Calibrating A Co Monitor: Using A Voltage Meter

Calibration is an important process to ensure the accuracy of readings from devices such as voltage meters and CO monitors. Calibration of a voltage meter can be done by comparing it to a known voltage source or a quality voltmeter. Some voltage meters also have a small potentiometer at the back that can be adjusted to calibrate the device. CO sensors, on the other hand, can be calibrated by exposing them to known gases with different amounts of CO2. This process is known as span calibration and is performed to combat sensor drift, which occurs due to the deterioration of the light source and detector in the sensor over time.

Calibration tools: a calibrated tristimulus colorimeter

A calibrated tristimulus colorimeter is a device that helps you calibrate your monitor's settings and generate a software-based calibration profile (ICC profile) to adjust the monitor's output to match an absolute reference. This process is often costly and inaccessible for most people, requiring an investment of hundreds of dollars in equipment and software or the hiring of a professional calibrator.

A tristimulus colorimeter, also known colloquially as a colorimeter, is used in digital imaging to profile and calibrate output devices. It captures a limited number of wideband spectral energy readings along the visible spectrum using filtered photodetectors, such as silicon photodiodes.

The colorimeter measures chromaticity coordinates in red, green, blue, and white colors, and these values are used to calculate LCD profile coefficients. The illuminant and observer conditions should be specified when citing a measurement, such as D65/10°.

There are various calibration methods for tristimulus colorimeters. One method involves using modern, high-accuracy detector standards instead of traditional lamp standards for calibration. This approach reduces uncertainties caused by imperfect realizations of the Commission Internationale de l'Eclairage (CIE) color matching functions. By employing stable light sources with different spectral power distributions, the colorimeter can achieve high accuracy with sub-tenths of a percent uncertainty.

The quality of a colorimeter can be assessed using the methods outlined in CIE publication 179:2007. Additionally, the calibration of a tristimulus colorimeter can be further improved by using matrix correction.

While the use of a calibrated tristimulus colorimeter is ideal for critical work in a professional setting, basic test patterns can also be used to enhance the picture quality and balance the image of a monitor.

Calibration settings: brightness, contrast, sharpness, colour temperature, white balance, ICC profiles

Calibration settings are an important aspect of monitor optimisation, ensuring accurate colour representation and enhanced picture quality. While professional calibration tools and software can be costly, basic test patterns can be used to achieve reasonably accurate results for personal use. Here are the key calibration settings to consider:

Brightness

The brightness setting affects how your monitor displays darker colours. If set too high, blacks will appear grey and the image will have reduced contrast. If set too low, blacks will be "crushed", resulting in a loss of detail as distinct near-black shades of grey are displayed as pure black. To calibrate brightness, use a near-black gradient test pattern and adjust the setting until the 17th step disappears, then go back one step to make it visible again.

Contrast

The contrast setting is similar to brightness but affects the brighter parts of the image. If set too high, brighter images will "clip", similar to crushing in the case of brightness. When set too low, the image will darken and lose contrast. To calibrate contrast, use a gradient pattern and adjust the setting until steps up to 234 show some detail, with the last few steps remaining faint.

Sharpness

Sharpness adjusts the appearance of edges on shapes displayed on-screen. A low setting will result in a blurry image, while a high setting will produce strange-looking edges and cause close, narrow lines to blend. To calibrate sharpness, set it to max and then lower it until no strange pattern forms between lines and shapes.

Colour Temperature

Colour temperature adjusts the overall tint of the picture, with cooler temperatures giving a blue tint and warmer temperatures producing a yellow or orange tint. A 6500k colour temperature, equivalent to midday light, is recommended as a standard for screen calibrations. However, some may find it too yellow and can adjust it to their preference.

White Balance

White balance refers to balancing colours across shades of grey to achieve absolute white or grey, which contains equal amounts of every colour. Accurate adjustment of white balance settings requires specialised equipment, so it is recommended to keep the default settings as incorrect adjustments can easily make things worse.

ICC Profiles

ICC (International Colour Consortium) profiles are used to modify the graphics card's Look Up Table (LUT) and gamma table, allowing the monitor to display more accurate colours. These profiles are created through a process called 'profiling', typically done by a hardware calibration device such as a colorimeter. While ICC profiles can improve colour accuracy, they may also result in a reduction in contrast and shade variety. It is important to note that ICC profiles are specific to individual units of the same monitor model, and using a profile from a different unit may not yield optimal results.

Calibration procedures: display and switch test, resistance measurements, DC and AC voltage measurements, DC and AC current measurements, logic function test, frequency measurements, capacitance measurements, diode check function

Calibration procedures for a CO monitor with a voltage meter can include the following:

Display and Switch Test

This test ensures that the CO monitor's display is functioning correctly and that the switches are responsive. Turn on the device and check that all the segments of the display are illuminated. Press each button or switch on the device and observe that the display responds accordingly.

Resistance Measurements

Resistance measurements are performed to ensure the CO monitor's circuitry is intact and functioning correctly. Disconnect the device from any power source and connect the voltage meter's test leads to the appropriate probe jacks, typically denoted as "Common" and "Ω". Touch the probe tips to the component or circuit being measured, ensuring a secure connection. Observe the voltage meter's reading, taking note of the unit of measurement (ohms, kilohms, or megohms). Compare this value to the expected value based on the manufacturer's specifications.

DC and AC Voltage Measurements

Voltage measurements are used to verify the accuracy of the CO monitor's voltage readings. Connect the voltage meter in parallel to the CO monitor's circuitry. Ensure that the voltage meter has a high input impedance to minimise its impact on the circuit. Measure the voltage at various points in the circuit, comparing the readings to expected values.

DC and AC Current Measurements

Current measurements are performed to ensure the CO monitor accurately measures electrical current. Connect the voltage meter in series with the circuit, allowing the current to pass through it. Measure the voltage across a known resistance in the circuit and calculate the current using Ohm's law (I = V/R). Compare this value to the reading on the CO monitor.

Logic Function Test

The logic function test verifies the correct operation of the CO monitor's logic gates or digital circuitry. This test will depend on the specific model and design of the CO monitor and may require specialised test equipment.

Frequency Measurements

Frequency measurements are used to ensure the CO monitor accurately measures the frequency of electrical signals. This can be done by comparing the readings of the CO monitor to those of a calibrated frequency counter or signal generator.

Capacitance Measurements

Capacitance measurements verify the integrity of capacitors within the CO monitor. This can be done using a capacitance meter, LCR meter, or other specialised test equipment. The measured capacitance values can be compared to expected values to ensure accuracy.

Diode Check Function

The diode check function ensures that diodes within the CO monitor are functional and properly polarised. Connect the voltage meter to the diode, first in the forward-biased direction and then in the reverse-biased direction. The meter should display a low resistance in the forward-biased direction and a very high resistance in the reverse-biased direction. Some multimeters have a dedicated "diode check" function that displays the forward voltage drop.

Calibration methods: zero point, span, 3-point, fresh air, automatic background

Calibration is the process of checking and adjusting an instrument's response so that its output accurately corresponds to its input throughout a specified range. This is done by exposing the instrument to an actual input stimulus of a precisely known quantity.

Zero Point Calibration

Zero point calibration exposes the sensor to 100% nitrogen, as 100% nitrogen means there is no CO2 present. This allows the "zero point" to be updated in the sensor's internal memory, accounting for drift over time as the sensor ages. The advantage of this method is that it removes any deviation from the sensor's zero point (0% CO2). However, it is not a complete factory calibration, as it does not account for the sensor's response curve.

Span Calibration

Span calibration, also known as 2-point calibration, is performed at the lowest and highest gas levels for which the product is rated. Typically, this is done at 0 ppm CO2 using 100% nitrogen, and a special gas mixture of nitrogen and CO2 called calibration gas. Span calibration is performed at the factory immediately after a CO2 sensor is manufactured. The sensor's reading for 0 ppm CO2 is recorded in its memory, and then it is exposed to the highest CO2 level for which it is rated. After being exposed to the second known gas, the sensor's response is also recorded. This allows for the creation of a calibration curve, which is a linear response to the gas concentration between the two calibration points. Span calibration is necessary to match the sensor's response curve to a known target gas.

3-Point Calibration

3-point calibration is performed at the lowest, midpoint, and highest gas levels for which the product is rated. For CO2 safety alarms, this involves calibrating at the 0.0% CO2 level, default alarm 1 level, and default alarm 2 level.

Fresh Air Calibration

Fresh air calibration is used when maximum accuracy is less important than cost and simplicity. Instead of calibrating at 0 ppm CO2 using 100% nitrogen, the sensor is calibrated at 400 ppm CO2, the agreed-upon average for outdoor air. This method is best for sensors or CO2 monitors that are used to measure air quality, are portable, and can easily be taken outdoors.

Automatic Background Calibration (ABC)

Automatic Background Calibration was developed by Senseair in Sweden to address the difficulty of calibrating wall-mounted CO2 sensors used in buildings to measure occupancy or indoor air quality levels. The theory behind ABC is that, at some point each day, a room will be unoccupied, and the CO2 level will return to 400 ppm, the same as outdoor air. By storing the lowest CO2 readings taken over time in the sensor's memory, an offset to 400 ppm can be calculated and applied to future readings. The advantage of ABC is that the sensor is virtually self-calibrating over its lifetime. However, it will not work properly if the sensor never "reads" 400 ppm fresh air, such as in an indoor livestock environment or an office building that is staffed 24/7.

Calibration requirements: calibrate once a month, calibrate once a year

Calibration is important to ensure that your equipment is working accurately and safely. The calibration process compares a measuring instrument with a measurement standard to establish the relationship between the values indicated by the instrument and those of the standard.

There is no one-size-fits-all answer to how often you should calibrate your equipment. It depends on the type of equipment, how often you use it, the accuracy required for your work, and industry standards or regulations.

If you are calibrating a CO monitor with a voltage meter once a month, this suggests that you are performing critical measurements and need to ensure a high level of accuracy. Monthly calibration is generally recommended for equipment that requires highly precise measurements and is used frequently. This could be necessary if you are in an industry such as medicine, where accuracy is crucial for patient safety.

Calibrating once a year is typically suitable for equipment used for a mix of critical and non-critical measurements. This balance between calibration costs and accuracy is often chosen by companies that don't perform critical measurements as frequently or require the highest level of precision.

• Manufacturer's recommendations: Check the owner's manual for the manufacturer's suggested calibration intervals.
• Critical projects: Calibrate before and after any critical projects to ensure the accuracy of your measurements and protect against potential recalls or out-of-tolerance conditions.
• Events or accidents: If your equipment experiences a physical impact or an electrical/mechanical shock, calibrate it to check its integrity, even if there is no visible damage.
• Age of equipment: Older equipment may require more frequent calibration as its accuracy can start to drift over time.
• Job requirements: Some jobs may require annual calibration of certified test equipment, while others may have different industry-specific standards or client requests for calibration.

To calibrate a CO monitor with a voltage meter, you would typically:

• Set your voltage meter to VDC and connect the probes to the CO monitor in parallel to test the output voltage.
• Adjust the voltage potentiometer on the CO monitor until the values match the voltage meter, assuming the voltage meter is the device closest to the real value.

Please note that this is a simplified explanation, and you should always refer to the specific instructions provided by the manufacturer of your equipment for calibration procedures.

Frequently asked questions