Submersible Pressure Transducers Improve Accuracy of Pump Tests
The Environmental Protection Agency (EPA) does not issue national guidelines regarding well yield rates, but U.S. states and municipalities often do. With water quality issues and regulations coming under closer scrutiny, more responsibility for substantiating information about ground water levels is being placed on the shoulders of local townships.
When new wells are drilled it is often recommend that a schedule of measurements be taken to monitor the lowering of water level (drawdown) when water is pumped from the well and the rise in water level (recovery) when pumping is stopped. Results for both recovery and drawdown must be recorded in the well driller’s water report in case problems or questions regarding compliance are raised later. Ideally, pump tests for private and domestic wells should be done for a minimum of two hours, while such tests for municipal, industrial and commercial applications require a minimum timetable of 24 hours.
More Accurate Tests
Until recently, the collection of water level data from pump tests has been costly, time-consuming and vulnerable to inaccuracy due to the manual data-gathering methods used. Such methods involved the lowering of tape-based or conductivity instrumentation every few minutes to take the required number of water level readings, recording them by hand, then later transcribing them at the office for official reporting purposes. This kind of procedure opens up the possibility of human error and endangers reliability, particularly in adverse weather conditions and “rush-to-meet-deadline” scenarios.
More Accurate Tests
An effective way to eliminate problems of this nature is to use a submersible pressure transducer. The device is lowered via a self-supporting cable into the well and connected to a datalogger at the surface. When the unit is supplied with DC power (9-30 vDC), it sends a continuous signal (i.e., 4-20 mA) to the datalogger that is proportional to the water level. The datalogger samples the signal at specified intervals, changes it to engineering units, automatically date stamps each measurement and stores it for later retrieval.
More Accurate Tests
Unlike conductivity probes and tapes, the submersible pressure transducer is lowered into the well one time. It most frequently is suspended slightly above the pump inlet to prevent the transducer from being sucked into the pump’s internal workings, and to minimize the chance of running the pump dry.
Selection and Use
Submersible pressure transducers come in all shapes and sizes, but their typical dimensions are 1 inch in diameter and 4 to 6 inches in length. They are generally constructed of metal such as stainless steel or titanium and weigh from 6 to 18 ounces. Inside the housing are a pressure sensor and a circuit board. The sensor is often piezoresistive silicon protected by a stainless steel diaphragm. Signal outputs include 2-wire (4-20 mA), 3-wire (0-5 vDC), 4-wire (0-100 mV) and SDI-12 (Digital). Circuit boards that handle the signal conditioning are usually surrounded by a potting compound for protection against water incursion and shock.
Selection and Use
The greatest majority of users prefer a 4-20 mA current output signal for two reasons. First, unlike voltage signals, the 2-wire 4-20 mA output signal can be sent many hundreds of feet without any line-loss problems. Second, a current signal is very resistant to electrical interference.
Selection and Use
It’s important to carefully consider the pressure ranges required before ordering because most submersible pressure transducers are not field adjustable. Therefore it is imperative that the transducer range specified will accommodate the maximum water level expected. To be on the safe side, it is commonplace to add an extra 10 percent to the highest pressure range expected.
Selection and Use
Users should also understand the different ways a depth transducer may be configured before selecting one, as its design may or may not conform to the specifications of the application.
Selection and Use
For illustrative purposes, let’s assume we are measuring the water level in a well with a total depth of 100 meters with the transducer installed 5 meters off the well bottom and a maximum expected water depth of 55 meters, relative to the well bottom. The transducer would be ranged for 0-50 meters, normally with 4 mA = 0 meters and 20 mA = 50 meters. However, some individuals are interested in knowing the distance from the top of the well to the surface of the water. In this case, the transducer output is reverse to that: 4mA = 50 meters of water above the transducer (45 meters below the surface) and 20 mA = 0 meters above the transducer (95 meters below the surface).
Selection and Use
Some transducers are available with lightning and surge protection, which protects the devices from threatening thunderstorms and over-voltage conditions. Protection schemes that provide a protector at the transducer end and at the instrumentation end are known to work best. Whether one uses lightning protection or not, users should be attentive to how the cable shield is grounded. In those devices where manufacturers tie the cable shield to the metal transducer housing, the cable shield needs to be connected to a true earth ground, not to the power supply ground. Also, all metal materials that come into contact with the transducer should be grounded to a common earth ground.
Submersible Cable
The way in which the submersible cable is connected, as well as how it is used during the application, can have an effect on the water level results gathered. Therefore, it is necessary for the installer or user to evaluate the application before determining configuration of the cable.
Submersible Cable
The transducer needs to be slowly lowered into the well so that the cable jacket is not punctured or cut. Damage to the jacket may allow water to enter the casing and eventually cause the electronics inside to fail. Some users prefer to order the transducer with a 1/2-inch MNPT fitting situated where the cable exits the transducers. Then a 1/2-inch MNPT x 3/4-inch FNT swivel PVC fitting is used to connect inexpensive garden-type hose to the back of the transducer. The inside diameter of the hose is sufficient to “snake” the cable through it.
Submersible Cable
Experienced users of submersible pressure transducers for pump testing will often suspend the cable using a cable hanger to ensure a quick, reliable installation. Sometimes anti-snag cones are used on the cable so that the transducers can be brought to the surface without getting hung-up on other cables that may be in the well. Also, inexpensive stainless steel weights may be attached to the nosecap of the transducer to keep the cable straight. Maximum pull-strength of the cable varies with manufacturer, but it can be as high as 200 lbs.
Submersible Cable
When it comes to specific requirements of a particular installation, it’s important to know how to properly terminate the cable. Many applications specify that the submersible cable be brought to a junction box at the surface and then less expensive non-submersible cable be strung to the datalogger.
Submersible Cable
The use of the junction box ensures a secure termination location for the cable vent tube, which provides atmospheric reference for the pressure sensor. This atmospheric reference prevents changes in local atmospheric pressure from affecting water level readings. Also, to prevent water from condensing in the vent tube, it is important to incorporate a vent filter or aneroid bellows. These devices are attached directly to the vent tube.
Dataloggers
Datalogger manufacturers provide many options for handling water level measurements taken by submersible pressure transducers. Because of this, manufacturers’ set-up instructions often require specific calibration information about transducer performance over pressure and temperature. This information should be provided by all reputable manufacturers along with the transducer.
Dataloggers
Most basic dataloggers have a built-in power supply that powers both the datalogger and the transducer. They will accept a voltage or current signal from the transducer and convert the signal to a common engineering level measurement (i.e., feet of water, PSI, kPsa). Most datalogging is done in one-minute intervals. The data can then be reviewed logarithmically using a spreadsheet program. However, some dataloggers can be set up to log at varying intervals so that spreadsheet logarithmic scaling of data is not required.
Dataloggers
Less expensive dataloggers encourage users to transfer the data gathered via a serial port to a computer spreadsheet program that can do all the graphing and calculations required. More expensive models will provide an abundance of analytic tools with the datalogger and may also include a means of reading the data on-site.
For More Information
Additional information about submersible pressure transducers and dataloggers is readily available from most manufacturers of this equipment. Such companies can easily be found by consulting the Thomas Register and the Instrument Society of America’s (ISA) Directory of Instrumentation at your local library, or by viewing their respective Internet web pages (www.thomasregister.com or www.isa.org).
