Can an EPI differential mass flow meter expand its measurement range?

Such flowmeters may not have such a wide measurement range; it is necessary to ask the manufacturer. If it’s done reluctantly, it won’t be very accurate either; in the cases I’ve tried, I just checked the approximate flow rate and then expanded it accordingly.

Thermal gas mass flow meters utilize the principle of thermal diffusion. Thermal diffusion technology is a method that offers excellent performance and high reliability under harsh conditions; its typical sensing elements include two thermistors (platinum resistance wires), one of which serves as a velocity sensor and the other as a sensor for measuring the temperature of the pipeline. When these two sensors are placed in a gas, the temperature sensor measures the temperature of the gas inside the pipe, while the velocity sensor contains a heating wire that is heated to a temperature higher than that measured by the temperature sensor (which corresponds to the temperature of the gas in the pipe), and this temperature is maintained at a constant level. The airflow carries away the heat from the speed sensor, reducing the temperature difference. To maintain a constant temperature difference, additional electrical power is required to compensate for the heating, and the amount of this compensating electrical power is proportional to the mass flow rate of the gas. From the principle, we know that the compensated electrical power is related not only to the flow rate of the gas but also to the specific heat capacity of the gas being measured (commonly known as its heat absorption capacity). To accurately measure gas flow rate, it is necessary to calibrate the flow meter using actual gas flow; in other words, the same gas that is being measured should be used for calibration. This is an important indicator for assessing the quality of thermal mass flow meters. Regarding the issue of expanding the measurement range for ammonia gas mentioned by the poster, for flow meters, expanding the range requires actual adjustment on the calibration device using the adjusted flow data. For thermal mass flow meters, the requirements are even stricter: not only must the range be changed on the calibration device, but the gas used for calibration must also be ammonia in order to make such adjustments. Of course, if the poster is not strict regarding the measurement data at the site and allows for a larger margin of error, air calibration can be used to adjust the range, followed by a correction for the air/ammonia coefficient; this approach can ensure an accuracy of 5%.

I mistook the gas medium just now. The poster mentioned that the gas medium being tested is hydrogen, so more needs to be said on this topic. As we all know, hydrogen is the lightest gas, but it also has the strongest heat absorption capacity; hydrogen of the same volume has 14 times the heat absorption capacity of air. Therefore, the poster needs to expand the measurement range. Technically, the maximum volume of hydrogen that can be measured is much smaller than that of air, so it is likely impossible to expand the range. The solution is to enlarge the diameter of the pipe, which reduces the flow velocity of gas at the same volume, thereby indirectly increasing the flow rate. This is not hard to understand; most flowmeters actually measure the flow rate of the fluid, and thermal flowmeters are no exception.

This type of flow meter is very unique; it is a flow meter for measuring hydrogen, and it registers a reading only when hydrogen passes through it, while no reading is obtained when using nitrogen.

The reason is that the thermal conductivity of hydrogen and nitrogen differs significantly, by about a factor of 7, and this difference increases further as the temperature rises. Therefore, the flow rate change can be detected when hydrogen passes through the flow meter, but it is not necessarily possible to detect such changes when nitrogen passes through.

A thermal flow meter has one characteristic: it is calibrated using a gas medium that corresponds to the medium being measured. A flow meter designed for measuring hydrogen cannot be used with other types of media.

This type of gauge cannot have its measurement range expanded; it is already calibrated at the time of manufacture. To expand the range, it must be sent back to the manufacturer