Submersible pressure transmitters work just like other pressure transmitters, 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 transmitters 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.
Most submersible pressure transmitters 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 transmitters 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 transmitters. 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 transmitters 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.
Submersible pressure transmitters 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 transmitters 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 transmitter 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 transmitters 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 transmitters.
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