Critical Performance

Technical Information.

Critical Performance Parameters

How do different sensor technologies compare?

Anybody trying to specify a new moisture analsyer cannot help but be struck by the paucity of transparent information from suppliers. Specifications and datasheets abound, but sadly all too often containing unrealistic performance claims that are ambiguous in meaning or, in some instances, plainly untrue.

MCM's ethos has always been to deliver the truth, and as such we oppose such misleading data. We believe that the quality of our instrumentation speaks for itself, and are confident that our technical advantages will be demonstrated, both theoretically and practically, by careful consideration of a few fundamental requirements.

The table below highlights several criteria that are critical to the collection of reliable moisture data, and shows how various moisture sensor technologies compare against one another, from a temperature controlled silicon sensor to a TDL. For more detailed information, refer to the footnotes at the bottom of this page or take a look at our documents What Makes A Good Hygrometer? and Specifying A Moisture Analyser.

 

  MCM TEMPERATURE CONTROLLED SILICON SENSOR ALUMINIUM OXIDE PROBE OSCILLATING CRYSTAL SENSOR ELECTROLYTIC SENSOR

TDL (TUNABLE DIODE LASER)

FAST SPEED OF RESPONSE – DRY TO WET
See note 1 below
Yes Yes Yes No Yes
FAST SPEED OF RESPONSE – WET TO DRY
See note 2 below
Yes No No No No
MASS TRACEABILITY OF CALIBRATION IN PPM[V]
See note 3 below
Yes No No No No
TEMPERATURE CONTROL
See note 4 below
Yes No Yes No No
ONLINE MOISTURE MEASUREMENT
See note 5 below
Yes Yes No Yes Yes
ABILITY TO DRY SENSOR ON DEMAND
See note 6 below
Yes No No No No
ABILITY TO VALIDATE MEASUREMENTS ON DEMAND
See note 7 below
Yes No No No No

 

1 & 2 It is critical to consider speed of response not just from a ‘dry’ to ‘wet’ condition, but also from ‘wet’ to ‘dry’. It is easy for any moisture sensor to become wet if starting from a dry condition, but many suffer from very long drydown times once saturated (in some cases, many hours). The ability to respond quickly from a wet excursion is essential for providing real-time process information and maximising the productivity of any moisture measuring application. For more on speed of response, click here.
3 The calibration of any hygrometer is only traceable if that calibration is performed against a Mass derived standard, measured as a moisture concentration in ppm[V]. Calibrations performed in Dewpoint °C are not traceable to Mass.
4 The sensitivity of any moisture sensor is affected by fluctuations in ambient or process temperatures. To guarantee traceability of the calibration, improve speed of response and improve resistance to contamination, temperature control is a pre-requisite. Temperature control should not be confused with temperature compensation, which is applied by some manufacturers whose sensor technology is not robust enough to withstand constant heating, and by its nature introduces errors to the measurement process. For more on temperature control, click here.
5 Not all moisture sensors are suitable both for portable spot-checking and continuous on-line analysis. In particular, consider the sensor’s speed of response (i.e.: its ability to provide real-time process data), long-term stability and resistance to contamination.
6 If the moisture analyser becomes saturated, either through process upset or during the connection / disconnection phase of spot-check analysis, then the ability to quickly dry the sensor back to a ‘zero’ condition is essential to the efficient collection of reliable data. A sensor drying feature also enables the operator to begin each measurement from a known, dry condition, thus greatly improving the repeatability of the sampling methodology and minimising errors of hysteresis. If the sensor drying feature is automated, then a further level of operator error can be removed from the measurement. For more on sensor drying, click here.
7 For most moisture sensors , the only way to test the responsiveness of the instrument is to disrupt the equilibrium of the sample gas and monitor the change in displayed reading. This process usually exposes the sensor to ambient moisture concentrations , after which the operator will have to wait for the natural drydown of the analyser. If an instrument has the ability of self-verification, by disturbing the equilibrium of the sensor instead of the process gas, then several benefits follow. Crucially, the operator can verify readings on-line, and the hygrometer will check the correct functionality both of the sensor and the internal electronics. Such diagnostic features provide significant levels of user confidence in the moisture analsyer, particularly in on-line applications where stable process conditions may be experienced for extended periods of time. For more on self-diagnosis, click here.