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FAQ - CALIBRATION AND METROLOGY

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Rules and regulations in metrology

What are the legal regulations in the metrology?
Each country regulate the legal status of metrology its own laws. Legislative regulations are in the countries differ from each other. Some measurements are so significant that only measuring instruments that meet specific requirements may be used. What measurements and what technical requirements must be meet is to be found in the legislation of the country. These are usually measuring instruments of consumption of electricity, gas, water, various billing measuring instruments etc. This can be found usually on the website of the National Metrology Institute. This meters are also called the legally controlled meters.

Is the list of legally controlled measuring instruments in all countries the same?
Not the same. Can be measurement with required legally controlled measuring instrument in one country but in another country is not required.

What requirements must comply the legally controlled meter?
Further information can be found on the website of the National Metrology Institute or it may be consulted with his staff. Usually it is a type approval for used application and then requirements on regular verification of meter. Legally controlled measuring instruments are verifies, other measuring instruments are calibrated. WARNING - This terminology may be different in different countries! Type approval process is very time-consuming and costly.

I have a meter that is approved as legally controlled in my country. Can I use it in another country?
You can not use it without other things. Discuss this matter with your relevant National Metrology Institute. It is possible that this meter will be approved in this country. Much depends on your supplied documents. However, it will also be that the local meteorological institute will require to make the entire approval process again.

What is the difference between verification of legally controlled meters and calibration of other of measuring instruments?
Calibration of measurement instruments is performed in accordance with EN 17025 standard and with common practices and rules in metrology. If the calibration certificate contains all requirements including confirmation of traceability of standards, it is internationally respected documents usually. Verification of legally controlled meters is performed according to specific rules, mostly by standards relating to the application.

Have some Comet products type approval?
Yes, some Comet devices from the Datalogger R/S and T-PRINT line are approval for storage and distribution of frozen foods. TÜV certificates are available at www.cometsystem.com


Calibration certificates

Is Comet system calibration performed according to standard EN 17025?
This standard contains complex requirements for calibration laboratories, the Comet system meets only a part of them. Comet is not accredited according to EN17025. Its calibration certificates are in accordance with the terms of this standard only.

Are the Comet products delivered with calibration certificates?
Yes, the final products are delivered with calibration certificates. For the partial components (eg. temperature probes) calibration certificates are not created. If device is the ordered with probes and it is clear from order which channels are connected to the probes, the calibration is performed with these probes. Calibration of physical quantities does not cover the entire measuring range of the instrument, examples of calibration certificates can be found here: http://www.cometsystem.cz/support/calibration. If this calibration certificate not meet your requirements, then we can deliver calibration certificate from the accredited calibration laboratory. Comet system s.r.o. is not accredited calibration laboratory.

Do I Need the calibration certificate from the accredited calibration laboratory?
Accredited Calibration Laboratory is independent laboratory that meets specified requirements and is permanently supervised. In some areas it is possible to supply only products with AKL calibration certificates (automotive, health, etc.). It depends on the application of meter, legislation or on your internal regulations. Also if the amount of calibration points from Comet is not sufficient (especially in physical quantities).

Can it be expiration date on the calibration certificate?
Calibration certificate describes the characteristics of the device at the time of calibration and says nothing how the device will be measured in the future. Is written in the EN 17025 Article 5.10.4.4.: A calibration certificate (or calibration label) shall not contain any recommendation on the calibration interval except where this has been agreed with the customer. This requirement may be superseded by legal regulations. COMET system does not write expiration time on your calibration certificates.

How to determine the calibration interval of meter?
The calibration interval is determined by the user the meter.
This must be taken into consideration:
- legislative requirements and internal company regulations
- manufacturer's recommendations which is given in the manual to the device. Unless otherwise stated, we recommend the following calibration interval
Temperature                     2 years
Relative Humidity            1 year
Atmospheric pressure    1 year
CO2                                    5 years
  - required accuracy of measurement (if the accuracy of the device is ± 0.4 ° C for two years calibration interval and user is sufficient to accuracy ± 1 ° C, this interval can be extended usually)

- trends from previous calibrations. If after several calibrations the meter remains deeply below acceptable limit, we can extend the calibration interval. If the trend is unfavorable, we must to reduce calibration interval. Repeated reduction of calibration interval indicates that the meter is not suitable for this application.
- working conditions, rate of use of measuring instruments, level of service, the risks associated with exceeding out of accuracy of measurement

Is it possible to calibrate the measuring instrument in situ?
Yes, it is possible but you have to agree it with your local calibration laboratories. The advantages are: usually it is not necessary to shut down the equipment and calibration better describes state of measurement in the specific operating conditions of the device. The disadvantages are higher uncertainty than in the laboratory, often limited range of calibration and generally higher price (it is necessary to arrive with special equipment).

What is the interpretation of the the term „accuracy“ of Comet products?
Comet system uses the classical model of accuracy expression. For example, if the thermometer measures of +23 °C with ± 0.4 °C accuracy, the actual temperature is in the range (22.6 to 23.4)°C with rectangular probability distribution.

What is meaning of „uncertainty“ term on the calibration certificate?
This term expresses the calibration uncertainty. It is not to measurement error or uncertainty of the device! The quality of the calibration depends on several independent things that are have different probabilities of occurrence (repeatability, uncertainty of standards, homogeneity of the temperature in the chambers, the resolution of the measuring instruments, the parasitic temperature dependences etc.). Using statistical methods are quantified these various effects and shall be included in resulting uncertainty, that is mentioned on the calibration certificate.

How to evaluate the results on the calibration certificate?
There are different ways to present the results on calibration certificates. Comet system always gives tables of measured values with their uncertainties. Generally, if the measurement error after adding uncertainty does not exceed the device specification, it is a statement that device "complies". If the measurement error itself is in the device specification, but after adding the uncertainty is out of limit, it is state "can not determine compliance or non-compliance." Other cases are already out of specification, the calibration certificate can not be created and the device can not be dispatched. If the measurement error does not exceed the specification, but after adding uncertainty is out of limit, then in case if the deviation is so small that the probability of device measured in its specification is greater than 95%, the calibration certificate is created and the device is dispatched.

What is it EA4/02 document and where it can be find?
This document describes the methodology for calculating of uncertainty in calibration. You can download it from here: http://www.european-accreditation.org/publication/ea-4-02-m

I have a separate calibration certificate for the device and a separate for probe. Can it be considered as a calibration certificate for the entire measurement chain?
No, generally is not true that separate calibration certificates for components to create a calibration of system as whole. Especially in critical applications (such as blood banks, etc.) is not possible to do it so. There is always necessary to perform calibration of the all measuring chain as whole. Calibration certificate has its full value if describes the relationship between the measured value and the indicated value what the customer use. If the chain is divided into several parts, the result can never give such weight as in the previous case. Some times can be expected very high probability that the problem does not occur (eg data logger with sensors 4mA to 20mA - measurement error of logger in compare to measurement error of temperature sensor is almost negligible), for some other devices it can not be accepted.

Is the Comet calibration certificate valid in other countries (eg. complies NIST)?
Yes, on the basis of arrangements CIPM MRA of 1999, the Czech Republic (by CMI) has become a part of this Agreement. Calibration certificates traceable to CMI are accepted in other countries (including NIST). More information is on www.bipm.org.

Can I require from Comet to proof of metrological traceability of uses standards?
On each calibration certificate is introduced that Comet has secured metrological traceability of your standards and confirms it with signature. If you find this argument as insufficient, ask our for calibration certificates from our standards. This is non-standard and paid service. Not all these calibration certificates are available in English.

What is the difference between „calibration“ and „validation“ ?
The aim of calibration is the most accurate to describe the behavior of the device, the goal of validation is to quickly and inexpensively verify that the instrument is not wrong. Validation is usually done in the time between scheduled calibrations.


Thermometers - calibration and measurement accuracy

Choosing of probe for temperature measuring in refrigerators and freezers.
For these purposes are often used Pt1000TG8 probes with different of probe body length. The probe cable length should be as short as possible. If the probe is connected to the transmitter to another interface (current loop, RS2485, Ethernet), it is commonly used probe cable length 2m max.
Sometimes it is necessary to measure the temperature of the liquid (eg blood bank). In this case, the probe is usually inserted into the saline bag. If cable probe is used, do not insert the junction probe/cable to the liquid, otherwise the probe will soon be destroyed. It is necessary to use a probe with longer metal body.
In refrigerators or freezers is significant fluctuation of temperatures and large volume temperature gradient inside. Sometimes it is necessary to significantly increase the response time of the probe. It can be done eg. with the metal thermowell (take care for its orientation - in order to drain any water condensation).

Problems during calibration of cable probes.
If the probe is calibrated in the liquid, then there must be its sufficient immersion to avoid influence due to heat dissipation by cable. At the same time there must be respected that junction probe/cable must not be long time immersed in the liquid. Sometimes, the probe is inserted into the appropriate thermowell or into eg. latex gloves.

I have problems with the calibration results of data logger with internal temperature sensors.
For a correct calibration of R/S data loggers is needed more time and the correct method. For this is elaborated detailed calibration manual that we send you on the request. When you using other methods the larger errors in calibration may be.

Problems with calibration of thermocouple probes.
If thermocouples are used for long-term measurements at high temperatures (eg 1200 ° C), the chemical changes of the material may affect the accuracy of measurement and thermocouple life. Therefore, it is better for these applications to get first experience with more frequent calibrations and next accordingly specify the calibration interval.
Evaluation device usually contains thermometer for cold junction temperature measuring. Therefore, this device must be installed in places where are minimal changes of ambient temperature (must not be placed in a forced air flow, eg from air conditioning)..


Humidity meters - calibration and measurement accuracy

Problems with the measurement of humidity in refrigerators and freezers.
When installing a hygrometer to the place with frequently temperature changes (eg. by switching of compressor of refrigerator), it may happen that relative humidity in the the space will cycle through almost whole range (eg. 10-100)% RH. This is a physical effect. Not a malfunction of measurements but it is improperly chosen cooling system.

The device does not measure humidity correctly, what to do with it?
It is possible send us device for repair or you can make adjustment yourself. You need the appropriate equipment for it and the manual for adjustment (on the request). If you need to adjust the relative humidity, make sure that the device is correctly measured temperature. If the device does not measure the correct temperature, humidity adjustment will be wrong! In addition, perform a visual inspection. If the transmitter or sensors are mechanically or chemically damaged, send it back for repair.

The device measures the correct temperature and correct humidity but not measured correctly calculated humidity values. What to do with it?
Check if the ambient pressure setting in the instrument correspond to reality (use TSensor SW).


Other devices - calibration and measurement accuracy

The device does not measure correctly the concentration of CO2. What to do with it?
First check if the pressure (or altitude) setting in the device is corresponded to the reality (use TSensor SW). The correct setting has a significant impact on the measured value.