The Importance of Instrument Re-calibration

Measuring instruments, including flow meters, should provide reliable, repeatable data especially when used:

• for mission-critical applications like energy management
• to make important decisions on the best measures to reduce energy waste
• for accounting purposes such as to get rebates from local utility companies or the government and to allocate costs.

If accurate data is so important, why do we still see so many un-calibrated flow meters in the field (i.e. meters without a valid calibration certificate)? Why do we still find metering equipment which has never been calibrated since it was first installed? Typical arguments we hear from the market are:

• Calibration is regarded as being cumbersome
• Calibration is expensive
• Calibration is time-consuming
• Calibration and maintenance is out of scope

Let’s take a look at these arguments in detail to see how valid they are:

Calibration is cumbersome

Calibration can be cumbersome … Yes, it can be! This is especially true when measuring flow in large pipe sizes above 3 inches where flow meters can become large, heavy and hard to remove and re-install. In fact, it can be nearly impossible to remove them to send them back. But there is good news. You can install a lightweight, insertion type meter, which can be easily maintained. There are even measuring instruments on the market where the critical part can be exchanged on-site. In other words; Choosing the right sensor technology is of utmost importance to make calibration as easy as possible.

Calibration is expensive

Indeed, there are costs involved for calibration. These are especially significant when going to a third-party lab since they often charge 500 to 1000 Euros per instrument. The costs for decommissioning and recommissioning measuring equipment, packing, shipping, handling, and customs, should also be considered.

So yes, calibration costs can be experienced as expensive. Consider this though; Flowmeters provide the input for your energy savings decisions, which may involve huge investments, like acquiring a new compressor or compressor control system. So, think about the consequences of buying the wrong compressor or running it with a less than optimal control system. What are the consequences of making a decision based on bad data? Not calibrating your instruments can lead to bad decisions.

We have seen uncalibrated instruments that were off by 100%. We also ran into a case where someone blamed the compressor for delivering 30% less airflow; but, after mediation by a third-party lab, the orifice flow meter tuned out to be improperly calibrated. This was for a large, > 750 kW, compressor running 24 hours a day and the cost for calibration would have been insignificant when the total cost to resolve the issues is considered. Is this not often the case; that calibration costs are not significant when looking at the total picture?

Calibration is time-consuming

The old-fashioned way to do calibration is to remove your instrument and send this to a lab or supplier. The calibration period plus the time for decommissioning, shipping and recommissioning can easily be three weeks.  During this period, you are without data from your instrument.

Nowadays there are manufacturers who offer sensor exchange services, which can be compared to changing the tires on a race car. After a short pit stop, you continue measuring normally. You do the exchange on-site without having to send the sensor back to a calibration lab. With an exchange, there is no waiting or wasting time. After the exchange, you simply return the sensor that was removed within a reasonable time frame.

Calibration is out of scope

We have seen cases where an energy monitoring system has been implemented, but after the system is up and running, sensor recalibration and maintenance is not included within anyone’s budget. Some are unaware that all sensors, that are part of an Energy Management System, should be maintained regularly. For each sensor type, a calibration interval should be defined and for companies with ISO-9001 certification, recalibrating sensors is mandatory. The requirement to maintain calibrated sensors also applies to companies with an ISO 50001 certified energy management system. In some cases, when energy rebates or cost allocation is involved, regular calibration is the law.

Conclusion

Most arguments not to calibrate don’t make good sense and for ISO-certified facilities, calibration is often required to maintain certification. So, be on the safe side and re-calibrate your instruments and sensors.

Calibrate

“to standardize (something, such as a measuring instrument) by determining the deviation from a standard so as to ascertain the proper correction factors”

Calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. The standard could be another measurement device of known measurement uncertainty It can be also a device generating the quantity to be measured. The calibration standard is normally traceable to a national standard held by a National Metrological Institute.

The outcome of this comparison can result in no significant error being noted on the device under test, a significant error being noted but no adjustment made, or an adjustment made to correct the error to an acceptable level. Strictly speaking, the term calibration means just the act of comparison and does not include any subsequent adjustment. In life, we are continuously calibrating, without even noticing it. For example, when you drive your car, you might use your GPS as a standard to ”calibrate” your speedometer. You can try this (safely, of course) to determine the deviation between the two.

Why calibration?

Calibration is a “necessary evil”. You simply cannot depend on a measuring instrument which is not calibrated on a regular basis. Sensors and electronics may drift, they might get polluted or mechanically wear out. These potential issues are something you will want to keep track of and to correct using a reliable, traceable reference standard.

For most instruments, the recommended recalibration interval is annual. The question of the best re-calibration interval can be disputed and it will depend on your specific situation. The interval depends on air quality, on the intensity of use, and on process parameters, etc.

• Air quality: When your air quality is poor, fouling of the sensor element of your flow meter can occur and it can even damage it. Your flow meter might need recalibration more often than once a year. ISO 8753.1 is a reference for air quality that might help you determine the best recalibration interval for your situation.
• Legislation/certification: If you are a company that is ISO 9001 or ISO 50001 certified, regular recalibrations are mandatory. The calibration interval for each instrument needs to be defined by company policy and then adhered to. You can start with an interval of once a year and modify it as needed to the specifics of your situation. Be sure to update your company policy when you change the calibration interval.
• Energy monitoring: If your goal is energy savings, you need to make sure your equipment is recalibrated on a regular basis to avoid basing decisions on incorrect data. We recommend an annual re-calibration for these applications.
• Auditors: Professional air auditors need equipment that is maintained, calibrated and in excellent condition. As an auditor, you might want to insist on recalibrating your equipment before every audit. Your customers must have confidence in your results, which should be traceable to a National Standard.
• Incidents: for example, if a flow meter is exposed to physical conditions which are out of specifications, it is wise to have the sensor checked and recalibrated. For example, when exposed to an extreme temperature above the maximum limit, the internal sensor could be damaged. Unless you do a calibration check, you will not know for sure that the sensor is working correctly.

A few final words

Calibration is key to maintaining the accuracy of your flow meters and sensor in general. Instead of looking for arguments against calibration, consider the consequences of not calibrating your instrumentation regularly.

Buying an instrument is not a “one-time” occurrence.  The need for calibration and maintenance must be considered as part of the cost of ownership. So, inform yourself about the calibration options and consider the total costs when purchasing a flow meter or other instrumentation.