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How Specific Gravity Affects Accuracy On A Float Level Transmitter

magnetic float for a level transmitterFloat level transmitters rely on matching the right float with the right specific gravity, or density, of your liquid. While there are other specifications that matter (i.e. chemical compatibility, temperature, and pressure), the accuracy of your float level transmitter is tied directly to specific gravity.

Each float has a reference point where the float is calibrated. This is essentially the point on the float where the measurement is taken. Getting the liquid level to line up with that point is ideal. If we can’t, we just do an offset.

A simple calculation will tell you how closely your liquid level will line up with reference point on your float. Here are the steps you need to take to figure it out.

1. Will It Float?

First, make sure the specific gravity (SG) of the float is less than that of the liquid. If it’s greater than or equal to, you’ll have a sink, not a float.

For example, a float with the same SG of the liquid may or may not sink to the bottom, and it may or may not float to the top. It will likely sit somewhere in between.

If the SG of the float is only slightly higher than the liquid then it will sink to the bottom, though it may happen slowly. If the SG is a lot higher, it will drop like a rock.

Remember to pay close attention to the temperature range of the liquid media, as the SG will change with temperature. Make sure the float will always have a lower SG than the liquid at all expected temperatures. More on this later…

2. Percentages

Once you know the float will float, now it’s time to find out what percentage of the float will be under the surface. This will tell you exactly where the surface of the liquid will be on the float.

The calculation is simple. Simply take the SG of the float and divide it by the SG of the liquid.

So a float with a .65 SG in water (SG=1) will be submerged 65%, whereas in methanol (SG=.786) it would be submerged 82.7%.

That’s .65/1 for water or .65/.786 for methanol.

3. Calibration

With that bit of information handy, you now know how far up the float the liquid surface will be. Now you need to determine how far it is from the reference point (sometimes called the calibration point). Use that distance to program an offset and you’ll end up with an accurate reading.

The offset can be programmed into some sensors. If yours can’t do it, then have your control technician program it into your PLC or controller.

Another option would be to compare your float level transmitter reading to a physical measurement and trim the sensor’s output to match.

4. Mind The Temperature Affect

Finally, beware of temperature changes. They change the SG of your liquid. Find a table of your liquid’s SG over a spectrum of different temperatures. Now find out what temperature range your liquid will experience in its tank or container.

Cross-reference the data in the table to your temperature range to find the SG swing that you’ll be dealing with. Next, attempt to establish an average or a normal and calibrate your offset to match. This will give you the best results.

While SG change with temperature variation is minimal, and typically within or near the sensor’s error band, compensating during commissioning of the sensor will give you better results.

If you really want to optimize your accuracy, you could create a table of SGs at certain temperatures, install a temperature sensor if needed (our MPX gives you a temp measurement), and use your control system to automatically compensate for you. It’s work, but it’s very doable if you can gather all the right information.

These four steps will give you the best accuracy from your float level transmitter. If you have any questions, please contact us. We’re experts on these sensors and can help you improve your performance and reliability.


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