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Measuring ambient conditions in the field

Our Swiss partner Rotronic and its versatile invention - the handheld relative humidity and temperature meter is an indispensable instrument with graphical display, audible and visual alarms, interfacable with all HC2 probes.

The importance of monitoring ambient conditions, in particular the accurate measurement of ambient temperature and relative humidity, is crucial for the correct functioning of measuring instruments and the evaluation of the results they produce.

Each measuring instrument has its own operating and storage area in addition to the specified measuring area,
but this is not the same for every type of instrument, and is always given by the manufacturer. These are operating temperature/humidity and storage temperature/humidity.

Measuring ranges and operating ranges

When we look at a typical specification of a measuring instrument, we read:

  • the measuring range (if it is a thermometer, this is the temperature range from to; if it is a relative humidity meter, this is the humidity range from to …). This information tells us in which measurement range our instrument can measure and what it is measuring. A common misconception – when measuring instruments have the possibility to connect different sensors, we need to be very careful, as the sensor is subject to a separate specification, often narrower than the instrument specification, and may even be tailored to the measurement site.
    • In precision meters, one or more narrower measuring ranges are often given alongside the full measuring range, in which the meter achieves the best values or different accuracy classes. Outside these areas, accuracy is poorer, which must be taken into account when evaluating the results.
  • ambient conditions, often expressed as:
    • operating temperature and relative humidity
    • storage temperature and storage humidity

Sophisticated, more precise laboratory instruments may have a very narrow band of optimum operating conditions, while industrial or less sophisticated instruments may have a very wide band. It is therefore up to the user to pay attention to what their instrument can do and under what conditions, and what it should not be exposed to.

Measuring instruments also have accuracy defined within the optimum conditions. To achieve this accuracy, ambient conditions must be within the declared range.

What does evaluation of results mean?

Most often, the user will read the displayed value from the display and take it as the correct value without hesitation. In metrology, this is called blind confidence in the result. We encounter measurements at the moment of birth and every day of our lives, so we take them for granted that they are accurate, or accurate enough for us to exist. It is only dangerous when this subconscious mindset is transferred to measuring instruments in industrial processes.

Evaluating the results means evaluating the read value – making a critical judgement: checking the specification of the meter, sensor and measurement system, checking the calibration certificates for the equipment, taking into account the correction and measurement uncertainty given by the calibration laboratory, taking into account our own measurements of ambient and storage conditions, taking into account our own various contributions to the measurement, taking into account our own contractor’s error, etc. That’s not easy! Evaluating the results requires tracking equipment, surroundings, measurements and personnel, as well as professionally trained operators with a wealth of experience. Staff experience is very important in evaluating results. We learn to evaluate the results and thus consciously overwrite the previously unconscious actions.

In this article, we want to bring you closer to the importance of monitoring temperature and humidity as environmental factors, since these two physical quantities are also the ones most often specified in manufacturers’ technical specifications and can in fact have a very large impact on measuring instruments, their accuracy, creep and lifetime. We should remember that everything can be carefully monitored, but what happens when we send a measuring instrument for calibration and it is -15 °C outside or +60 °C in the car? Do the ambient conditions of our measuring instrument still cover this range? What about high humidity, sensitivity to vibration, etc.?

Not only when they are in operational status, we monitor the measuring instruments 24/7 to ensure their optimal performance.

Below we have compiled some of the most important contributions to user measurement uncertainty that we see making the biggest difference to lay users (note: this is intentionally lay content, not directly related to calibration procedures; however, the content is professionally validated by staff authorised to interpret it).

1. Impact of temperature fluctuations

When measuring the ambient temperature of the environment in which you are using your instrument, you need to measure in close proximity to the instrument with an ambient conditions meter (Figure 3).

It is also important not to expose the measuring instrument to too rapid temperature fluctuations during use, as this will reduce its accuracy. As a result, our measurement results may unknowingly deteriorate.

2. Tempering process

For all meters, it is important to temper the meter before starting to measure or use the instrument. This means that when the measuring equipment is transferred from an environment with different temperatures to the place of use, the measuring instrument is exposed for a sufficient time to the ambient conditions in which it will later be used. This is most noticeable when the instrument is exposed to outside temperatures in winter and then brought into a warm room. Before using such an instrument, and to ensure the best possible accuracy of the measurements, you should allow enough time for the instrument to internally equilibrate to the ambient temperature – to temper itself (note: pay particular attention if there is a lot of air flow, draughts, etc.).

3. Taking the correction into account

When reading the results, the correction and measurement uncertainty from the calibration certificates must be taken into account and a confidence level must be given.

4. Repeatability

When we take measurements, we always ask ourselves what I need to write down and take into account so that I can repeat the measurement under the same conditions if necessary. Once this question has been answered, the basis is laid for a process that is essential to ensure that measurements are always taken in the same way. But if the conditions are different, our result will be different, and rightly so. The personnel taking the measurement itself has a huge impact on the result, so writing down the procedure is always the right direction (a procedure is nothing more than an instruction manual – it’s up to you to decide how detailed you need it to be).

5. Staff

As mentioned in the article, people take measurements for granted, and the results shown are equally so. This may be perfectly adequate for simpler measurements and private life, but it is not sufficient for process and industrial conditions, especially not in the age of automation and digitalisation, when everything is sensor-based. All those measurements that drive automated systems, all those measurements that are part of control systems, all those measurements that are part of quality control or health measurements or development or regulatory control, etc. must be carried out by professionally trained staff, called metrologists. At the same time, metrologists need the right skills, equipment, resources and conditions to carry out their work independently.

Our experience

In ELPRO Lepenik & Co., d.o.o., Calibration Laboratory we use a hand-held meter from Rotronic. On a Rotronic meter, type HygroPalm HP23 is connected to a temperature and humidity probe of the type HC2A-S .

Figure 1: HygroPalm HP23 monitorsambient conditions in the laboratory
Figure 2: HP23 meter with connected probe HC2A-S
Figure 3: an example of how Calibration laboratory uses a meter and probe in the field

HP32 HygroPalm ročni merilnik ROTRONIC

  • ročni (prenosni) merilnik relativne vlage in temperature
  • komunikacijski vmesniki: micro USB
  • programska oprema: Hygrosoft
  • baterija: 3.7 V, 1000 mAh  (integriran)
  • izračuni relativne vlage, temperature in psihrometrični izračuni

HC2A-S standardno tipalo za vlažnost ROTRONIC

  • senzorji:
    • vlažnosti: Rotronic HT-1 /
    • temperatura: PT100 1/3 DIN razreda B
  • obseg uporabe: -50 … 100 °C / 0 … 100 % RV
  • točnost pri 23 °C: ±0.8 % RV / ±0.1 K

☏ Call us: +386 2 62 96 720

They are always available for opinions and explanations:

Aleksandra Lepenik, expert in temperature measurements

Measurement expert Zoran Lepenik

You are invited to read related articles:

By Blaž Tomaž Vertačnik, Aleksandra Lepenik; October 2023

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