Ice Point Humidity Generator

MSL
Measurement Standards Laboratory

Technical Guide 4 Ice Point Humidity Generator

Ice Point Humidity Generator
Practical and reliable humidity sources for checking hygrometers in industrial situations can be difficult to come by and use. Humidity sources such as saturated and unsaturated salt solutions offer one method of checking hygrometers but they are time consuming and a reliablereading can take up to 6 hours. Removing and checking the hygrometry system off site may also be undesirable. With quality systems becoming more common there is a need for organisations to ensure their hygrometers are checked regularly. The Ice Point Humidity Generator offers the opportunity to check hygrometers using a readily achievable reference point similar to using the ice point for checkingthe 0 °C point on thermometers (see MSL Technical Guide 1). This guide describes a procedure for producing a humidity generator that supplies humidified air with dew-point at the ice point temperature of 0.0 °C. This conditioned air can be used directly to check hygrometers that measure dew point. If the hygrometer measures relative humidity (RH) then the temperature in the immediate vicinity ofthe probe needs to be known accurately. Knowledge of the dew point and chamber temperature enables calculation of the RH in the test chamber (see Table 1). The procedure utilises a prepared ice point as the saturator temperature controlling medium. MSL Technical Guide 1 outlines the procedure for producing a suitable ice point.

Figure 1: The saturator coil showing the water trap and inlet andoutlet tubing

Test Chamber
Figure 2 shows examples of test chambers. A small container (e.g. plastic box with an air-tight lid or perspex tubing sealed at one end) is required in which to place the hygrometer and temperature probes. A tubing connector inserted tightly in one end of the container is connected to the outlet of the saturator coil. Small holes (e.g. 4 x 3 mm) for ventilation need tobe made at the opposite end of the container away from the air inlet. Holes for the hygrometer and thermometer also need to be made in this end of the container. The open end of a tubular chamber serves adequately for this purpose. Insulate the chamber with foam or similar to minimise internal thermal variations within the chamber (see Appendix).

The Equipment
Saturator
Figure 1 shows thesaturator coil. It consists of about 4 m of 6 mm plastic tubing wrapped around a stainless steel former. Six turns from the bottom a small test tube is inserted in the line to act as a water trap. The air is directed to the bottom of the coil and moves up through the spirals then exits from the top. The water trap ensures that minimal moisture is present in the tubing above the trap and preventsmoisture carryover past the coil for a number of hours. The generator relies on the air supply having a dew point above 0.0 °C ensuring the system acts as a condensing saturator. If it is found that the dew point is below 0.0 °C, that is, the RH is less than 26 %RH at 20 °C (which is unlikely in most parts New Zealand) a damp sponge under the air pump and close to the pump inlet will help to raisethe dew point of the air entering the saturator.

Aquarium Air Pump
An air supply of about 1 litre / minute (about 2 cubic feet / hour) is required. This can be achieved using an aquarium air pump (see Appendix). The air is pumped through the saturator coil to the test chamber where the hygrometer probe is inserted.

Insulating Container
An insulating container such as a vacuum insulated flaskor polystyrene flask (e.g. yoghurt maker) 300 to 400 mm deep and 80 to 100 mm diameter is ideal.

Measurement Standards Laboratory of New Zealand • FAX 64 (0)4 569 0003 • email: msl@irl.cri.nz • www.irl.cri.nz/msl/

11. Insert the humidity and temperature probes and wait until stable readings are achieved. The time for a stable reading will depend on the type of probe. Ensure sufficient...