Application Stories: Condition Monitoring

A London hospital is using a Microlink 751 system to monitor the conditions of their low temperature liquid nitrogen freezers. These freezers are used to store blood and bone marrow samples of patients in the hospital. The temperature of these freezers is critical as any warming of the samples could render them useless and hamper the treatment of the patient.

Each freezer has several thermocouples within it at different levels, to provide a temperature profile of the vessel. The thermocouples are linked to an isothermal box and 751 USB data acquisition unit. Windmill software runs on dedicated PCs and constantly monitors and records the temperatures within the freezers.

The hospital sets alarm levels which allow warnings to be raised should any temperatures cross a pre-determined threshold. Colours change on screen for example, but more importantly a warning beacon outside the freezer room is activated - alerting any nearby staff to the situation. This is achieved by the digital outputs provided by the 751 unit, which switch on the beacon.

Another use of the alarm levels with digital outputs is the linking to an Autodialler. Like the beacon this is activated by a switch on a digital output. However, in this case the unit dials up a selection of pre-programmed telephone numbers and plays a recorded message to the recipients, alerting them to the situation.

The Autodialler is also used in combination with the Windmill Alarm Logger software. This allows delays to be inserted between an alarm being registered and an output being switched - useful in reducing the instances of false alarms. If, say, the temperature was to momentarily rise when the freezer lid was opened the alarm would not activate unless the temperature remained over the limit for a certain time, for example if the freezer lid were to be left open. Alarm Logger can also bring up a window on screen to warn of the alarm and require authorised manual intervention to cancel the alarm. This action is then saved in a file so it can be seen when alarms occurred and who was responsible for deactivating them.