How To Specify A Mass Flow Controller

The specification process

1. Most importantly, it is essential that we understand the application without making assumptions.
2. Secondly, we need to clarify the technical details although this in itself can be something of a challenge. More on this later.
3. Lastly it is equally important to understand the essential driver for the customer. Again, our tendency is to assume that the customer requires the best technical solution possible, however, this is not always the case. On occasion it might be that the delivery time is more important and whilst the solution needs to be fit for purpose it is more important that it can be delivered within a customers required time frame. Equally it could simply be that the customer needs nothing more than the lowest price possible and whilst contrary to our engineering-led approach it is nevertheless valid. These various priorities need to be understood and met if the customer is to realise their required solution.

Simple Solutions

We can now return to the technical details required when looking to specify a mass flow controller. As a bare minimum we should look to understand the basic process details. These are:

• Supply pressure
• Process pressure after the MFC
• The gas type being controlled
• The desired flow rate

From this basic data a device can be configured that will work well within most straight-forward applications even though a number of assumptions need to be made. Standard construction will be offered and the customer needs to fit in with whatever these are. This is, however, rare as there is nearly always some level of nuance that needs to be understood. We therefore need to take the level of questions to the next level.

Improved Customer-Centric Solutions

Supply pressure

Is this fixed or could it be adjusted higher or lower? Sometimes by asking to adjust the supply pressure it allows the specifying engineer to select a more cost-effective valve type. This concept is an essential part of customer-centric support.

Process pressure after the MFC

Is there a requirement to minimise pressure loss? Again important for valve selection but also to meet overall system pressure loss targets.

The gas type being controlled

Are multiple gases required on the same device? If yes then will they be controlled individually? With the potential for up to 130 gases loaded onto the one MFC this might allow the customer to purchase just one MFC rather than multiple devices. This can result in significant cost saving. Also, would creating an onboard gas mixture be useful? As Alicat devices operate without conversion factors the creation of mixtures retains the highest level of accuracy.

The desired flow rate

What is the highest flow rate anticipated? What is the lowest flow rate hoped for? With a flow measurement capability across 10,000:1 one Alicat MFC can be selected rather than perhaps three. Once again cost savings can be achieved. For clarity it is worth confirming the engineering units being used; Ln/min, SLPM, SCCM, mlm/min, etc. Fortunately the user is capable of changing these units should they change in due course.

The further process details of gas temperature and ambient conditions are useful as a cross-check.

Additional process questions

In addition to these process questions there are a number of possibilities for the particular construction of a device. These are:

• Alicat devices have fast control response (30ms) that is user selectable; would a factory pre-set response time be helpful?
• Knowing their process, would the customer prefer a particular elastomer material? For pharmaceutical and biotech applications should they be USP Class VI ?
• What level of measurement uncertainty is acceptable? This would include accuracy, repeatability, reproducibility, etc. If possible, all potential errors should be considered when applicable.

• To avoid changing out a pre-existing wiring loom Alicat can copy exact on other manufacturers pin-out. Would this be useful?

Answers to the above now give us a very good understanding of the most common applications. There are opportunities to match the requirements of the application to the construction of the correct MFC whilst offering over-all cost saving. It can be seen that asking the correct questions can help the customer arrive at the best solution for them.

This, however, is not the final level. Even deeper questions can be asked to be of greater benefit to the customer. These are aimed at achieving an even deeper understanding of the process to be able to offer even more helpful advanced configurations. These can be seen as:

Higher Level Solutions

• Would the device be better set as a volumetric controller rather than mass?
• Would the application be better served by the device being set as a pressure controller complete with flow and temperature measurement ? Alicat devices can be user set to control any of mass flow, volumetric flow or pressure whilst still measuring the other parameters (and temperature).
• Would bi-direction flow control be useful?

• Would batch control help with dispensing a specific amount of gas?
• Would a remote valve location be useful? The valve would be electronically, but not mechanically, connected to the flow measurement section. This sometimes helps with fitting the device into tight space envelopes.
• Would compensation for height above sea level help to set all global research to the same base-level standard?
• Would a ramp function help with configuring a specific rate increase/decrease?
• When being used in situ does the flow travel from Right-to-Left or Left-to-Right > configuring for this will ensure the correct orientation of the device and far simpler plumbing.
• Unlike thermal by-pass mass flow controllers Alicat MFC's can be oriented in any direction. For balloon gases they can therefore be installed vertically with no impact upon the zero-flow condition or span.
• Would variable dead-band control or flow averaging improve the process?
• Alarm functions are available and are user configurable.
• In certain electrical situations a separate ground (not a common ground) can be helpful.
• Over-pressure protection is available complete with valve closure.
• Upon power up the device can be pre-set for automatic gas selection.
• Upon power up the device can be pre-set for a specific set-point value.

In Summary

As can be seen there are a great many configurations and pre-sets that are intended to simply make the users' life easier or enhance the work they are doing. In truth there are still many options that are possible (for example ultra-light weight construction, customised calibration points and data-streaming modifications to name but three) but these are delving into almost unique specializations. Asking such deep questions might not always be required but the value to the user when they are applicable can make the difference between a good process solution and a truly exceptional one. Whilst we strive for this excellence please bear with us when perhaps sometimes we ask too much!

Get in touch

We are more than ready to help with your specific application, so please do not hesitate to contact our engineers at sales@pctflow.com or call 01953 609930.

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