TECHNOLOGY

Over the past 30 years, Rense Instruments has developed an impressive array of products related to the measurement of Humidity.  The basic technology is relatively simple in theory, but the adaptation and refinement of this technology for application in real-world situations is the key to the company’s success.

Sensors and Transmitters

Most humidity sensors are based on a similar concept, in which the amount of moisture present in a polymer layer varies the capacitance of a sensing element. The associated circuitry translates this capacitance change into a frequency change which is directly proportional to humidity,

Relative Humidity (RH) is defined as the actual partial pressure of water vapor present, divided by the saturation vapor pressure at that temperature.  In other words, it is the amount of actual water vapor present expressed as a percentage of the maximum water vapor that the gas is capable of holding, at a given temperature, without condensing.

Warmer air is capable of holding more moisture, hence RH is heavily influenced (inversely proportional) by temperature. See the psychrometric chart for a graphical representation of temperature-humidity relationships.  For this reason, it is important to accurately know the temperature of the process being measured.  To illustrate this point, at moderate to high humidity, a 1°C temperature uncertainty results in a 6% RH uncertainty.

Since RH measurements need to be “corrected” for temperature through the use of a temperature sensing element, it is fairly common for RH transmitters to also provide temperature outputs.  Rense RH transmitters can be provided with RH outputs only (HX prefix), separate RH and temperature output signals (HT prefix), or less commonly, temperature signals only (XT prefix).

Outputs are available in a range of standard voltage and current formats (such as 0-1VDC, 0-10VDC, and 4-20mA), as well as via RS-232 and RS-485.  Rense also produces many products with integral LCD displays.  When analog outputs are required for interchangeability with older products, but better accuracy and linearity are necessary, Rense offers several digitally-corrected products with analog outputs.

Test & Calibration Equipment

Rense produces several test and calibration instruments to complement our line of sensors and transmitters.  This includes handheld meters, which are attached to Rense transmitters to provide ambient and remote RH and temperature measurements, as well as humidity and climate chambers to calibrate the sensors themselves.

As with the sensor product line, it is the application of straightforward technology, refined by years of research and experimentation, which differentiates Rense products.  For example, it is critically important to minimize thermal and humidity gradients within a test chamber when testing and calibrating sensor products.  Meticulous placement of heating/cooling elements (Peltier devices), saturators, drying lines, circulation fans, insulation and seals is necessary to achieve acceptable levels of measurement uncertainty.

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