FAQ - Thermical massflow measurement

Moist gas of up to approx. 80 % relative humidity has only a slight impact on the measuring accuracy. However, make sure that no condensation can take place in the measuring device. The condensation could settle on the sensor which could lead to inaccurate measurements. In the event that a measuring device has come into contact with condensation, it can for the most part be rinsed with a dry gas, such as N2 or (X)CDA.

A horizontal mounting position is always recommended. This can be upright, sideways or upside down. With a vertical mounting position, dependent on the type of gas and at gauge pressures above 5 bar, a non-constant zero-point offset can develop. This effect is caused by convection in stationary media.

The following situations can cause problems:

• Strong heat sources, or ambient temperatures.
• Strong sources of electromagnetic radiation such as spark discharges.
• Humid environments and the associated condensation. It leads to damage of the electronics components.
• Particularly in the case of flow controllers, strong vibrations will cause unstable control.
• In general, aggressive environments reduce the life span.
• Liquid running backwards can penetrate into the measuring instrument. An elevated mounting location generally helps, or using check valves.

When operated properly, most devices do not require any routine maintenance. We recommend that the calibration is checked after 12 months, however. In case of MEMS based flowmeters they are most likely within tolerance as that type has virtual no drift and excellent multi year stability.

As an end user, you determine the calibrating interval. People often use once per year or in case of very clean gas, once per 2 years.

The following questions can be helpful:

How important is it to maintain the accuracy of the measuring device?
What is the maximum allowable deviation?
Can the user determine the deviation of the measuring devices himself by means of a reference?
Is a periodic calibration from an accredited body required?

The following influences can lead to an increased deviation:

1. No stable flow, caused by pulsating pumps.
2. The gas condenses or carries along liquid or dusty particles. Applications with ambient air are particularly susceptible to contaminations. A filter needs to be provided here.
3. The effective pressure deviates considerably from the calibrated pressure (+/- 0,2%/bar).
4. The gas or gas mixture is not consistent with the calibrated gas.

The thermal mass flow meters and controllers directly measure the number of gas molecules (=mass), which flows past the sensor. The thermal flow measurement principle is therefore insensitive to changes in pressure and temperature. The thermal mass flow meters are calibrated on two temperatures and a temperature sensor in the gas stream compensates any minor temperature error due to temperature effects in the electronics. Due to this compensation there are almost no measurable temperature effects in the range of between 0…50 °C.

The most common causes of faults concern the way that devices are connected to the gas supply. The following guidelines make life a lot easier:

• The pipes must be absolutely clean. Please flush them with N2 or (X)CDA before installing the measuring instruments!
• Use appropriate pipe materials (pressure rating, durability).
• Even when connected to fixed pipework, we recommend that the devices are mounted on a base(plate) using the appropriate mounting holes.
• From 50 ln/min, please allow the following flow-calming sections: 10 x diameter before inlet, 5 x diameter after outlet.
• Use appropriate fittings (material, size, thread, pressure loss).
• Malfunctions can be caused by unstable pressure controllers, pumps that oscillate, and volumes before and after the measuring device that are generally too small. Installing a gas reservoir with 2 liter volume in the feed pipe solves many measuring problems.
• The size of the pipe must be matched to the measuring/control device. A diameter that is too small results in an increased pressure drop. From 60 ln/min, we recommend a pipe inside diameter of at least 10 mm.
• We recommend that a bypass system is used. It makes maintenance work like calibration easier. This is particularly important where the gas supply must not be interrupted.

We always recommend that a filter is installed before the measuring devices. It often happens that solid matter such as welding residues, metal or plastic chips, rust or sealing tape affect the function. In pressurized-air applications using compressors, the air must be dry and free of oil. Please ensure that a suitable processing unit is located before the devices. In the case of gases from cylinders, no special filtering is needed.

1. Residual ripple should be of the smallest possible value (preferably below 50mV).
2. Proper grounding of your electronics is essential for a safe situation, but also for accurate measurements. Improper grounding is a common cause of failure.

Removing contamination on-site may be possible, depending on the type of contamination. As a first step, we recommend flushing with N2 or dry air. If it is contaminated with liquids (like oil), pure ethanol (100%) can be used. Please rinse after cleaning the device with valve position 100% open with dry air or nitrogen for approximately 15 min. to dry all liquids. With a flow controller, it is helpful if you operate it by software to open the valve.

It may be necessary to repeat the process cycle of wet-dry cleaning several times. Depending on how clean the result has to be, flushing with clean gas may take a very long period. In these cases a setup with particle counters and an inline purifier is advised. If required, Teesing can deliver this service.