Most sensor manufacturers specify the calibration frequency of their instruments, however this can vary greatly with different environments and applications.  The simplest answer to this question is you should calibrate as often as necessary to insure that your system is working properly.  Usually it is advisable to track the response of new sensors more frequently until you understand how that particular sensor responds to your environment and application.  A calibration log can then be used to gauge whether to shorten or lengthen the frequency of calibration.  Also, a sensor should always be calibrated after a traumatic exposure, i.e., dropped in mud, left in a pickup truck on a hot day, etc.  Portable instruments usually require more frequent calibrations than fixed systems.

Back to Top

FAQ2 -  What flow rates should I use to calibrate my sensors with?

Most diffusion sensors require a low flow rate, either 0.5 or 1.0 LPM, to recreate diffusion conditions about the sensor.  Sample draw instruments usually require a higher flow rate, either 1 or 2 LPM, to overcome the draw rate of the sample pump.  Our CAL 2000 units can be used in either the diffusion mode with its internal pump or in sample draw mode with the pump disabled and the flow monitored by the internal mass flow controller.  Our VersaCal also has an adjustable flow, although not over the range of the CAL 2000.  Our CAL 2000 LT in contrast is available only with a set flow rate of either 0.5 or 1.0 LPM.

Back to Top

FAQ3 -  Are there any sensors that do not need calibrated?

All sensors should be calibrated periodically to insure they are in proper working order.

Back to Top

FAQ4 -  What is the difference between calibration and "bump testing"?

"Bump" or functional testing of a sensor is used to quickly determine if that sensor responds to a gas sample, usually of a higher concentration than the alarm point.  Calibration requires a known concentration of gas to be applied to the sensor until it comes to equilibrium.  This usually requires significantly more time than a "bump test" and a more accurate gas sample.

Back to Top

FAQ5 -  What is an "interference ratio"?

Some sensors will respond to more than one gas.  Different gases usually give differing signals for the same concentration.  The ratio of the signals is commonly known as the "interference" or "response" ratio.  If the interference ratio of two gases on the same sensor is known, either gas may be used to calibrate that sensor.  This is primarily important when one of the two gases cannot easily be obtained for calibration purposes.  An example is electrochemical ozone sensors that also respond to chlorine.  Chlorine is much easier to obtain and far more stable than ozone and is often used in its place.

Back to Top

FAQ6 - The manufacturer says I can calibrate my chlorine dioxide sensor with chlorine.  What is the difference between calibrating it with ClO2 and CL2?

Many sensors have an interference ratio to other gases (see FAQ5), allowing this interfering gas to be used to calibrate the sensor.  However, it is usually best to calibrate the sensor with the gas being detected if at all possible.  Interference factors may vary from sensor to sensor, or change as the sensor ages.  For example, while some ClO2 sensors do respond to chlorine, others do not and need to be calibrated with chlorine dioxide gas (cross sensitivity to chlorine is often undesirable when monitoring for chlorine dioxide).

Back to Top

FAQ7 - Why should I use your calibration gas generators instead of the cylinders I’ve been using?

Our calibration gas generators are typically much less expensive for multiple calibrations of corrosive gas sensors.  In addition, we do not have the stability and shelf life issues that cylinders do.  Also, some gases such as chlorine dioxide simply aren’t available in cylinders.  Growing concerns regarding the shipment of hazardous materials (such as calibration cylinders or permeation tubes) also make our gas generators an increasingly attractive alternative.

Back to Top

FAQ8  -  Can your instruments be used to calibrate any sensors, regardless of manufacturer?

Our instruments generate the actual gas desired in air,  therefore they will work with any brand or type of sensor, as long as it is supposed to respond to that gas.  For instance, a bromine sensor may be calibrated with our chlorine generator if the sensor responds to both bromine and chlorine.  Check with the sensor manufacturer to determine which sensors may be calibrated with our generators.

 Back to Top

FAQ9  -  What is the difference between electrochemical generation and permeation tubes?

Permeation tubes store the gas like cylinders, but in its pure form.  The pure gas then permeates into an air stream to give the final concentration.  Electrochemical generators do not store the gas, but rather release it from the electrolyte or electrodes as needed.  Since the gas is generated in small volumes, they are much safer to handle than permeation tubes.

Back to Top

FAQ10 - What is the difference between your QuickCheck Vials (QCV) and permeation tubes?

Permeation tubes contain a pure form of the chemical being generated inside the tube, and the gas constantly  permeates out at a continuous rate.  For this reason permeation tubes are hazardous to ship and store.  Our QuickCheck Vials contain a small amount of the chemical in a sealed ampoule, designed for only one days calibrations.  No gas escapes our QCV until they are used, giving them long shelf lives and the ability to be shipped and stored as non-hazardous items.

Back to Top

FAQ11  -  Do you offer NIST certification for your instruments?

NIST only offers one primary standard in gas mixtures, that being 25 ppm hydrogen sulfide in nitrogen.  We utilize this primary standard to certify all of our hydrogen sulfide instruments.  For other gases, like chlorine, we have developed wet chemistry tests that use all NIST traceable components.  This is similar to cylinder manufacturers who have utilized NIST traceable scales to weigh their cylinders before and after they have been filled.

Back to Top

FAQ12  -  Which instruments can I get NIST certified?

We currently offer NIST certification on our CAL 2000, CAL 2000 LT and EC Cal gas generators as well as the generating cells for each of these units.  We also offer NIST certification on our VersaCal and HydroGen units.  The VersaCal and HydroGen are certified as complete units.  Our standard NIST certification is + 10%.  We achieve this 10% accuracy by certifying the instruments to + 5% and the generating cells to + 5%.  Thus for 10% accuracy it is possible to swap generating cells between instruments (note that cells cannot be swapped between different types of instruments).  We also offer a more precise + 5% NIST certification on all of these units for those who require a more narrow range of operation.  To certify to this level we must match a cell with a specific instrument and test and calibrate the pair.  If the cell is used in another instrument it is no longer + 5% accurate.

Back to Top

FAQ13  -  Are your instruments affected by temperature changes?

Electrochemical generation itself is not temperature dependent, but changes of air density with temperature can affect the output if not compensated for.  Any of our instruments that use volumetric flow have either compensation built-in or have instructions on how to manually compensate for temperature and altitude changes.  Our Cal 2000 uses a built-in mass flow controller to automatically compensate for air density changes due to temperature, altitude, barometric pressure, etc.

Back to Top

FAQ14  -  Are your instruments affected by humidity?

Our generators use the ambient air to blend with the gas and are not affected by changes in humidity.  In fact, most sensors should be calibrated under their normal operating parameters which includes varying humidity.  All cylinder gases are 0% humidity, a condition the sensors never see in normal operation.  One effect of lower humidity situations is that our generating cells can lose water through dehydration.  Note that only water is lost, not electrolytes.  Thus for the CAL 2000 and CAL 2000 LT cells it is easy to simply refill the generating cell with either distilled or de-ionized water.  For our sealed cells such as the EC Cal and the MiniCal, the cell can be stored in a container with a moistened cotton ball so that water can be absorbed back into the cell.

Back to Top

FAQ15  -  Can your instruments be shipped by air?

Yes, our instruments contain primarily salts and water in small amounts that are sealed in leak-proof cells.  Since no gas is stored or generated when the instruments are turned off, they are safe to ship, handle and store.

Back to Top

FAQ16  -  Can you swap generating cells in your instruments?

All of our current instruments have the ability to read what type of cell is installed in the instrument and change its operating parameters accordingly. Most of our earlier instruments were designed to only use one cell at a time.  Thus, any CAL 2000 cell can be used in any CAL 2000 instrument.  However, cells cannot be swapped between different types of instrument.  For example, a CAL 2000 cell will not work in an EC Cal instrument.

Back to Top

FAQ17  -  How long do your cells last?

Our cells consume either the electrolyte or electrode during normal operation.  The "life" of the cell is dependent upon how large the cell is and how much gas is being generated.  Our smallest cells only provide about 2 hours of gas while some of our larger cells generate gas for up to 100 hours.  Since there is no shelf life on most of our cells, this translates into years of normal operation.

Back to Top

FAQ18  -  Does the electrolyte used in your cells have a shelf life?

Most of our electrolytes have an unlimited shelf life, the exception being chlorine dioxide.  Chlorine dioxide electrolyte is unstable in a liquid form.  For this reason chlorine dioxide electrolyte is shipped in a two part form of powdered electrolyte and distilled water.  In powder form the electrolyte has a very long shelf life.  When ready to use the two parts are mixed together.  Once the chlorine dioxide electrolyte is mixed into a liquid form it is stable for one to two weeks.

Back to Top

FAQ19 - Does ACD offer a 4-in-1 calibration blend for my confined space gas detector?

Many confined space gas detectors are configured to detect combustible gases, oxygen and one ore more toxic gases, commonly carbon monoxide and hydrogen sulfide.  To meet this calibration need many cylinder manufacturers provide a 4-in-1 blend of these four gases.  ACD cannot provide all four gases, but we can reduce your cost for this blend.  Hydrogen sulfide is the one reactive gas in this mixture, and its presence makes the gas more expensive with a shorter shelf life.  The CAL 2000 instrument can be configured to take in a 3-in-1 gas blend and then add hydrogen sulfide to it, making the 4-in-1 blend.  Using 3-in-1 gas gives much better pricing per liter of gas as well as a longer shelf life.  Contact your ACD representative for more information on this.

Back to Top

FAQ20 -  Can you generate carbon monoxide gas electrochemically?

No, carbon monoxide and most organics cannot be generated electrochemically.  In addition, carbon monoxide is relatively inexpensive and very stable so it is primarily used in cylinders.

Back to Top

FAQ21 - Are there any gases that are best stored in cylinders?

Cylinders work very well for higher concentrations of non-corrosive gases.  These gases and concentrations are often very difficult or impossible to generate.  Examples of these gases are carbon monoxide and methane.

Back to Top

FAQ22 - Can all of ACD’s QuickCheck Vials (QCV) be used in all instruments?

Currently ACD offers three instruments for use with the QCV.  These are the QC-50, the QC-100 and the VersaCal.  The QC-50 and QC-100 can use all QCV offered, while the VersaCal can use all except HCl.  The high accuracy PID used in the VersaCal does not respond to certain chemicals such as HCl. 

Back to Top

FAQ23 - What is the shelf life of ACD’s QuickCheck Vials (QCV)?

The QCV from ACD currently come with a one year warranty.  However, our expected shelf life is much greater than that.  The vials contain liquid in sealed ampoules.  As long as these ampoules remain sealed the liquid should not degrade, and the sample should be good for a number of years.

Back to Top