Alarm Systems

Alarm Systems

When a blood refrigerator malfunctions and the
temperature goes out of range, the temperature
change occurs relatively slowly. This is because of
heat capacity of the product stored in the
refrigerator, insulation of the refrigerator and other
factors. Because of this fact , manufacturers design
alarm systems to respond to slow sustained
temperature changes and to filter out rapid transient
changes. In order to accurately test these alarms,
the test procedure must simulate an actual
temperature failure.

All blood refrigerator temperature alarm systems
consist of some type of temperature sensor,
signal-processing circuitry, and usually have a
display device, typically a digital temperature
readout. The most common type of temperature
sensor in use today on alarm systems for
refrigerators is the Resistance Temperature
Detector, or R.T.D. Frequently, the sensor is
encased in a much larger stainless steel immersible
probe. All temperature sensors have a characteristic
response time to changes in temperature. This
response time is primarily a function of the mass of
the sensor and sensor material. In order to compare
one sensor to another, the response times MUST be
similar or the temperature must be kept stable long
enough for both sensors to equilibrate. The larger
the sensor probe, the longer the response time, in
general.

Rule #1: Only use sensors (or thermometers) with
similar response times to verify temperature alarms,
OR KEEP TEMPERATURE STABLE LONG ENOUGH
FOR BOTH SENSORS TO EQUILIBRATE.

The relative response times of two sensors, (or a
sensor and thermometer), can be checked by
placing the sensors in a container of liquid at a
stable temperature and letting them equilibrate for
10 minutes. Then place both of the sensors in a cup
of ice water and measure the time it takes for each
to reach 0 degrees C. (If the sensor is connected to
a display, the time will also include the response
time of the display circuit).

Alarm circuitry has changed over the years from
fairly straightforward analog to
microprocessor-based alarms. Most alarm circuits
still transmit the sensor signal to the alarm without
much delay, however some microprocessor-based
alarms introduce a delay between the time the
sensor senses the alarm condition and the alarm
activation.

On alarm systems with display devices, many
manufacturers have intentionally slowed the rate at
which the temperature display updates the
temperature information. Slowing the sample rate
provides a stable display free of insignificant
fluctuations that can generate unnecessary
warranty service.

The thermal response time of the sensor, added to
the display response, equals the total response time
of the alarm system. It is the total thermal response
time that makes alarm verification so difficult.

Most blood banks use a method of verifying the
alarm set points that involves cooling the sensor
with an ice bath and comparing the alarm display to
a mercury thermometer. The sensor is then placed
in a water bath that is slightly above the high set
point and the alarm activation temperature is
checked against the mercury thermometer. Different
response times of sensors and slow responding
displays, cause problems when trying to verify
alarm set points, allowing staff to believe that the
alarms are functioning correctly, when they may not
be.

Rule #2: Use a heating /cooling device that changes
the alarm sensor temperature slowly enough to
cause the digital display to change by one tenth of a
degree Celsius each time the display updates.
In other words, the temperature display should
increase or decrease by 0.1 C, i.e. 4.0, 3.9, 3.8, 3.7
etc, not 4.0, 4.7, 4.5.

It is extremely difficult, if not impossible to
accurately control the rate of temperature change
using a water bath. Lack of adequate temperature
control is the most common problem associated
with alarm Q.C. Using a reference thermometer with
a different response time than that of the alarm
system is the second most common problem.

Rule #3: Making adjustments to alarm set points or
display calibration based on results from a test
using a thermometer with a different response time
than that of the alarm system, or with a change in
temperature too fast for the display, will result in
the system being calibrated incorrectly and it WILL
NOT display correctly and / or alarm at the desired
temperature.

Alarm system fundamentals

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