer will be on the back of the split system, so if anything in my design goes bad, it will result in the loss of rear brakes, vs loss of the front.

Actually ... I wonder what would happen if I put the transducer after the RWAL unit in my pickup... then when my anti-lock brakes for my rear axle kick in, the pressure will fluctuate resulting in a pulsed signal from the transducer that would mimic (with lag) the anti-lock behavior of the rear brakes.

Of course, this isn't really for driving in snowy conditions but rather for emergency braking. Most of you have probably had first hand experience when your trailer brakes lock up solid with a heavy load. The trailer just slides around all over the place and it seems like ABS during emergency braking could reduce this tendency.


When all is said and done, I should have a schnazzy brake controller all for about $6 plus the cost of the transducer. Now the only question is ... will it work? :eek: :)

Fasttrack
02-01-2010, 07:27 PM
Fasttrack,
A couple of things. First, define cheap. What accuracy do you need, cheap and accurate are mutually exclusive. As for materials compatability, most good xdcrs have stainless wetted materials, no problem with your hydraulics. Very low pressure xdcrs/xmtrs are typically the devices that can't handle liquids. However, if it's not a welded diaphragm (data sheets don't always tell you that) then the manufacturer is using a seal of some sort, typically an O-Ring in the cheap units, may not be compatible. Will the pressure be pulsating or suddenly increasing or decreasing? That will really mess with some units. You'll need to install a snubber or dampener also. As for a little air entrained in the cavity of the sensor, at 1K psi it will introduce some error, but that's something that can also be handled, but most users don't worry about it . Don't forget you can take a current loop unit, i.e. 4-20 mADC, and measure the voltage drop across a resistor. From your dearth of detail, almost any unit in your pressure range would work for you, for a moment or two.

Cheap means <$100. Accuracy is of no concern at all. The output signal will be calibrated based upon a trailer brake system - that is, the output signal won't be "measuring" pressure, it will just be turning it into an electronic signal that will reproduce a similar degree of braking action in a trailer with electric brakes.

Pressure will change quickly - in some cases very quickly and may also pulse quickly. I hadn't thought about that hurting the unit. Even if the unit has some small lag or "averages" the pulsing hyrdaulic pressure, that will be acceptable. I have designed a circuit that allows me to tweak the signal a great deal.

JanvanSaane
02-01-2010, 07:50 PM
I am not sure how it would work on the low end but a common rail diesel injection system uses a pressure switch in the rail to tell the computer how much pressure is in the fuel rail. The lowest pressure I have seen one one register is about 1500PSI. Makes sense they would pick up lower pressure but I don't know for sure. john

Evan
02-01-2010, 08:44 PM
Take a flexible brake line with a stainless steel braid jacket. Insulate it with a length of heat shrink tubing. Slide it into a length of stainless tubing that just fits snugly over the hose. Connect wires to the braid and the tube. You now have a capacitance sensor that will change value as the hose bulges under pressure. It works and is stable, I have used a similar device in another pressure application.

bill jones
02-02-2010, 07:38 PM
-I happen to have a few zero to 800psis transducers which are described here:
-----------------------------------------------------------------
ZERO TO 700psis SENSORS
- Texas instrument #101HP2-28, JD #AT228298, SN 19-03-06-1090 zero to 700psis sensors.
-psis apparently means that the sensor refers to a standard atmospheric baseline of 14.7psia.
-has a 3 wire roundish oval shaped connector on the end of about 6" worth of wires---to where you could cut the connector off and
use common GM weather pack connectors.
-the fitting end is a #4 AN O-ring-----which is 7/16-24 thread and needs a recess for the O-ring.
-if obtaining a fitting is going to be a problem I can supply a steel or a brass adapter fitting for $20----to connect into a female 1/8 npt
hole.
-tested this at 4200 elevation where the barometer is about 12.6psia
-I have GM weather pack connectors, pins and the rubber seals.

0 = .51volts
5# = .53
10 = .56
20 = .61
30 = .67
40 = .72
50 = .78
60 = .84
70 = .89
80 = .95
90 = 1.00
100# = 1.06

100# = .98
200 = 1.54 (these red numbers are using a different gauge and an argon welding bottle with a bypass type
300 = 2.10 regulator-----where the inlet was restricted with a .014" hole}
400 = 2.69
500 = 3.19
600 = 3.71
700 = 4.30
800 = 4.84

-I have 8 only at $25 each
----------------------------------------------------------------------
-I also have a pretty good selection of other ranges that are shown here:

http://ryanbrownracing.com/Bill_Jones_Page_29.html

-to buy any of these there is a couple of stipulations listed in there also.
-------------------------------------------------------------------
-The zero to 800psis photo is the one with the green triangled wire connection end and the stainless steel housing.
----------------------------------------------------------------------
-If there is any interest here I may also have fairly simple plans to provide the 5VDC power for the input--but I'll have to look around and see if I can find'm.

A.K. Boomer
02-02-2010, 09:41 PM
Take a flexible brake line with a stainless steel braid jacket. Insulate it with a length of heat shrink tubing. Slide it into a length of stainless tubing that just fits snugly over the hose. Connect wires to the braid and the tube. You now have a capacitance sensor that will change value as the hose bulges under pressure. It works and is stable, I have used a similar device in another pressure application.




Now see, This is the Evan I love,,, (why don't we have those little faces with the hearts around them? I asked for them along time ago:p )

No chance of fluid exchange Fasttrack ---

This is the part of Evan's thinking where you skip that nasty little first word and just flat out call the guy a savant --- still - keep in mind you don't want any vibrations so don't drive on bumpy roads or run the engine - also, only drive in the temperatures that you initially calibrated the unit for OR YOU COULD BE KILLED!

Fasttrack
02-02-2010, 09:53 PM
Now see, This is the Evan I love,,, (why don't we have those little faces with the hearts around them? I asked for them along time ago:p )

No chance of fluid exchange Fasttrack ---

This is the part of Evan's thinking where you skip that nasty little first word and just flat out call the guy a savant --- still - keep in mind you don't want any vibrations so don't drive on bumpy roads or run the engine - also, only drive in the temperatures that you initially calibrated the unit for OR YOU COULD BE KILLED!


I was concerned not only with the vibrations but with the plausibility of hermetically sealing the variable capacitor against road salt/grime/etc. Certainly not impossible (after all many transducers are nothing more than variable capacitors) but if I can get a pre-made unit, I would prefer to do so.

Also, we are talking about fairly small capacitances here. Apart from accurately measuring a minute signal, I was also concerned with the effect of mud/etc caking up on the outside tubing. If you think that won't matter, try it! It will make a difference :eek:

Again, this would be a very good idea in many applications, but for this application, I would prefer a premade unit.

I hadn't considered the change in temperature ...

Evan
02-02-2010, 10:26 PM
The hose may move around slightly inside the tubing but it won't change the capacitance at all no matter how it vibrates or moves. Think about it. For any part that moves away another part moves closer by the precise same amount. The capacitance only changes when the hose bulges. It makes a very clean signal. Temperature won't make much difference either since the braid and the tube are the same material and have the same CLE. Sealing the ends of the tube is trivial.

As for dirt on the outside, seal the assembly inside a piece of heater hose. I don't know why you are so concerned about the sensor when the rest of the system is just as critical and you are building that.

Fasttrack
02-02-2010, 11:28 PM
The hose may move around slightly inside the tubing but it won't change the capacitance at all no matter how it vibrates or moves. Think about it. For any part that moves away another part moves closer by the precise same amount. The capacitance only changes when the hose bulges. It makes a very clean signal. Temperature won't make much difference either since the braid and the tube are the same material and have the same CLE. Sealing the ends of the tube is trivial.

As for dirt on the outside, seal the assembly inside a piece of heater hose. I don't know why you are so concerned about the sensor when the rest of the system is just as critical and you are building that.

Yes but I have greater confidence in my ability to produce the other components. ;)

Sure vibration doesn't matter, so long as the dielectric doesn't deform. Like I said, I'm not arguing whether or not it would work. It's just that I found a pre-made pressure transducer on ebay for 99 cents ;)

A.K. Boomer
02-02-2010, 11:57 PM
The hose may move around slightly inside the tubing but it won't change the capacitance at all no matter how it vibrates or moves. Think about it. For any part that moves away another part moves closer by the precise same amount.

Sorry - not buying it, if the inside tube had a spring loaded spiral wind then I can see it being much closer to your statement, but due to the nature of this inside tube slipping into the other in the first place it means that there is some fair amount of clearance -- this buy the way will do nothing but get more as time goes buy - not only that - even with slight clearance the inside tube can seek an equilibrium with certain frequencies (vibrations) - (esp. if installed in a vertical position )


Temperature won't make much difference either since the braid and the tube are the same material and have the same CLE. Sealing the ends of the tube is trivial.

Temperature won't make much of a difference?
Didn't you just say "assemble with heat shrink tubing"?



As for dirt on the outside, seal the assembly inside a piece of heater hose. I don't know why you are so concerned about the sensor when the rest of the system is just as critical and you are building that.


Sometimes its worth leaving stuff like this up to entire companies who actually do nothing but build this stuff,,, it usually ends up allot more practical instead of having all kinds of crap hanging off your vehicle that either gets snagged on something or has so many quirks its just not worth it ------- Im just sayin....

wierdscience
02-03-2010, 12:06 AM
Cheap means <$100. Accuracy is of no concern at all. The output signal will be calibrated based upon a trailer brake system - that is, the output signal won't be "measuring" pressure, it will just be turning it into an electronic signal that will reproduce a similar degree of braking action in a trailer with electric brakes.

Pressure will change quickly - in some cases very quickly and may also pulse quickly. I hadn't thought about that hurting the unit. Even if the unit has some small lag or "averages" the pulsing hyrdaulic pressure, that will be acceptable. I have designed a circuit that allows me to tweak the signal a great deal.

What about $125?

http://www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Pressure_Sensors/Pressure_Transmitters/PTD25-20-3000H

beanbag
02-03-2010, 12:10 AM
The hose may move around slightly inside the tubing but it won't change the capacitance at all no matter how it vibrates or moves. Think about it. For any part that moves away another part moves closer by the precise same amount. The capacitance only changes when the hose bulges. It makes a very clean signal.

The capacitance of two non-coaxial tubes is not independent of the offset. I don't know what is your method of measuring capacitance, but I had a very similar setup, and it was very susceptible to triboelectric effects (vibration).

But then again, I was measuring a femto farad signal and the coaxial capacitance was 10-100 pico farads :p

Evan
02-03-2010, 12:25 AM
Triboelectric effects are electrostatic charging effects produced by friction. A sealed and bypassed metallic container such as a tube within a tube that is bypassed with a meg or so will not build up an internal differential charge at all. Both electrodes will be at the same potential on average. Triboelectric effects are at the heart of how a phtocopier works as well as differential voltages between charged insulators. (see electret)

However, it's hard to beat 99 cents.