Temperature Sensors

The GCS Temperature measurement system.

Product Information Sheet in .pdf format
Introduction

The GCS temperature measuring system is based on a fundamental design that has been used successfully for over 20 years in Cold and CA fruit and vegetable rooms throughout the world. The system described here is an updated version that uses modern measuring technology to provide a very accurate and reliable system.

All temperature measurement systems depend fundamentally on the quality of the measuring sensor. The GCS system uses a precision Thermistor network to provide an accurate, high stability and linear output. The fundamental resistance is high which dramatically reduces installed errors due to wiring resistance and simple multicore cables can be used. The sensor is embedded into a rugged aluminium housing which has 3 layers of moisture sealing and encapsulation.

The sensors are connected through an accurate and proprietary interface to an industrial PLC control system. This provides interconnection with industry standard SCADA display and recording systems or can be used with an interface provided to a Excel based recording and analysis system.

Probe Specification.

Response time.

Two sensors selected randomly from the production line were tested by immersing in a beaker of iced water. The following results were obtained:

It can be seen that the reading becomes stable after 60 seconds and therefore easily meets the 3 minute requirement 2.3.2b in the Mexico work plan.

Accuracy:

A production batch of 32 sensors were tested in a stirred ice and water mixture. The temperature was monitored with a standard thermometer as the exact 32F is not usually obtained due to impurities and an imperfect crushed ice mixture.

It can be seen from these results that 2 sensors were just outside of the +/- 0.2 degF accuracy limits and were rejected.
This accuracy is very much better than the +/- 1 F required by the specification 2.3.2a

Wiring errors.

One of the major and significant benefits of the GCs system is the small errors produced by the interconnecting wiring over considerable distances.

The extent of these errors can be shown in graphical form as follows:

It can be seen that the errors due to wiring are not a significant factor in a correctly installed system.

Probe identification.

Each probe is provided with a unique serial number for recording and certifying calibration.

Long term stability

We have experience of probes using the same measuring sensor still installed after 20 years of service and undergoing regular calibration checks to better than 0.2 DegF

Linearity

The design of the thermistor network within the probe combined with the interface within the measuring electronics provides a substantially linear output over the temperature ranges of interest in fruit storage applications. The linearity characteristics are shown in the graph below.

Secure temperature probe.

There is a possibility that some storage operators may substitute temperature readings from rooms other than the one being monitored and certified. To reduce the possibility of this happening we can provide a security probe to install in each cold store.

The security probe has a double output. The measurement output is the standard GCS280 sensor as described above. The second output transmits a unique 5 digit serial number to the PLC measurement system. To ensure the probe and security number are linked, the number is only transmitted when the temperature falls below a preset temperature. (say 38 degF). In any subsequent record it will be obvious that the number and measurement are the same because the time stamped recording will show the transition down to the storage temperature and back again at the end of the period.

The sensor and security circuit are encapsulated together in an aluminium probe making it impossible to tamper without damaging the complete unit