There is lots of information in the molbox1+ and molbox RFM user manuals regarding BPR when using sonic molbloc elements with a molbox1+ or molbox RFM flow terminal. This article will explain why this might be seen when initially turning on the molbox or when you first start flowing, as it can be perceived as a hardware error message or similar.
Will start by saying a flashing BPR message is completely normal for sonic molblocs. Sonic molblocs do not produce accurate flow readings until the flow of gas has reached the speed of sound (sonic) at the current conditions. Because of this, the flow value is not displayed until sonic flow is achieved. Instead, the BPR warning is displayed to let the user know the sonic molbloc has not met its proper Back Pressure Ratio to produce accurate flow measurements yet.
The simple solution, without even understanding what is really going on in the flow system is to just continue to increase your pressure regulator on the sonic molstic until the BPR message goes away and until the flow value that is then displayed is no longer flashing. Be mindful that normally you want your main gas supply regulator on your gas source to be set at about 75 psig as this acts as a safety net so that you cannot increase pressure any further when using the system, as otherwise there is potential to easily overpressure the pressure sensors that are inside the molbox and/or perhaps other low pressure rated tubing, etc.
Now that there is a very basic understanding of what is needed to get past the BPR message to display a flow rate rather than the BPR message, let's understand more about why this is required and potential challenges.
In most applications it is recommended to use our laminar molblocs up to the maximum possible laminar flow (typically 100slm) and then only use sonic molblocs for higher flows. The reason for this is due to the BPR requirement and the limitations it produces with not being able to allow flows down to 0 flow. Our Sales team typically configures flow systems with this in mind unless there is a specific reason sonic molblocs are desired (which is rare). This does not mean that some sonic molblocs cannot be used for low flows, it's just that typically using a laminar is much easier in general and fewer molbloc elements are needed to fully cover a range of low flows.
You might see the terms back pressure ratio (BPR), choked flow, critical flow, or sonic flow and it is important to realize these all essentially mean the same thing. Sonic molblocs are essentially just a "nozzle" (specific sized opening) with pressure being read on each side of the nozzle (at least initially).
What the BPR message you initially see is telling is that the ratio of pressure on the upstream side of the nozzle is too close to the pressure that is on the downstream side of the nozzle and sonic flow has not been achieved. This makes sense because at zero flow the ratio is 1:1 as you likely have atmospheric pressure on the upstream side as well as the downstream side. As you start to introduce flow with the pressure regulator on the molstic this increases the pressure on the upstream side, but the downstream side would remain at atmospheric pressure, so thus the ratio might be 1.1:1 once you get a fair amount of flow through the system. It is generally not important to know or monitor this as long as you understand that you just need to keep increasing the flow/pressure until you start to see a flow value being displayed. Remember most sonic molblocs that are ordered are for high flow rates so you won't likely see this until a lot of flow is coming out the system.
The next question that naturally comes up because it is related to BPR, is at what flow you will likely start to see a flow being displayed and thus accurate flow readings to start taking measurements? Let's start by saying it is certainly not at 0 flow, will be a high amount of flow on most sonic molblocs, and gets higher as the flow range of the molbloc goes higher. This needs to be understood so you know which of your sonic molblocs is the proper one for the flow test points you need to set and/or when you need to switch to a different molbloc to achieve other flow rates. It is best to get familiar with using the flow rate charts in the user manual as they show the flow ranges possible:
To keep this already complicated and long discussion as short as possible will make the assumption that the molstic has the Standard Pressure (SP) calibration and will use the N2 chart as the numbers are the simplest to view and also very close to what would get if have the default Air calibration. Will also use the 5E2-S in this example as it is the most common to start using once go beyond the 100slm that laminar molblocs are typically used to cover, but yet still has some overlap. We recommend highlighting the molblocs you have as shown in the example below for quick reference as many of these details are often hard to remember later:
The key to the above chart is the "Minimum without Vacuum" column that is highlighted. This tells us at about what flow rate the BPR message will stop being displayed and thus where accurate flow readings can start. So, the minimum the 5E2-S can do in N2 (very similar for Air) is about 67slm.
The column furthest to the right that is highlighted is the maximum flow that is obtained with a standard molbox and the Standard Pressure calibration type. So, the maximum the 5E2-S can do in N2/Air is about 300 slm. This column was manually edited as there was a change over time where we calibrate them up to 600 kPa now. As a result, keep in mind the maximum will actually be the midpoint of what the 500 kPa and 700 kPa columns show in the chart in the user manual, so would manually edit it just like has been done above in the highlighted chart.
It could be important to note the above charts use the unit of slm@0C . It is often more common to be using units of slm@70F and the difference between these two is about 7.7% which is significant. In other words, the 5E2-S range in units of slm@70F would be about 72 slm to 323 slm if the devices you are testing are using the 70F reference temperature instead.
Their is a higher pressure molbox1+ to obtain higher flow rates with the high pressure calibration type using the same molbloc (columns not highlighted above to the right) and there is also the option of using vacuum to obtain lower flow rates (with the Standard Pressure or Low Pressure calibration) also using the same molbloc (columns not highlighted above to the left). Most our customers do not opt for these configurations so won't discuss these topics further in this article, but keep in mind those capabilities as very useful in some applications.
There are a few things that further complicate achieving the above highlighted flow rates.
- The first is that if you are testing a device that is placed downstream of the sonic nozzle it can produce a back pressure. To avoid this, it's common to place the device being tested upstream and thus the sonic molbloc outlet remains directly open to atmospheric pressure. There are minimal concerns of the device being tested running upstream and contaminating a sonic molbloc since it is just a nozzle, but still be mindful that don't want trace liquids or contaminants in this type of system in general. You typically can't have something like a needle valve or highly restrictive device downstream as otherwise your minimum flow at which the proper BPR will be met will be too high taking up most if not all of the useable flow range. This is another reason why it is often just best to run the device being tested upstream if it can have some elevated pressures on it. Running the device upstream also allows that device being tested to have much higher pressures it might be designed for on it (typically using a backpressure regulator) that the molbox cannot handle.
- The differences in atmospheric pressures in different locations will also have an effect on the exact flow rate the proper BPR is obtained, but in most cases this is not too significant (still something to realize).
- In many cases it is not realized that there are significant considerations for the supply gas source being sufficient, large diameter plumbing, high flow regulators, temperature considerations, and similar might be needed to even achieve the sonic flow rate on many sonic molbloc's. This is also a deeper topic but mentioning it here as it might be what is causing you to never reach the proper BPR even though seems like you have a lot of flow going through the system. In cases where the larger diameter plumbing in not available/practical or similar the workaround is often to elevate the supply pressure up above what would normally be the 75 psi before starting any flow . However, be extremely cautious if turn in/increase the supply regulator more as once you start to decrease flow or if you were to accidentally shutoff flow with the isolation valve on the sonic molstic or similar the pressure will go up higher than what is being displayed on the gauge/molbox when flowing. As a result can easily accidentally over pressurize the molbox, one of the tubes being used to deliver the flowing gas, or similar. If ever have to turn the supply regulator in more than where it was originally producing the 75 psig before you started flowing, just reduce the supply pressure regulator down as you go down in flows to avoid this but it takes paying close attention to the pressures at all times (on the molbox can use the P&T button) so this in general is NOT recommended unless clearly understand the risk in doing so.
The final question that normally comes up when considering sonic molblocs and the associated BPR limitations is, what would be the next molbloc I should purchase if want to go to even higher flow's? We will continue to assume you cover 0 to 100slm with laminars and have the 5e2-S sonic molbloc that thus continues your flow coverage up to about 300slm with no gaps. If take a closer look at the charts above you will see every other molbloc is either shaded or not across its row in the chart. What this is attempting to tell you is once you select either a shaded or non-shaded molbloc the other molbloc's you will likely want would be the same to provide some overlap on the low flow end while giving you much more upper end flow coverage. So, the next logical molbloc selection after the 5e2-S would be the 2e3-S as it starts at about 248 slm (good overlap with the 5e2-S max of 300slm) and goes up to 1,200slm (the midpoint between the 500kPa and 700kPa columns).