Detecting Combustible Gases with the VQ548ZD / VQ549ZD

The VQ500 Series is a comprehensive range of small, completely certified, flameproof sensor heads with low power consumption, a high quality pellistor pair optimised for the detection of combustible gases or % volume gases in thermal conductivity mode. They have an outline identical to the modern miniature electrochemical cells and are able to be used in portable instruments.

There are specific sensors in the VQ500 series which can detect most combustible vapours or gases and ammonia at LEL levels. The thermal conductivity sensor version is capable of detecting most % volume gases with different thermal conductivities than that of air.

Key Features

The key features of the VQ500 series are:

  • Optimised for combustible vapours and gases and vapours
  • Thermal conductivity versions are available
  • Shock resistant
  • ATEX certified II 2G Ex d IIC Gb (Ta = -40 to +55°C) –check original
  • ATEX certified I M2 Ex d I Mb (Ta = -40 to +55°C)
  • IECEx certified Ex d IIC Gb (Ta =-40 to +55°C)
  • IECEx certified Ex d I Mb (Ta = -40 to +55°C)
  • CSA certified – File 107498
  • UL recognised – File E186043
  • Certified flameproof component
  • Integral pellistors
  • Low power
  • Standard miniature body size
  • Poison resistant – hydrogen sulphide and silicone

General Parameters

All tests are performed at ambient temperature, pressure and humidity at a flow rate of 0.5l/min within the test equipment of the manufacturer.

  • The linearity of combustible sensors is linear up to 3% volume methane or balance air
  • The thermal conductivity depends on the manufacturer
  • Operation, storage and shipping in packaging (pots):
    • Temperature is - 40 to +55°C
    • Humidity is 0 to 80%RH non-condensing
  • Response time at T50 is less than 10s
  • Long-term signal drift is less than 5% LEL/month
  • Long-term zero drift is less than 5% LEL/month

Important Points

Points to note include the following:

  • The response and drift determined has been done with 1% methane in the test equipment of the manufacturer.
  • The theoretical figures and the actual response to flammable vapours may be a bit different. These k factors are just for guidance and it is important to calibrate the sensors to the actual vapour concentration needed.

Design Consideration

Design considerations include the following:

  • The supply voltage polarity may be reversed without any damage. There is an effect only if the polarity of the output signal is reversed.
  • The sensitivity is determined in the standard test jig of the manufacturer without any filter or other material between the sinter and the gas.
  • If the sensor while operating has been exposed to gas concentrations higher than the lower explosive limit, calibration must be checked. It is possible that the sensor be permanently damaged in these cases i.e. if the limit is exceeded.
  • The manufacturers suggest that the sensors be run in a Wheatstone bridge circuit (Figure 1).
  • Gas flow should not be directed to the sinter.
  • It is suggested that the sensors function in constant voltage mode, even though the sensors can be operated in constant current mode.

Wheatstone bridge circuit

Figure 1. Wheatstone bridge circuit

Outline A

Figure 2. Outline A

Outline B

Figure 3. Outline B

Instructions

The instructions pertaining to hazardous area installations (reference European ATEX Directive 94/9/EC. Annex II, 1.0.6). These instructions pertain to equipment covered by certificate number SIRA 01ATEX1073U:

  • The VQ500 Series Gas Sensing Head is component approved and must not be used as a stand-alone item in a dangerous area without any protection.
  • In order to classify the apparatus based on temperature, in the one VQ500 Series Gas Sensing Head is used, on any outer surface the temperature is not more than 64°C.
  • The VQ500 Series Gas Sensing Head cannot be rated as a safety device (EHSR 1.5).
  • There are no user-serviceable parts in the component.
  • The component certification only permits operation at ambient temperatures between -40 and +55°C and must not be operated outside this range.
  • The VQ500 Series Gas Sensing Head shall be used along with the apparatus offering an intrinsically safe supply with a maximum output power (PO) not more than 2.5 W.
  • The end-user/installer must know that the certification of the VQ500 Series Gas Sensing Head is based on the following materials used in its construction, which are perfect for most common applications – These are – Enclosure-stainless steel, Sinter-stainless steel and Bushing – Epoxy resin
  • The certification markings for the VQ500 Series Gas Sensing Head are: II 2G Ex d IIC Gb and IM 2 Ex d l Mb

Key Parameters

The key parameters are shown in Table 1

Table 1. Key parameters

Sensor Type Operating DC Voltage (V) Sensor Current (mA) Sensitivity (mV/% vol.) Offset (mV) Poison Resistance Outline (page 4) Description
VQ542R 4.25 ± 0.1 50 to 60 30 to 50 (1% methane) ±10 Some A Low power, general purpose sensor
VQ542RD 4.25 ± 0.1 50 to 60 30 to 50 (1% methane) ±10 Some B Low power, general purpose sensor
VQ542ZD 4.25 ± 0.1 50 to 60 30 minimum (1% methane) ±20 Silicones B Low power, silicone poisoning and shock resistant sensor
VQ545ZD 3.0 ± 0.1 67 to 80 20 minimum (1% methane) ±20 Silicones B Low power, silicone poisoning and shock resistant sensor
VQ546M 4.25 ± 0.1 50 to 60 — 4.0 minimum (2.5% methane) ±15 Not required A Thermal conductivity sensor, methane negative output
VQ546MR 4.25 ± 0.1 50 to 60 4.0 minimum (2.5% methane) ±15 Not required A Thermal conductivity sensor, methane positive output
VQ547TS 3.0 ± 0.1 40 to 50 21 minimum (1.5% ammonia) ±30 Some A Low power, LEL ammonia sensor
VQ548ZD 3.0 ± 0.1 67 to 80 20 minimum (1% methane) ±20 Silicones and H2S B Low power, silicone poisoning, H2S and shock resistant sensor
VQ549ZD 4.25 ± 0.1 50 to 60 30 minimum (1% methane) ±20 Silicones and H2S B Low power, silicone poisoning, H2S and shock resistant sensor

Certifications

The VQ500 Series holds certification as a flameproof component to EN60079-0:2006 (including amendments A1 and A2) and EN60079- 1:2007 by SIRA Certification Services (SCS), England. The certificate number is SIRA 01ATEX1073U.

According to the Canadian Standards Association, the VQ500 series has been given a component certification to be used as part of an intrinsically safe portable combustible gas detector or enclosed in a remote sensor housing. It meets the CSA standard C22.2 No. 30-M 1986 and UL standard 1203 as a flame arrestor for Class 1, Groups A, B, C and D.

The VQ500 series is recognised by the Underwriters Laboratories Inc. as components in intrinsically safe single- or multi-gas detectors to be used in Class 1, Division 1, Groups A, B, C and D hazardous locations. The VQ500 also meets the requirements of UL913, fifth edition.

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