How to accurately measure and transmit tank level data to a PLC with sensor offset - Tank setup details included

If you're not getting the readings you need, it could be due to incorrect configuration. Many level transmitters can be customized to send analog signals based on "strapping tables" within their settings. For instance, you can adjust the transmitter to send 5.23 mA when the water level is slightly above the tip (assuming the tip is 0.2m high) and 18 mA when the water is at 2.6m. It can even be set to send 4 mA if water is at or below the tip. While your equation may be correct, make sure to use the appropriate values like 5.23 and 18 instead of 4 and 20. At 3m, the transmitter could send slightly over 20 mA, but most AI cards are capable of reading up to 21 mA for accurate measurements at higher levels. Without the probe reaching lower depths, you won't get precise readings, but the displayed tank level will decrease from 0.2m to 0.0m, which is more accurate than showing 0.2m when the tank is empty. Remember, just because you can work around a design issue doesn't mean you should. Take a closer look at the transmitter's capabilities before making changes to the PLC code.

• 08-11-2024
• Aardwizz

Before programming anything, it's crucial to address the underlying design issue you're facing. It seems like you're using a Guided Wave Radar (cable or rod) in a tank, which may lead to inaccuracies below 0.2m due to the sensor no longer touching the product. Understanding how the sensor/transmitter is set up is key, as guided wave radars are typically installed in a bridle on the side of a vessel/tank. If your process can maintain at least 0.2m of product, the application should work fine. However, calibrating the sensor to detect product when it's not in direct contact is possible but may not be very accurate. Working closely with the customer to understand the design and process can help alleviate uncertainties. In situations where a tank is constantly filled and emptied, it's important to ensure the product level doesn't drop too low to prevent pump damage. If the product goes below the 0.2m mark, indicating an "invalid" level and alerting the operator can be a solution. Consider using a through-air radar for better accuracy, depending on the tank's contents and the customer's budget constraints.

• 09-11-2024
• nicherbert

If the device in question is radar or another transmitter-based distance measuring tool, it raises the possibility that a 0m (equivalent to a 4mA signal) reading indicates the level being at the level of the transmitter. In contrast, a 0.2m tank level corresponds to a 2.8m (3m - 0.2m) reading, and a 5m (equivalent to a 20mA signal) reading would imply the level is 5m below the transmitter, which is not feasible in the current setup. My query pertains to how a device with a 2.8m sensor has a measurement range of 0-5m. The simplest and most intuitive formula for linear measurement is (InputSignal - LowLevelSignal) / (HighLevelSignal - LowLevelSignal) = (MeasuredLevel - LowLevel) / (HighLevel - LowLevel), which highlights the linear relationship between input signals and measured levels within their specified bounds. This equation can be rearranged to solve for the measured level based on the input signal, resulting in the formula provided in the initial post (InputSignal - LowLevelSignal) / (HighLevelSignal - LowLevelSignal) x (HighLevel - LowLevel) = MeasuredLevel + LowLevel.

• 09-11-2024
• drbitboy

Monitoring tank levels using pressure transmitters is a common practice in various industries. When dealing with submersible transmitters, it is advisable to position them a few inches above the tank bottom to prevent interference from any settled debris. To achieve accurate measurements, I calibrate the sensor to its full range in engineering units and adjust for the offset corresponding to the sensor's distance from the tank bottom. In the case of water tanks, this offset may be a negative value if the sensor is installed in a pipe within a vault situated several feet below the tank floor.

• 10-11-2024
• OkiePC

To optimize sensor performance, adjust the upper range to 3.0 meters, lower range to 0.0 meters, and offset to 0.2 meters. The input should range from 4mA to 20mA between 0.0 meters and 3.0 meters. Implement an alarm or control mechanism to alert or prohibit filling beyond 2.6 meters.

• 10-11-2024
• Robb B

In order to understand the situation accurately, it is crucial to be physically present onsite, in front of the tank, to take measurements and observe real-time activities. Speculation without firsthand experience can lead to inaccuracies. While your efforts are commendable, being physically present is essential for a comprehensive understanding.

• 10-11-2024
• padees

It is quite common for a lower signal to perform better than a higher one, especially when the concept of unity is not fully understood. Instead of simply solving the issue, this situation presents an opportunity to educate and enlighten the original poster. By gaining a better sense of proportion, they can likely resolve the problem on their own.

• 10-11-2024
• drbitboy

Thank you for your advice and input. The tank's shape is not round, and the sensor being used is a submersible pressure sensor, as mentioned by OkiePC. In order to calculate the measured level, I convert the sensor readings to engineering units, adjust for the sensor's location relative to the tank's bottom, and then use a formula incorporating the sensor's low and high level signals. Is this the process you were referring to, OkiePC?

• 10-11-2024
• colesha

When dealing with in-ground clearwells, a common practice is to utilize a submersible 10psi transmitter with a 4 to 20mA output. The transmitter is scaled linearly in the PLC, where 4mA corresponds to 0 and 20mA corresponds to a specific measurement in feet. For instance, at 20mA, there would be 23.1 feet of water above the sensor, even if the clearwell itself is only 14 feet deep. It is crucial to regularly check the analog signal for any out-of-range conditions and promptly address them to avoid any potential issues. One key tip is to set the lower alarm limit to a value slightly below 4.0 mA, such as 3.8 mA, to allow for a margin of error during normal operation while still being able to detect any malfunctions or sensor failures. By setting up the PLC scaling for the maximum range of the sensor, it simplifies the replacement process in case a sensor needs to be swapped out. Furthermore, incorporating a variable offset that can be easily adjusted can help fine-tune the measurements for greater accuracy. For those already working with sensor ranges in meters, scaling it appropriately in the PLC and adding a customizable offset can streamline the process further. It is also recommended to fill the tank to the overflow level periodically to verify and adjust the offset as needed, ensuring precise measurements and efficient operation of the system.

• 10-11-2024
• OkiePC

Hey there, members! Just a quick update - I will be stopping by the site next week to conduct the evaluation, as well as the installation and calibration process. Looking forward to getting everything set up and running smoothly!

• 10-11-2024
• colesha