ENMET Corporation incorporates a variety of sensor technologies in our gas detection instruments. Sensors have certain properties that we try to utilize to increase the product performance and user safety. There are four basic types of sensors that we use.
MOS (Metallic Oxide Semiconductor)
MOS sensors are broad range in their response nature. We may set up a sensor for a given gas, or vary certain sensor parameters to optimize for a certain gas, but they are inherently non-specific. A single sensor can be used to monitor for one gas or many gases. These sensors are particularly useful when the atmospheric hazards are not entirely known, such as sewer entry, or when there may be a multitude of gases present and a instrument is used to screen for the presence of any one of them.
An MOS sensor consists of a heated mixed metal (iron, zinc, tin) oxide element that decreases its resistance substantially in the presence of many gases and vapors. An instrument measures that change and displays it as a concentration of gas present.
There are a variety of different types of MOS sensors that can used for %LEL monitoring of flammable hydrocarbons, ppm level of toxic hydrocarbons and a variety of other toxic gases such as carbon monoxide, hydrogen sulfide, refrigerants, and ammonia.
The version MOS sensor in an instrument, the gas that is used to calibrate the MOS sensor, and other factory set sensor parameters determine the response characteristics of that sensor. Even though the sensor is broad range, the levels at which it responds to other gases, may not be ideal. Contact your distributor or ENMET Customer Service for performance expectations in a specific application.
Field Service Life for MOS Sensors: 3-5 years
Hot-wire (Catalytic Element)
Hot wire sensors are broad range in their gas response. A sensor may be calibrated for a certain flammable gas, but will detect others if they are present.
A Hot-Wire sensor consists of two heated elements, one a compensator and the other a catalyst coated active sensor head (detector). In the presence of a gas, the sensor's detector element actually burns a small sample of the gas, increasing the resistance of the wire to which the sensor is bonded. These elements are put into a special electronic circuit (often called a Whetstone Bridge) that is able to measure that resistance change. The instrument interprets that change and displays it in the form of a gas concentration usually in %LEL (percent of the lower explosive limit).
Hot-wire sensors are traditionally used to detect %LEL levels of flammable gases and vapors. They have been used for detection of ppm concentrations of the same compounds but require active sampling and other instrument design considerations.
Hot-wire sensors have very predictable cross-sensitivity performance. Given a known gas to calibrate the sensor, it is relatively easy to predict a sensor's performance to another gas. While the response is predictable, the levels at which it responds to other gases, may not be ideal. Contact your distributor or ENMET Customer Service for performance expectations in a specific application.
Field Service Life for Hot-wire sensors: 2-4 years
Electrochemical
Electrochemical sensors are specific or at least selective to a particular gas.
An electrochemical sensor consists of a membrane, electrodes and electrolyte that are chosen to detect the target gas. Gas diffuses through membrane and reacts with the electrolyte and electrodes to generate a current signal based on the concentration of the gas present. The instrument interprets that signal and displays it in the form of a gas concentration.
Since electrochemical sensors are based on chemical reactions it is always possible to have certain compounds react very similarly. That's why some electrochemical sensors can be very specific like oxygen, (not many gases react like it) and others are less specific (more cross sensitive to a family of acid gases or oxidizers).
Electrochemical sensors are used to detect oxygen concentrations or ppm levels of various toxic gases. The detectable gas concentration range, the degree of specificity and sensor lifetime vary from gas to gas.
Field Service Life for Electrochemical Sensors:
Gas
Expected Field Service Life (years)
Chlorine
1.5
Hydrogen sulfide
2.5
Hydrogen cyanide
1.5
Hydrogen chloride
1
Sulfur dioxide
2.5
Hydrogen fluoride
1
Ozone
1
Oxygen
1.5
Carbon monoxide
2.5
Fluorine
1.5
Hydrogen
2.5
Nitrogen dioxide
1.5
Nitric oxide
1.5
Ammonia
1
NOTE: There are a number of factors including temperature exposure, gas exposure and humidity levels that can affect a sensor's life. The values above are typical, but not a guarantee of performance.
