Is Losing Business Worth the Risk?
Having an instrument not meeting specifications is bad for the owner or operator of an instrument and bad for the manufacturer. It can be costly. The owner may need to take special action to mitigate the
problem, perhaps even recall work done. The manufacturer may lose customers’ trust. Clearly, it is a situation to be avoided. Without due consideration, out-of-tolerance conditions may be much too likely to occur.
A calibration is a comparison of measuring equipment against a standard instrument of higher accuracy to detect, correlate, adjust, rectify and document the accuracy of the instrument being compared.
Typically, calibration of an instrument is checked at several points throughout the calibration range of the instrument. The calibration range is defined as “the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values.” The limits are defined by the zero and span values. The zero value is the lower end of the range. Span is defined as the algebraic difference between the upper and lower range values. The calibration range may differ from the instrument range, which refers to the capability of the instrument. For example, an electronic pressure transmitter may have a nameplate instrument range of 0–750 pounds per square inch, gauge (psig) and output of 4-to-20 milliamps (mA). However, the engineer has determined the instrument will be calibrated for 0-to-300 psig = 4-to-20 mA. Therefore, the calibration range would be specified as 0-to-300 psig = 4-to-20 mA. In this example, the zero input value is 0 psig and zero output value is 4 mA. The input span is 300 psig and the output span is 16 mA. Different terms may be used at your facility. Just be careful not to confuse the range the instrument is capable of with the range for which the instrument has been calibrated.
What are the Characteristics of a Calibration?
Calibration Tolerance: Every calibration should be performed to a specified tolerance. The terms tolerance and accuracy are often used incorrectly. In ISA’s The Automation, Systems, and Instrumentation Dictionary, the definitions for each are as follows:
Accuracy: The ratio of the error to the full scale output or the ratio of the error to the output, expressed in percent span or percent reading, respectively.
Tolerance: Permissible deviation from a specified value; may be expressed in measurement units, percent of span, or percent of reading.
As you can see from the definitions, there are subtle differences between the terms. It is recommended that the tolerance, specified in measurement units, is used for the calibration requirements performed at your facility. By specifying an actual value, mistakes caused by calculating percentages of span or reading are eliminated. Also, tolerances should be specified in the units measured for the calibration.
All test equipment readouts are accurate only to a certain level of uncertainty, or tolerance. The best test equipment has LOW uncertainties, but NO test equipment can give you a 100% correct output or input reading – there is a tolerance for all equipment.
For example, the Bird 43 series wattmeter and wattmeter elements have uncertainties of ±2% of the full scale on the meter and ±5% of FULL SCALE on the elements. As an example of how this can affect your accuracy and readings, let us assume that you have a 15 RMS FM transmitter in front of you. You have a Bird 43 wattmeter, a 50 watt FULL SCALE element and a 100 watt dummy load to connect to the output of the Bird 43 to measure the output power of your transmitter. You connect the transmitter to the input of the Bird 43, drop the element in and orient properly, and connect the ‘dummy load’ to the output. Energizing the transmitter, you see that the needle comes to rest at 14.9 watts on the Bird Wattmeter dial.
Many field technicians I know would consider this 14.9 watt reading to ‘gospel’ in that the transmitter is obviously putting out approximately 1/10 watt low.
But is this necessarily the case?
The answer is no, this is NOT necessarily the case, if you examine the uncertainty (measurement tolerance) of the Bird 43 and element used. For the sake of argument, let us assume the Bird 43 and this element you are using were calibrated together, so the 2% uncertainty of the meter proper is “washed out”. This leaves us with an uncertainty of up to ± 5% of full scale on any reading taken with this wattmeter and element combination. What does this mean to your 14.9 watt reading? This means that the actual power being put out by the transmitter could be anything from 12.4 watts to 17.4 watts. (All readings are ±5% of FULL SCALE – that is the uncertainty of any reading is ± 2.5 watts.)
One time, we had calibrated a communications test box – one of those pieces similar to an IFR 1200 or Motorola 2600 communications test set. The customer called up, stating that he was seeing up to a 3 dB difference between the transmit levels and the receive levels he had in the unit.
I reprinted the test data from the calibration so that I had it in front of me when I spoke with the customer. While on the phone, the customer told me the frequencies he had been testing at, and the differences he was seeing. I noted to myself that I was seeing the exact same differences he was seeing at these frequencies – but the unit was in tolerance, and had passed calibration. I then asked the customer if he was aware that the receiver level specification for this unit was ± 4 dB, and the generator specification for this unit was ±4 dB. Also, was he aware that the generator and the receiver were specified independently of one another?
Silence on the other end of the line for a moment. Then, “No, I did not realize this. In other words, you are telling me that to perform the measurements I desire to perform, I need better test equipment?” I agreed with the customer. I asked him if he had a specification sheet for his equipment, he did not have one – so I faxed him the specifications for his communications test box from the service manual we had in the technical library.
With the receiver being ± 4 dB uncertainty, and the generator being ± 4 dB uncertainty, there could be a difference in readings between the generator and the receiver of 7.99 dB and the communications test box would still be within manufacturer’s calibration tolerances.
As you can see, it is of critical importance to know the uncertainty (accuracy of measurement) of the test equipment you are using. Calibration verifies, or adjusts the equipment until the inputs/outputs are at or within those aforementioned manufacturer’s levels of uncertainty.
Thus – know the capabilities and limitations of the equipment you are using to perform whatever task is before you.
Mark Price, Sr. RF Technician
JM Test Systems, Inc.
Calibration Service
Since 1982 JM Test Systems has been providing NIST traceable calibration services to our customers. We are dedicated to a single goal: provide the best possible service for both our products and our customers.
ISO/IEC 17025 Accredited by A2LA ISO/IEC 17025 accreditation is your assurance that our work meets the highest standards including:
- Documented and controlled procedures
- Controlled calibration environment
- Trained technicians
- NIST traceability and standards of suitable accuracy
Our A2LA audits include compliance with ANSI/NCSL Z-540, ISO-10012-1, and Mil Std 45662A
Call JM Test Systems today for a quote at 800.353.3411 or send us a message.
