Sierra Clearly Differentiates Through Accuracy & 4:1 Primary Standards

Use our stand against accuracy specsmanship games and our 4:1 primary standard calibration practices to your advantage in your next competitive situation.

Among our scientific instrument competitors, namely those that compete with the 100 Series, 800 Series and 50 Series, we have seen a great deal of discussion about accuracy and a great deal of “specsmanship” in the industry. Customers are generally confused as to what an accuracy statement really means to them. Accuracy goes far beyond a simple statement on a spec sheet where one competitor’s number is better than the other. Sierra’s firm stance on accuracy statements is no specsmanship, be honest and conservative, and back specs up with testing data and world class calibration equipment and methodology.

First, we want to remind you that our industry-leading Smart-Trak® 2 is available with improved calibration accuracies. When accuracy is critical,  your customers have a choice of the standard +/- 1% of full scale using our innovative Dial-A-Gas® Technology  or our premium A1 accuracy spec using actual gas calibration for accuracy of +/- 0.5% of full scale (available in the Smart-Trak 2 price list as the A1 option). A Smart-Trak® 2 with a premium A1 calibration is one of the most precise mass flow controllers on the market, because its design was focused on getting the physics correct. Excellent stability and performance results from a patented, inherently linear Laminar Flow Element (LFE) design, advanced Capillary Tube Platinum Sensor technology, a proprietary Frictionless-Hovering control valve and 4:1 Primary Standard Calibration.

Instrument design is only half the battle. Our philosophy is that the purpose of a flow meter is to transport excellent calibration accuracy to the field, thus providing the most accurate flow measurement information to the end user. In the case of an MFC, excellent calibration also means accurate flow control.  As a result, the accuracy of information provided by our flow meter to the customer depends a great deal on its calibration. Sierra maintains nearly 40 primary standard piston prover calibration systems worldwide.  Each and every capillary thermal instrument we manufacture (100 Series, 50 Series, 800 Series) is calibrated on a NIST (National Institute of Standards and Technology) traceable primary standard (our proprietary Sierra Cal=Bench® Piston Prover as well as our Cal=Trak product) with strict adherence to the 4:1 rule.

So, what is the 4:1 rule? You may not have heard of it because our competition prefers it that way. Sierra maintains the ISO 17025 mandated 4:1 ratio of propagated transducer uncertainties at every calibration step. What this means is that in a calibration accuracy propagation chain, the subject mass flow meter or controller device under test (DUT) can only be calibrated by a “standard ” that itself has an accuracy approaching one-fourth that of the DUT. This level of commitment to flow measurement accuracy requires that Sierra uses only primary standard piston provers with better than +/-0.2% accuracy.  Many of Sierra’s provers, especially those designated for the Smart-Trak 2 premium accuracy calibrations, (the A1 option) are better than +/- 0.125% accuracy.

All piston provers at Sierra are themselves calibrated using NIST (National Institute of Standards and Technology) traceable measurements of every parameter involved. Sierra does this in order to drive the initial uncertainties to the lowest possible level. As a result, this allows Sierra to claim (and prove) our standard Smart-Trak 2 accuracy statement of +/- 1% of full scale and A1 premium actual gas calibration accuracy of +/- 0.5% of full scale.

What makes “Primary Standard” calibration so much better than “Transfer Standard” calibration?  A primary gas flow standard uses only units of volume, which can be broken down into length and time to measure flow. Since both length and time are clearly defined by international standards, these are called primary measurement units. Sierra’s Cal=Bench piston prover uses a tube of fixed volume and measures the rise time of a piston inside the tube to determine the volumetric flow rate. By adding highly accurate temperature and pressure measurement, mass flow rate is obtained.  Many competitors use transfer standards as a calibration device–typically another flow meter of higher accuracy than the DUT. A transfer standard is typically a 2:1 device with accuracy only twice as good as the DUT.  And by definition, transfer standards are NOT based on primary units of measure, thus they can never be called primary.

Since many competitors, like Brooks, use transfer standards (i.e. Molbloc), as a calibration device, they must rely on specsmanship.  Misleading accuracy statements from the competition are in small print next to their accuracy number. Misleading statements may read, +/-1.0% of Full Scale (at calibration conditions).  This means that the supplier only claims the accuracy while it is in their calibration lab and on their standard, not while in the field in the actual application.  Other examples of misleading statements sre: “at operating conditions” or “at laboratory conditions.” These companies clearly do not follow the 4:1 rule. Once the instrument goes into the field, the accuracy specification is in question, and more importantly, the information the flow meter provides can be less dependable.

Sierra wins a lot of business because we do not make “at operating conditions’ type statements in our specs. Use our stand against accuracy specsmanship games and our 4:1 primary standard calibration practices to your advantage in your next competitive situation.

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