From on-site calibration to on-board compensation of pressure and temperature

The importance of volume flow measurements to air quality and operating costs has been underestimated for many years. The observations below clearly illustrate this point:

1. 1. Although verification measurements are usually per- formed during the start-up process for any ventilation system, it is the case that almost no other measure- ment is “fudged“ as regularly as the volume flow to be ac Inspection measurements prove that deviations (usually shortfalls) of up to 40 % are not unusual.

2. Until a few years ago, there was simply no practical and sufficiently accurate process using a handheld device for obtaining instantaneous volume flow mea- surements in air ducts, which often have highly in- dividual inflow and outflow layouts. Nevertheless, this gap has now been plugged by the universally recognised standard DIN EN 12599. However, in cer- tain cases, it is well worth checking that the company performing the work really understands and is able to execute this process.

3. Even if a fixed system was installed with the intention of obtaining continuous volume flow measurements, it was almost always implemented “half-heartedly“ with no on-site calibration and adjustment. Although the air duct routing forced designers to create adven- turous or even risky inflow and outflow routes, instru- ments for measuring volume flow (and regulators) were installed at completely unsuitable locations. The absence of adequate inflow and outflow routes gen- erates asymmetric flow profiles at the measurement locations – and these profiles often change over time.

In this type of measurement scenario, failure to per- form on-site calibration and adjustment results in a large measurement error.

4. Even at critical measurement locations in process air technology applications (e.g. in drying technology), designers neglected the fact that volume flow mea- surements change significantly when temperature and static pressure values change.

But maybe that‘s quibbling. After all, why take such enor- mous trouble to measure volume flow if perhaps it does not really matter?

That may be true for some applications. Yet there is a growing recognition that excessive volume flow in air conditioning and process air systems sends operating costs spiralling upwards. Not only do ventilator fans con- sume extra energy in moving the surplus air around the system but expensive air conditioning processes (cool- ing, heating, humidifying or dehumidifying) generate fur- ther costs by treating unnecessarily large volumes of air.

On the   other hand, operators and planners have rec- ognised that too little volume flow also creates problems as it results in an inadequate air supply. In other words, it is no longer possible to ensure the required air supply in the zone of use with negative consequences for the per- sons or products affected. Moreover, an insufficient vol- ume flow can also lead to hygiene problems – from oxy- gen deficiency to accelerating bacterial growth in moist, slow-moving air.

The measurement technology specialist halstrup-walcher identified this problem and responded immediately by of- fering a complete package solution:

 

1. 1. The company supplies pressure transmitters, which are perfectly tailored to the requirements of volume flow measurement, such as the newly launched P 34 differential pressure transmitter.This provides the fol- lowing functions:

1. a) compensation of pressure and temperature values with the aid of an on-board static pressure sensor and analogue temperature input that can be used in a variety of ways (optional, if required)

1. b) optimum support for the start-up tec The volume flow   can be assigned to the differential pressure using any of the three methods described below:

• by entering a k-factor (volume flow = k * root (differential pressure))

• by entering a volume flow-differential pressure value pair (e.g. after determining the design at the operating point) or

• by using an individual 20-point curve. This is particularly advantageous during calibra- tion / adjustment of complete test sections.

These options dramatically simplify start-up oper- ations and therefore also the accuracy of a wide variety of applications.

2. Selection, delivery and (on request) installation of the appropriate primary element.The simple X-grid is not always the right solution. The key is to find the cor- rect compromise between the available budget and the accuracy required. Some measurement locations also require the installation of a simple flow straight- ener, without which the obtained results would be completely unreliable. Most suppliers of measure- ment technology today do not provide design and consulting services of this kind as these are time consuming and require expertise.

3. On-site volume flow calibration: we always offer and include on-site calibration for every volume flow measurement point (in accordance with DIN EN 12599). This takes into account, and for the most part compensates for, the   individual deficiencies of the measurement location. Based on this calibration, the pressure transmitter is then adjusted to achieve optimum precision.

4. Volume flow calibration in an accredited laborato- ry: not only primary elements (determination of the k-factor) but also balometers, ventilators and com- plete test sections (sections of air duct with primary elements and suitably adjusted differential pressure transmitters) can be calibrated. This   is particularly interesting for machine and plant designers because customer-specific air duct designs can   be obtained with factory calibrated volume flow measurement lo- cations, if necessary including the appropriate flow straighteners. Incidentally, our accredited laboratory can also be used to calibrate mass flow.

Clearly, the difference lies primarily in being able to ob- serve all the relevant aspects of the volume flow mea- surement as a whole. In practical applications, the ac- curate, stationary (permanent) measurement of volume flow is essential for maintaining air quality and keeping operating costs under control. Volume flow has awoken as a measurement parameter and deserves this attention.

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