transmitter unit transmits each temperature datum to the receiver unit twice due to the low probability that two consecutive transmissions by the transmitter unit will not be correctly received by the receiver unit this method enables the recovery of approximately 95 of the data that would otherwise be lost due to missed packets collisions and crc errors it would be possible to transmit two previous temperature data along with the current datum and this would allow the system to provide a complete record even if two consecutive transmissions were lost in this case three memory cells would be required in the transmitter microcontroller the invention is not restricted to transmitting just one or two previous temperature data along with the current datum but it will be appreciated that each increase in the number of temperature data increases the length of the data packet and leads to a higher probability of overlap among transmissions from other sensors and a higher probability of collisions although the transmitter unit described with reference to fig1 transmits the most recent temperature datum and the immediately preceding temperature datum it would be possible particularly if the output device 68 provides a record for subsequent viewing and immediate warning of a temperature value passing out of range is not critical to transmit for example the previous datum and the datum that immediately precedes the previous datum further it is not essential that the data that are transmitted in a given telemetry slot be consecutive data it is necessary only that each datum be transmitted at least twice and that.