WL3100 Dry Pressure Transducer

• Temperature Compensated Measurement• LCD Display• SDI-12 interface• 4-20mA interface

(at the same time)• Option : Extra SDI-12

By : Steve LaurenceHydrological Services P/L

Using a Bubbler to Measure Pressure :

Deriving the Relationship :A bubbler (or submersible) water level transducer measures the water pressure (relative to atmospheric pressure) and calculates the water depth. In the transducer, a simple formula is used :

Water Level = Measured Pressure x User Factor ( + User Offset )

Deriving the Relationship :We are taught that : “Force = mass x acceleration” the amount of force that a body of water exerts is dependent upon the acceleration due to gravity. The mass of an object is : “mass = volume x density”  so now we have : Force = mass x acceleration due to gravity = (volume x density) x gravity = (area x depth x density) x gravityRelating back to pressure :

“Pressure = Force / Area” (substitute the Force from above) = (area x depth x density) x gravity / area (area cancels)

Pressure = depth x density x gravity Solve for depth : Depth = Pressure / ( density x gravity) 

Water Depth = Pressure x (1 / (density x gravity))

 Therefore : User Factor = 1 / (density x gravity) 

NOTE : We have now shown that the User Factor is a function of water density and gravity !!!

You might think they are both constants, but you would be wrong !!!!!!!!!

The User Factor we use is 0.101972 m / kPa

Water Salinity :• As water salinity increases, the density increases• The User Factor is proportional to 1 / density • Therefore the User Factor will decrease as salinity increases.

LCD Menus :

Navigate with the “Scroll” and

“Select” buttons

Setting Up 4-20mA :• The 4-20mA circuit is optically isolated from the rest of the electronics• Therefore it must be powered by the loop• Set the 4mA Level using the WL3100 LCD menu• Set the 20mA Level using the WL3100 LCD menu

(You can use –ve levels)

ML420 Data Logger measuring the 4-20mA

To conserve power, the data logger switches power to the 4-20mA circuit only when it wants to make a measurement.

4mA / 20mA Level0.0 / 10.0 m

Sampling Modes :

When “Continuous Mode” is set to “NO” the WL3100 only takes a sample when : - The water level is displayed on the LCD or - An SDI-12 Measure command is received or - The 4-20mA loop power is detected. When “Continuous Mode” is set to “YES” the WL3100 takes a sample :- Every 12 secs. (Continuously)

(This is useful if you use the SDI-12 command 0R0! as it will immediately return the most recent measurement made within the last 12 secs – which makes the WL3100 SDI-12 interface the same as the AD375A SDI-12 interface. It can also be useful when using the 4-20mA interface, as the 4-20mA loop current will be accurately updated 100mS after loop power is applied – without having to wait 12 secs for a measurement to be made !!)

WL3100 S/W Rev 7.1 (and lower)

Sampling Modes :WL3100 S/W Rev 7.2 (and higher)

When “Sampling Mode” is set to “Single” the WL3100 only takes a sample when : - The water level is displayed on the LCD or - An SDI-12 Measure command is received or - The 4-20mA loop power is detected. When “Sampling Mode” is set to “Contin+Avg X” the WL3100 takes a sample : -Every 12 secs. (Continuously)(When a sample is requested, via the display or via SDI-12 or 4-20mA, the last X samples are averaged. If it is known that the water level will change very slowly, it is advantageous to set X to a high number, so that the last say 20 samples will be averaged.)

Sampling Modes :5 Sample Average Example :

Full Temperature Compensation : Each WL3100 undergoes a full Temperature Compensation

Calibration at our factory in Liverpool.

10 units at a time in our Environmental Chamber

-20C, -10C, 0C, 10C, 20C, 30C, 40C, 50C, 60C, 70Cfor a 1 hour soak, for everything to stabilise

At each temperature we do a 7 point pressure calibration over the required range : 0%, 15%, 35%, 50%, 65%, 85%, 100%

Ruska 7250i pressure calibrator is accurate to 0.005%(for example 10.000m x 0.005% = 0.5mm)

Calibration of each unit takes over 24 hours

Field Calibration Check HS40 :Navigate to the WL3100 “View Pressure” displaySet River Line Select to “Close” positionSet Calibrate Valve to “Calibrate” positionSet the Pump Up calibrator to “Vent” Connect Pump Up calibrator to the Bubble Rate Test Port

Calibrator Pressure is now read on the WL3100

Remove Pump Up calibrator from Bubble Rate Test PortSet Calibrate Valve to “Open”Set River Line Select to “Open”