Submersible Pressure Transducers
How They Work
Submersible pressure transducers work just like other pressure transducers, except they’re inside a water-tight housing, and they read in feet or inches of liquid (adjusted to specific gravity) rather than PSI. This is done with a simple equation. PSI is converted to feet by multiplying it by 2.31 and then dividing it by the specific gravity.
There are 2.31 feet of water in a 1 inch square column of water that weighs 1 pound, hence the conversion factor. We multiply by the specific gravity (density) to get a true height of whatever liquid it is. For example, petroleum oil has a specific gravity of about 0.8. So a 1 pound, 1 square inch column of petroleum oil would be about 2.89 feet high.
If you want inches, instead of feet, we simply multiply by 12. This conversion math is done in the firmware to give you a native level reading.
Our submersible pressure transducers can be in any pressure type you need. The standard is gauge pressure, or PSIG. This is achieved with vent tube that runs from the reference point chamber, through the cable, and to the panel box where the wire is tied into the control system. While this solves a problem, it presents another one: condensation.
Condensation can build up in a vent tube, eventually dripping water out of the vent tube and directly onto the sensor electronics, causing failure. This can be eliminated fairly simply, by keeping moisture out of the tube. In the past we have done this with a desiccant cartridge. It was too large and eventually needed replacement. So we developed a better answer, a breathable waterproof membrane on a small cap. Unlike competitor solutions, this filter cap does not require heat shrink to seal it onto the vent tube. It uses an o-ring instead, and is about the size of a pen lid.
The pressure sensor itself uses a diaphragm with an attached electrical current. As the diaphragm flexes, the resistance on the small circuit changes in direct proportion to the pressure. This technology is called piezoresistive, and is a proven method for digital pressure sensors.
Features
Most submersible pressure transducers on the market stick to a simple mold: piezoresistive sensor in a submersible housing. Beyond that, there are few options and alterations.
Built In Lightning Protection
Our PT-500 series submersible pressure transducers are made with you in mind. For example, we know that lightning strikes are a big problem for submersibles because they’re often attached to outdoor electronics and submerged below ground. Most others recommend buying a separate surge protector that can cost as much as $300. We decided to build the protection right into the sensor.
Multiple Cage Options
We also know that adding a protective cage adds quite a bit to the cost of the sensor. With you in mind, we made a PT-500-P30 cage that is removable, so you could reuse it on replacement sensors that you buy without a cage - saving you money. We also have a traditional cage, laser welded to the body of the sensor.
Field Zero Adjust
Another major oversight from other manufacturers is the zero adjust. This handy option is common with other pressure sensors, often done with a potentiometer adjustment screw. It would be difficult to seal such a method on a submersible pressure transducer. Therefore, many have removed the option altogether.
Not us. You can adjust the zero in our PT-500 series submersibles with a simple magnet. Simply touch the side of the housing to move the zero point up or down as needed. It couldn’t be simpler.
Chemical Compatibility
Our submersible pressure transducers are very compatible with most chemicals. The standard housing for PT-500 series is 316L SS, and we are currently testing a PVC model. There are three different cables to choose from: Polyurethane, PVC, and Hytrel®. That means we can take care of your needs in just about every circumstance.
Hytrel® is a registered trademark of E. I. du Pont de Nemours and Company.
Applications
Submersible pressure transducers are incredibly versatile and inexpensive. They can measure the level of just about any liquid within their temperature range. Because of this, they’re often used in waste liquids where the exact chemical makeup is ill defined. They’re also becoming popular for battery powered applications because of their low current draw.
Lift Stations
A wastewater lift station is a critical piece of the waste treatment puzzle. It is a pump station for waste lines, collecting it from either gravity fed lines or from another lift station upstream. Once the lift station well is full, it activates pumps that push the waste down the line, until it reaches a waste treatment facility.
Submersible pressure transducers are a central component of a lift station because it drives the pump control and prevents hazardous and expensive overflows of waste, called Sanitary Sewer Overflows (SSOs). Other level sensing technologies are used as well, but the submersible pressure transducer is very common because of it’s durability, simplicity, and low cost.
Leachate Tanks
Landfills are constructed using large barriers to keep the waste from contaminating the soil and water supply below. These barriers hold in everything, liquid and solid. To keep the moisture in the waste from forming a rather unpleasant pool at the bottom the landfill, the leaching liquid is drained and processed as waste. This liquid is called leachate.
It’s exact chemical composition is variable - a direct product of whatever is tossed into the landfill. Our submersible pressure transducers offer the chemical compatibility and longevity needed to work in these underground pits.
Low Power, Low Cost Tank Level
Submersible pressure sensors will be a staple in the industry for many years due to their low cost, versatility, and low power consumption. This makes them an ideal candidate for remote tank level monitoring as well - where battery power is required much of the time.
Power consumption is an increasing concern, one that is difficult to resolve with many other level sensors because of the nature of their operation requirements. Inferring level from a pressure sensor is much easier on power consumption. The sensor isn’t exciting a signal, but simply monitoring the change in a small electrical circuit. The response time - from turning on the sensor to getting a valid reading - is much faster as well, further reducing the power consumption of our submersible pressure transducers.
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FAQs
Are they resistant to caustic chemicals?
Yes. Our standard model, the PT-500, is made with 316L stainless steel. We're also testing a new PVC model, the PT-503. Between the two of them, they cover a lot of chemicals, both inert and caustic. If you're interested in testing our upcoming PT-503 PVC model, contact our sales department.
back to topHow do I protect against electrolysis?
Electrolysis is a chemical reaction to current running through your liquid. This is the unfortunate result of a ground loop - when multiple grounds have a difference of electrical potential and create a flowing current between them. You have two options to eliminate electrolysis:
- Get rid of the ground loop
- Use a plastic material, instead of a metal
To get rid of the ground loop, you'll need to ensure that your sensor is only attached to a single ground. The electrolysis is probably happening because the submersible sensor is case grounded to the liquid around it, and the current is flowing from the sensor housing to the nearest ground point. All you have to do in this situation is to disconnect the ground wire of the sensor at the wiring terminal inside the control box.
If this makes you nervous, then you can always test our upcoming PT-503 PVC submersible pressure transducer. This sensor is currently in a BETA release, and is appropriate for testing purposes only. Contact our sales department to learn more.
back to topDo you have lightning protection?
Yes. Lightning protection is built right into the Series PT-500 with reverse polarity and surge protection per IEC 61000-4-5. Most others require external surge protectors ($300+) to provide the same protection.
back to topWhat is the max depth?
For the Series PT-500l, it's 450 ft. If you are testing the PT-503, it's 346 ft. The Series PT-510 and Series PT-510W top out (bottom out?) at 46. This is not a limitation of the sensing elements and diaphragm, but of the sensor housing.
back to topHow do you protect the diaphragm?
We have two good options for protecting the diaphragm:
- Nose Cone
- Cage
Typically, the nose cone is sufficient. However, in turbulent or high flow environments with suspended solids, the cage is recommended.
back to topWhat is a vent tube and why is it important?
The vent tube is a component of a vented (psig) submersible pressure transducer's cable that vents the chamber behind the diaphragm to atmospheric pressure. This is critical to the operation and accuracy of the sensor. The sensor's reference point is atmospheric pressure, meaning it automatically adjusts to changes in atmospheric (barometric) pressure - or that it tells you how much pressure is applied to the sensor in addition to atmospheric.
It's important to keep the vent tube both dry and unobstructed. We offer two options for keeping the tube dry: a desiccant cartridge and a hydrophobic, breathable vent cap. Make sure you have one of these installed on your vent tube to keep the sensor's electronics safe.
Blocking or obstructing the vent tube is also detrimental. If the sensor is not properly vented, then accuracy will suffer as the atmosphere's pressure will begin to change the way the sensor's diaphragm behaves. Sealing off the vent will fix the sensor's reference point to one pressure, leaving it vulnerable to changes in elevation and weather. As submersible pressure sensors are typically low pressure instruments, these small changes in the atmosphere can have a noticeable affect on your level reading.
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