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Understanding Volumetric based Mass Flow Units: Difference between ulm/uccm vs. slm/sccm on a molbox

Understanding different flow units can be challenging in Flow Applications. Much of the confusion is due to the fact that Flow can be expressed as mass or volumetric based. This is whole other topic but related and further info can be found here: 


For this article we will focus specifically on the differences of selecting units like slm/sccm vs. ulm/uccm directly from the molbox front panel.  It is very important to understand this topic as molbox/molbloc and many other flow devices are mass flow devices, but it is very common to use volumetric based units to describe mass flow.


We will focus on slm vs. ulm as the same applies to uccm vs. sccm or other similar units. If we break down the slm unit it will start to make more sense. The unit slm is Standard Liters per Minute. Standard means there is some pre-defined “standard” or reference value this unit is based off of. Liters is a measurement which ultimately defines how much fluid will fill a volume of one liter and thus why this unit is a volumetric based unit.  Minute is the amount of time to the flow will be quantified in.


From the above break down the only thing that is still not clear is what the Standard component is referring to.  We know from basic physics that pressure and temperature are very important for volume measurements so this is what we need to define as a “standard” when comparing two devices to ensure it is an apples to apples comparison. Luckily it is universally accepted in most cases that the pressure "standard" is standard atmospheric pressure (14.696 PSIa, 101.325 kPa, etc.) so we are left with just figuring out what reference temperature needs to be used. There is no universally accepted standard for temperature as many different industries/manufacturers use different temperature “standards” unfortunately. It should also be noted that this temperature has nothing to do with the temperature of the lab or even the temperature of the flowing gas, it is simply a reference attached to the unit of measure.  


We choose 0C as our standard temperature to align with what most national laboratories use. As a result, choosing the unit of slm on the molbox means the flow unit is based off a reference temperature of 0C. This can add a lot of confusion as the large majority of industries use a different standard such as 21.11C (70F), 24.4C, 25C, etc. With the limited amount of room on the molbox display it was decided upon to create a unit called ulm (user defined liters per minute) to allow a different standard temperature to be entered. When selecting one of the user defined units it will prompt you to enter the desired standard/reference temperature.


The next question is typically how do you determine what the standard temperature should be. This is determined by the device you are testing. It is often printed on the label for the device or if not you will need to contact the manufacturer of that device to determine what the standard/reference temperature is. Need to be aware that many manufacturers allow the same device to be ordered/adjusted to different standard temperatures so need to check each device you calibrate rather than assume they are all the same. Once the standard temperature is verified you will then enter the same temperature after selecting ulm or the desired user defined unit. Only if the device you are testing is a 0C standard temperature device would it be ok to select the slm unit, as otherwise it will not be an apples to apples comparison.


It is very important to understand what the standard temperature value should be used as the difference is very significant. As an example, if you select 0C reference temp (or slm unit) on the molbox rather than ulm with a device under test that has a standard temperature of 21.11C there is about a 7.7% difference in flow (100 vs. 107.7523 slm) !!! Having a mismatch in standard temperatures between the reference and the device under test is the most common mistake made in flow calibrations. It is also not uncommon to find devices that were previously tested wrong and perhaps had their output adjusted to a different standard temperature than what it should be due to a lack of understanding on this topic.


To help avoid a lot of confusion on the molbox it is often recommended to just remove units like slm, sccm, etc. from the list of units displayed on the molbox menu and replace them with ulm, uccm, etc. By doing this you can still enter the 0C reference temperature that slm uses when it is needed or any other standard temperature. This also forces the user to think about the proper selection rather than it being "hidden" and perhaps thus overlooked.   


It should also be noted that when using our COMPASS software this is a bit less confusing. We are not limited as much on space for the units so we do not use the ulm unit or similar. You will see the units listed out as slm@21.11, slm@0C, etc. in this software to add further clarity.

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