General Questions about Thornton Products - METTLER TOLEDO
Know How

General Questions about Thornton Products

Know How
General Questions about Thornton Products
General Questions about Thornton Products


What is the difference between repeatability and accuracy?

Repeatability is the closeness or consistency of a sequence of measurements taken on a constant parameter. Accuracy is the closeness of measurements to an accepted standard, often expressed as the inaccuracy or error. High accuracy requires good repeatability. However, repeatable measurements may all have large errors as long as they are consistent. 


What is the system accuracy of my equipment?

System accuracy is usually the sum of the errors from the different components in most cases, the instrument and the sensor.

  • Conductivity - Instrument accuracy is typically +/-0.3% of reading for 770-series instruments and +/-0.5% of reading for 200-series instruments. Conductivity sensor accuracy is typically +/-1% of reading.
  • pH - Instrument pH accuracy is +/-0.02 pH. pH sensors are calibrated on site so the accuracy of calibration depends on the tolerance of the standard buffer solutions used for calibration. However, the actual measurement accuracy in a process is highly application-dependent and could vary from +/-0.1 to +/-1 pH, depending on composition, temperature, pressure, sensor fouling, etc.


Does the 200CR (or other model) come in an explosion proof or intrinsically safe version to meet a Class I, Division 1 location requirement?

Thornton instruments are used primarily in safe locations for water treatment. However, occasional users need installation in hazardous areas. A user has installed the 200CR in a windowed explosion-proof box with explosion-proof conduit running to a sensor with NPT threaded top.

Others have used intrinsic safety barriers with the 200CR. Typically two are required, one for the AC conductivity signal and one for the DC temperature signal. On-site temperature calibration is required at startup to compensate for barrier resistance. Because conductivity sensors consist of simple electrode contacts and a 1000 ohm RTD, they qualify as simple apparatus and do not require agency approval.

Because locations seem to have their own preference for barrier brand, we have not standardized on a recommendation of barrier. 200pH instrumentscannot be used with intrinsic safety barriers because of incompatible grounding.

Class I, Division 2 locations do not require explosion-proofing or intrinsic safety barriers although they may be used. Division 2 requirements can be met by apparatus that is non-incendive: apparatus that does not have any high temperature components or open contacts that would arc in normal operation that could ignite a flammable gas. Thornton conductivity and pH sensors operate at very low power and can be considered non-incendive although they have not been evaluated by an independent agency for this.


What does a display of asterisks on my 770- or 200- Series instrument mean?

Asterisks are an indication of a sensor or measurement error. It means the measurement is out of range which could be caused by: sensor disconnected or wired wrong, no sample at the sensor, sample conductivity/resistivity too high or too low for the sensor and instrument to measure, sensor failure, etc. Also check the temperature measurement. If the temperature measurement shows asterisks indicating a problem with that measurement, then any temperature compensated measurement will also display asterisks even if it would otherwise be okay.

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