- Model 2505 - Mercury Vapor Primary Calibration Unit

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The Tekran Model 2505 Mercury Vapor Calibration Unit is an ultra-precise and accurate closed-vessel, saturated gaseous mercury source. The 2505 utilizes a thermoelectrically cooled mercury reservoir and dual temperature sensors calibrated against a US National Institute of Standards and Technology (NIST) traceable source producing exceptionally accurate Hg delivery. The mercury vapor is delivered manually using a high precision, digital gas tight syringe. The unit registers the reservoir temperature and calculates the amount of mercury in a user defined injection volume. The device is portable, operating from either 12 VDC or a standard wall outlet. The Tekran 2505 is the most advanced system for ultra low-level mercury vapor calibration, since NIST and other standards organizations have yet to provide the means, methods, or reference materials to assist with this type of measurement.

The Model 2505 can be used to calibrate any low level gas phase measurement system. This unit is ideal for performing field and laboratory audits on the Model 2537 analyzer’s automated, internal calibration system. When performing analysis of mercury in gas matrices with the Model 2600-NG analyzer, the Model 2505 is used as a primary calibration standard for routine calibration. The Model 2505 is frequently used for system investigations such as evaluation of sample line transport by spiking at the inlet, confirming accurate instrument response or for other specialized research applications requiring an accurate and precise Hg source.

  • Advanced design contains an integral microprocessor, liquid crystal display (LCD) and keypad
  • Dual temperature sensors with a resolution of 0.001 OC
  • Thermoelectric cooler provides capability of withstanding high temperatures
  • Automatic calculation of the amount of mercury contained per unit volume at a given temperature
  • Three power options: 100-240 VAC, 12 VDC, or via internal battery (no active cooling)

One of the greatest difficulties in the monitoring of low levels of mercury in the atmosphere or in process gases is calibration. Low-level (ng/m3) mercury vapor gaseous standards are not stable and cannot be supplied in cylinders. Neither the US National Institute of Standards and Technology (NIST), nor any other organization provides any means, methods, or reference materials to assist with this type of measurement.

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The usual method of calibration used by researchers is saturated mercury vapor injection1 . This requires keeping a container filled with dry air and a small amount of liquid mercury at a constant, known temperature. The vapor pressure of mercury as a function of temperature is well documented. (See Graph at Left) The function is exponential, with increases of approximately 8% to 10% per degree centigrade. Accurate measurement of vapor temperature is thus critical to ensure overall accuracy.

Gas tight syringes with Teflon7 tipped plungers are used to inject known amounts of mercury into the analytical system for calibration. The saturated mercury vapor must be kept below ambient temperature in order to prevent condensation of liquid mercury within the syringe. This can impact precision and may cause serious contamination of the analytical system.

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The simplest implementation of a calibration source involves filling a vial with some mercury and insulating the entire assembly with foam. This is a very straightforward solution that virtually anyone can build. However, there are several problems with this basic approach.

  • The source is not chilled below ambient. This can allow the condensation of microscopic droplets of liquid mercury within the needle and barrel of the syringe, causing severe contamination of your analytical equipment when injections are made.
  • The source is not temperature controlled and the internal temperature is subject to constant change. Lengthy settling times are required for the air/mercury vapor mixture to reach equilibrium.
  • The entry of heat through the body of standard glass thermometers can severely impact the measured vapor temperature reading.

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A more sophisticated implementation of the method utilizes a chilled water bath. This also suffers from several problems.

  • Chilled water baths are bulky, take significant amounts of power and are not easily transported.
  • Lengthy settling times (on the order of days) are required to let the source stabilize after transport. In addition to the time taken to reestablish temperature of the apparatus, a lengthy period is required for the air/mercury vapor mixture to reach equilibrium.
  • The entry of heat through the body of standard glass thermometers can severely impact the measured vapor temperature reading. Many implementations only measure the temperature of the water bath, causing inaccuracy due to heat entry through the top of the flask.

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The requirement to periodically verify the performance of analyzers operating in the field added a new sense of urgency to solving the problems that exist with conventional sources. Most organizations have QA/QC protocols that require periodic, on-site audits of all air monitoring instrumentation. It was found soon after initial deployment of the Tekran Model 2537 ambient air analyzer that a major cause of disagreements between units operating in the field was differences in the customers’ vapor sources used to characterize the internal calibration source of the analyzer.

In response to this demand, Tekran introduced the Model 2505, a temperature controlled saturated mercury vapor source. A thermoelectric cooler allows precise control of the reservoir temperature. The unit measures the reservoir temperature, accepts an injection volume, and calculates the amount of mercury in the injection. The device is portable, operating from either 12 VDC or line power.

The Model 2505’s dual temperature sensors are calibrated against a NIST traceable source. Each Hamilton Digital Syringe comes with a NIST traceable certificate. Thus, all input variables and injection amounts are traceable in the Tekran Model 2505 system. This unit is an ideal primary calibration standard for performing field and laboratory audits on the Model 2537 analyzer or for calibrating any gas phase analytical system. The Model 2505 solves the problems inherent with chilled temperature baths.

  • The source weighs less than 5 kg. (10 lb.) A convenient carrying handle allows field transport.
  • The unit may be kept powered up at all times, allowing the chamber to always maintain a constant temperature. This ensures that the vapor constantly remains at equilibrium, eliminating lengthy stabilization periods. The Model 2505 can operate from an automotive cigarette lighter using the adaptor cable supplied.
  • The unit has two sensors. One is used to control the isothermal block containing the vapor chamber. The second is housed within the chamber itself in order to measure the precise vapor temperature. The sensor leads are pre-cooled to minimize heat entry.

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