So, how accurate can a transmitter be, anyway?
This is a question I frequently hear from both my customers and my prospects. While I understand why my customers ask this question, the real question they should be asking is, “How repeatable is your transmitter?”.
What’s the difference? Glad you asked.
Repeatability refers to how closely something – an instrument, for example – will reproduce a measurement given the same test conditions.
Accuracy, on the other hand, refers to how well that same something measures up to a different assessment of the same thing. When it comes to consistency measurements, accuracy typically refers to how well a particular transmitter measures up to a lab assessment of the same stock.
The lab assessment could be anything, but it is usually some variant of the TAPPI 240 method and this is where the problem comes from. The TAPPI 240 method specifies a repeatability of 10% for that test, which means that 95% of the time, the lab test, if executed as specified, will yield results within 10% of each other. So, for a nominal test of 4.0% consistency, a second, properly executed test of the same stock sample should yield a number between 3.6 and 4.4%. Of course, the repeatability statement also says that 5% of the time , or once out of twenty tests, you could get a number that’s worse than that 10% limit.
What makes this really scary is that very few laboratories actually execute the TAPPI 240 test as described in the procedure. Many labs take short cuts – I once saw one guy try to squeeze dry a sample by stepping on it, for example - which means that the repeatability for the manual lab test may actually not be as good as 10%.
That’s the reason why we manufacturers prefer to talk in terms of repeatability rather than accuracy. While we can never be sure how accurate our transmitters will be relative to the procedures your lab uses, we can be very certain about how our transmitters will respond, given the same stock conditions. In the case of the TECO StockRite series of consistency transmitters, that repeatability is 0.0025 of the full scale range. So, if your transmitter is set to read from 2% to 6%, for example, the TMC6000 system will repeat to within no greater than 0.01% (-4.0 * 0.0025).
Which is pretty doggone repeatable, if you ask me.
So the correct answer to the question “How accurate is your transmitter?” is that we are highly repeatable.
