|Pressure transducers used in industry today take on a rather typicalappearance.|
A round, tubular stainless steel body with a pipe fitting on one endand a cable coming out of the other end.
Stainless is used because of its high strength and resistance to corrosion.
What is inside?Starting at the pipe threaded end, the opening or port has a stainlesssteel diaphragm inside that protects the sensor element from the mediabeing measured (i.e. water).
As we continue our journey through the transducer we come to the otherside of the diaphragm where one side of the sensor element is. The actualelement is a strain gauge; that is, a resistive element whose resistancechanges with the amount of strain placed on it. This variable resistorforms one leg of a bridge circuit. The other side of the strainelement is the reference port that the measuring port is compared to.All transducers have two sides; sometimes the other side has its own pressureconnection and the device is called a differential pressure transducer.
The insides of a modern pressure transducer
Further on in our voyage, we come to small circuit board.
The two voltage out points from the bridge circuit are fed to an amplifierthat changes the very small voltage into a 0-5V signal or most commonlyto a 4-20 mA signal. This signal is fed out the cable (sometimes alongwith a vent tube) which finishes our voyage.
Standard Pressure Units and ConventionsTo communicate the amount of pressure requires the use of an agreed onscale and convention. Just as the conventions and units for temperatureare referred to as Fahrenheit, Centigrade or Kelvin; industry uses PSID,PSIA,PSISG and PSIVG notation for pressure.PSI (Poundsper Square Inch) is the pressure unit while the last letter(s)refers to the convention used to measure the pressure. You would use PA(Pascels) units instead of PSI in the metric system.
Four Pressure Conventions
By nature, pressure is the difference between two points or ports.Pressure between two ports (or pipe connections) is differential pressure.A designation of 10 PSID means 10 Pound per SquareInchDifferential.
If one of the ports is a vacuum, it's convention is designated absolutepressure. When you see 30 PSIA, it means 30 Pounds perSquareInchAbsolute.
Connecting one of the ports to atmospheric pressure lets us measurewhat is called gauge pressure. This is the scale most water peoplewill see because it easily converts to "Feet of Water". You will see itprinted on a pressure gauge as PSIG.
To complicate matters further there are really two different gauge pressureconventions; sealed gauge and vented gauge. Sealedgauge pressure measurements are made by connecting one of the pressureports to a sealed chamber that has atmospheric pressure in it (writtenPSISG). The problem with a sealed gauge pressure scale is, that the samenumber of "Feet of water" will read differently depending on the localbarometric pressure. This can cause readings to fluctuate as much as 4feet of water! If the pressure units are noted as PSIG (PSI gauge), itis a good bet that it really reads PSISG (PSI sealed gauge) instead ofthe better PSIVG (PSI vented gauge).
An example of how the sealed gauge convention can cause problemsmay help. Lets imagine a clear well 10 feet deep. Imagine further, thatwe calibrate a stop point one foot from the top on a high barometric pressureday. On the next low pressure day the clear well will over flow!
The solution is simple-use the Vented Gauge convention(writtenPSIVG). The vented gauge conventioncompares the pressure to a local "vent" open to local atmospheric pressurethus eliminating any changes with barometric pressure changes.
Transducers are built to measure only one convention at a time (witha few exceptions). When purchasing a transducer you must be sure to orderone that will measure using the correct convention for your purposes. Inthe water industry that will almost always be PSIVG. A submersiblevented transducer is easily recognized by the small tube that is in thecable with the wires.
One other note. Some manufactures will claim that the jacket of theirsubmersible cable will act as a vent tube. The reality is if the fittingthat keeps the water from entering around the cable entrance is tight enoughto keep out water it also restricts air movement through the cable.
At best this is a vented gauge device with lots of hysterisis; at theworst it is a sealed gauge device. (If this is a non-submersible transducerwith only a foot or two of cable it's not a problem.)
What PSIG means in practicePSIG, as an industrial standard notation, is supposed to mean PSIVG. Butunfortunately companies put PSIG on their transducers when it is a 'sealed'type. A sealed gauge unit compares the port pressure to an average airpressure while the vented type compares the port pressure to local airpressure via the port. Always ask if the transducer is of the vented orsealed type before buying.
Transducer Electrical OutputThe output of a transducer is naturally a voltage. Most water applicationsconvert the voltage to a 4-20 mA current loop. This provides a good loss-less,low impedance connection to control equipment. By sending the signal asa current there is no error from voltage drop introduced from the resistanceof the wire what ever the length is.
Most applications call for a 0-5 VDC, 4-20 mA or a mV output. But ifnecessary, transducers can be ordered in, 0-10 VDC, 0-2.5 VDC, 0-100 m,ratio metric mV/V (custom ranges are available at a price).
4-20 mA OutputTo connect a 2 wire 4-20 mA transducer take the + (red) lead of the transducerand connect it to the + terminal of the power supply. Connect the - (black)lead of the transducer to the + input terminal of the readout. Connectthe - input terminal of the readout to the - terminal of the power supply.
0-5 Vdc outputTo connect a 3 wire Vdc output transducer connect the - excitation (black)lead of the transducer and the - terminal of the power supply to the -input terminal of your equipment. Connect the + lead (red) of the transducerto the + terminal of the power supply. Connect the signal lead (white)of the transducer to the + input terminal of your equipment.
What does 4-20 mA mean?
Current vs. PressureThe term "4-20 mA" literally means 4 to 20 milli-amperes. A signal of zerowould be sent as 4 mA. For example, if there was no water in your tower,or you had the transducer disconnected, the transducer would transmit 4milli-amperes written as 4 mA. The 4 mA current represents zero gauge pressure.By sending a zero pressure as 4 mA instead of 0 mA we avoid confusing abad connection (0 mA) with zero pressure (0 mA). A Full ScaleOutput(FSO), as in 100 PSI applied to a 100- PSI transducer would cause 20 mAof current to flow.
One more example just for drill:
How much current would flow if we apply halve of full scale or 50 PSIto a 100 PSI transducer?
The current ranges from 4 to 20 mA or a 16 mA span. Halve of 16 mA is8 mA plus the 4 mA at zero gives us 12 mA.
Reverse SignalFor some applications, it is important to know how far the water is fromthe top of the tank or the surface of the ground. If specified by the customer,the factory can set the transducer so that zero pressure reads full scaleelectrical output and maximum pressure reads zero output.
Accuracy and Resolution
The accuracy of transducers is usually quoted as plus or minus a percentof full scale output ( 1% FSO). Using this 'tolerance' we can calculatethe maximum amount of error in the measurement. For example:
A 100 PSI transducer with a 1% FSO rating would read within 1 PSI orabout 2 feet of water.
There are two kinds of tolerance: tolerance due to changes in temperature(thermal error) and tolerance at a given temperature (static accuracy).
It can be seen, by the above example, that over a wide temperature rangethermal error will dominate the total error.
Do you need accuracy or resolution?If resolution is all you need for repeatability don't over specify. Ifyou are doing level control and use a vented gauge transducer there islittle reason to look at anything other than thermal error (which you willfind won't matter in almost all level control applications). (Most controlequipment is usually not much better than 1% )
Also remember that a submersibles are mostly used at small depths --10-20' and 1% of 20' is only 2.4"!
Suspension and Mounting Pressure Transducers
Ensure no damage occurs to the cable as cable damage represents themost frequent cause of transducer failure. Good cables are quite flexibleand care needs to be taken when bending the cable that you do not crimpthe vent tube inside. Do not bend the cable tighter than a 1 inch radius.
If you require a compression fitting to secure a Tefzel or polyurethanejacketed cable as it enters a junction box don't use more than 15 ft/lb.Or you may damage the cable or pinch the vent tube.
One of the better ways is to suspend the submersible transducer in aperforated 1 1/2" or 2" PVC pipe or attach the transducer (using a optional1/2" M NPT fitting) to a rigid conduit. Submersible pressure transducerscan be attached to a rigid conduit and the cable run through the conduit.Submersible transducers can be fitted with a 1/2" NPT male conduit fittingwhere the cable exits the transducer. This fitting then mates with standardrigid conduit. You could also use an optional bracket to clamp the transducerto a fixed object (i.e., wall, ladder, step). Suspending the transducerwithout any protective pipe or attachment device can invite damage.
Installers often use cable hangers (Chinese finger grips) used for hangingelectrical cables (available from most electronic supply stores). The cablehanger slides onto the cable and can be positioned anywhere on the underwatercable by pushing the ends together. Once positioned, the cable hanger providesa snug grip on the underwater cable. When mounting the transducer in awell casing, the cable hanger can be secured to a eyebolt attached to thewell plate or the side of the well casing. The cable hanger loop is thensecured to the eyebolt. In still wells, the loop-end of the cable hangercan be attached to an eyebolt mounted in the still well shelf.
Although it is possible to screw a transducer directly onto a pipe,it may not be a wise decision. For an example, in water systems you canthink of the water pipes as the best ground available to the power gridwith miles of buried metal pipes. Also think of a spherical shaped watertower. Your physics teacher would tell you that a water tower resemblesa Van de Graaff generator in its shape and ability to build up a largestatic charge before dramatically discharging. Currents of up to 200,000Amperes are not uncommon. The local ground potential will change dramaticallycompared to the electric utilities ground. Even if the transducer survivesit may be bad news for the control equipment that it is attached to. Ifthe pressures and media allow you to, it is best to connect using a nylontube of several feet (to provide a high impedance water channel). Placethe transducer body in a short length of PVC pipe to insulate it from 'groundsources'. The most important thing to remember in applying transducersis to avoid introducing a 'ground source' where it does not belong.
Isolate metal bodied transducers!
Isolating transducer bodies is easy:
- Connect the transducer with 10' of 1/4" or smaller nylon tubing.
- Place the transducer body in a short length of PVC pipe.
- Keep the transducer up high where it will not become flooded.
Hot Tip On Handling Transducers
Cable Lengths And Voltage DropsThe maximum length of cable used with pressure transducers dependent uponthe type of electrical output of the transducer. For a 0-5 VDC output,a maximum cable length of 100 feet is recommended as voltage outputs areprone to interference.
The 4-20 mA outputs can be transmitted much longer distances dependingupon such factors as wire size, length of wire, power supply and voltagerequirements of any devices to be powered. The 22 AWG conducting copperwire in the polyurethane jacketed cable has a resistance of 16.45 ohmsper 1000 feet. Using Ohms Law (E = IR) (E = voltage, I = current and R= resistance), one finds that a 20 mA signal requires .329 volts to driveit along 1000 feet of 22 AWG copper wire (E=16.45 x .020).
To find out how much voltage is required to drive 4-20 mA signal 10,000feet, just add the minimum excitation voltage to the voltage drop offeredby 10,000 feet cable (10 x .329=3.29). The resulting voltage requirementis the minimum excitation voltage + 3.29V. If you also have other loop-poweredinstrumentation on this circuit you will have to add their voltage dropsin order to find the minimum supply voltage.
|To covert from||multiply By||To Get Unit|
|PSIVG||2.3||feet of water|
|feet of water||.434||PSI-SG|
|PSI||.06895||Bars or kPa|
More pressure conversions
|Milli-ampere current loop to pressure||Pressure to current loop milli-amperes|
|P = (ma -4) FSO/16||ma =4 + 16 P/FSO|
|where: P = the pressure, ma = the milli-ampere outputof the transducer FSO = Full scale output|
Not All Pressure Transducers Are Created EqualOften the cost of diagnosing and servicing a control system because ofa failed transducer is many times the cost of the transducer. I have quotedtransducers and lost the business for the difference of a few dollars only.Later, the customer comes back with tales of electronic failures, excessivethermal drift or leaking cables. Sometimes the devices just don't meetthe claimed specification. Turns out that the manufacturers of poor transducersseem to thrive - they make more money because they are always selling replacementsunits. This is one of the reasons we got out of the Transducer business.
Use of 'dry air' transducers on compressed air lines can be a big mistake.Condensation and compressor oil can ruin these transducers. If you arenot sure that the media is a dry gas use an all media transducer.
The most common cause of submersible failure is moisture on the backsideof the transducer. This is most often due to poor quality cable jacketsand/or seals to the transducer. We won't sell the SO cable jackets as asubmersible cable because the rubber degrades over time, and can becomeporous. SO jacketed cables are meant for extension cords not underwateruse.
The cable to transducer seal is also very important. It is easy to makea seal that last for a few months but quite a different story to make onethat lasts for years.
(C) Copyright 1994-2000, Transtronics, Inc. All rightsreserved
Transtronics® is a registered trademark of Transtronics, Inc.