Selection of level gauge

If the density changes, the error of the differential pressure type will be quite large!

I wonder what’s wrong with the radio frequency admittance? If the material is clean and its density remains constant, a differential pressure level gauge should work; temperature is not an issue, as capillary pressure transducers can be used to avoid high temperatures.

RF admittance transmitters always experience zero-point fluctuations, requiring adjustment every few days, which poses a problem for instrument technicians; it’s unclear whether this issue occurs elsewhere as well

Is the double-flange differential pressure level gauge related to the density of the medium? Could you explain that?

Differential pressure level gauges are closely related to changes in the density of the medium, as the positive pressure side of such gauges measures the sum of the pressure of the gaseous medium above the liquid surface and the pressure of the liquid itself. The pressure of the liquid is determined by multiplying the height from the pressure sensing point to the liquid surface by the specific gravity of the liquid; when the density changes, so does the specific gravity, and consequently the pressure as well. Additionally, dual-flange capillary level gauges cannot be used to measure temperatures at any level, as this depends on the filling material inside the diaphragm box. I believe the maximum temperature is around 300 degrees, perhaps around 250 degrees at most; I’m not entirely sure. You can check with manufacturers such as Rosemount, EJA, or E+H.

But generally, the pressure measurement on the positive pressure side refers to the pressure of the gaseous medium above the liquid surface; the situation described by the original poster only occurs when the liquid level exceeds the upper limit for detection； After another change in density, the density of the liquid on the low-pressure side of the level gauge will also change, so there should be no issue affecting the measurement data.

The need to continuously adjust the zero point of radio frequency admittance adds a lot of work. Due to the high temperatures, the temperature tolerance of the level gauges used for measurement is limited. Using pressure transfer tubes may be a better solution; level gauges with sapphire chips might work better as well. It would be ideal to have the density of benzene provided, so that the level gauges can be calibrated at the time of manufacture

For low flow rates, metal rotor flow meters or glass rotor flow meters can be used. For high flow rates, vortex flow meters or differential pressure flow meters are recommended

To change radio frequency admittance to differential pressure, it is first necessary to create another port on the device, which is somewhat troublesome. Moreover, if changes in temperature and pressure have a significant impact on the density of benzene, temperature and pressure compensation is required! Could radar level gauges (rod type or cable type) be considered? Benzene at 200 degrees Celsius can be measured using a gauge with a dielectric constant of around 2. If a guided-wave rod is used (a cable-type radar level gauge capable of withstanding high temperatures can be considered when the measurement range exceeds 4 meters), it is possible to carry out measurements without having to add additional openings, and changes in temperature and pressure have little impact on its measurements (almost negligible). Things become even simpler if the interface flanges are the same.

Ultrasonic level gauges are worth considering

An electric float level gauge could be considered. If there are no significant density changes.

There’s no need for additional openings; the equipment already has two holes, so installing a double-flange level gauge isn’t a problem. Moreover, differential pressure level gauges are relatively inexpensive. One should consider not only practicality but also cost-effectiveness – running a business these days is tough

If cost-effectiveness is a consideration, I agree with the view from the 12th floor that it’s cheaper, but the measurement range of the float is relatively small – will it meet the process requirements? The price of double-flange level gauges is also high; you can ask for quotes and compare them (of course, this depends a lot on the manufacturer). Radar level gauges are expensive, but I think they are more suitable from the perspective of future maintenance. High temperatures place high demands on the filling fluid for capillaries, which ordinary manufacturers may not be able to meet.

For a level gauge that uses a magnetic flip switch for remote transmission, just inform the manufacturer of the density and height when placing the order.

That’s right, the density on the high-pressure and low-pressure sides is the same. But our differential pressure level gauge measures the pressure difference, which is then converted into a level height difference. To use an exaggerated example: P = pgh. If the initial density is 1 and the pressure difference between the high-pressure and low-pressure sides is 2, then the liquid level value will be 2 ； If the density becomes 2 at this point and the pressure difference between the high-pressure and low-pressure sides remains 2, the liquid level should be 1. However, by ignoring the change in density, you obtain an incorrect liquid level value of 1. If the range of density variation is within your allowable error, using differential pressure measurement is fine.