Altrac's standard temperature probe is a waterproof digital temperature probe using two wires, data and ground.
The head of the probe contains a Maxim Integrated DS18B20 1-Wire Digital Thermometer. This sensor has a measurement range of -55°C to +125°C (-67°F to +257°F) with ±0.5°C Accuracy from -10°C to +85°C. The sensor is factory calibrated against a NIST traceable standard.
Common Questions and Answers for the Altrac Probe:
Why is my device showing the error "Temperature Probe Fault"?
This probe is either unplugged or has been severed. Please inspect the probe head, probe wire and any other wires attached to the Altrac system for cuts, places melted or other damage. Also check that all connectors are plugged in and fully seated (locking mechanism is "clicked" in).
Why is my probe data cutting in and out?
The wire for this probe has likely been damaged. Please inspect the probe head, probe wire and any other wires attached to the Altrac system for cuts, places melted or other damage. Also check that all connectors are plugged in and fully seated (locking mechanism is "clicked" in).
Why doesn't my wind machine respond the same way as it did with the old probe?
The size and probe material can make a large difference in how it responds to changes in air temperature.
For example, the Altrac probe is smaller and has thinner walls than an Orchard-Rite analog probe. The size difference between the probes means that the Orchard-Rite probe will be slower to respond to changes of air temperature. This means that an Altrac probe will get colder faster, so will start a wind machine sooner than an Orchard-Rite probe and it will also warm up faster and stop a wind machine sooner than an Orchard-Rite probe. See picture below for a visual comparison.
How should my probe be positioned and covered?
Your Altrac probe should have be mounted in the fruiting zone of your crop and be covered by some sort of shield to prevent water, dirt or chemical sprays from interfering with it metal probe head.
The probe should not touch the sides of its shield, if possible. If the shield is a metal shield it is very important it does not touch the metal shield, as metal shields will affect the probe readings.
Why doesn't the Altrac probe agree with my other (handheld, pickup, etc.) thermometer?
This question is difficult to answer conclusively because of how digital thermometers work, how air diffuses temperature changes into the sensors, differing elevations between thermometers and where the thermometer is mounted (either mounted or handheld).
The short answer to this question is:
- Any substance on the Altrac probe can affect readings. Check that the Altrac probe is not covered with water or some other substance and is mounted correctly in the canopy
- The Altrac probe is factory calibrated by our supplier using instruments certified by the US Federal Government (NIST). The calibration does not drift and the probe cannot be recalibrated.
- It is extremely unlikely that two separate Altrac probes would be calibrated incorrectly, so swapping probes can prove that both are working correctly.
To do a homemade check on an Altrac probe use an ice bath. Combine a small amount of water and ice and then insert the probe. We would expect a probe in an ice bath to be between 32F and 33F. It will not be exactly 32F due to the water being slightly above 32F and the 0.9F accuracy limit of the sensor.
To compare the Altrac probe against a handheld thermometer measuring air temperature, you'll need to put the two devices next to each other and leave them together long enough to ensure they are both measuring the same air at the same time. To do this accurately, we would recommend:
- Temporarily mount the handheld probe next to the Altrac probe using a zip tie or similar
- Walk away for 10 minutes or so to ensure your breath, body heat, etc do not affect the reading
- Ensure the Altrac probe has taken a 2+ measurements in the app
- Walk back and read the handheld probe and compare to the Altrac reading
We do not consider data from a pickup truck sensor to be accurate enough to form an credible comparison to the Altrac probe unless the differences are very large (20F). Pickup probes suffer from a number of accuracy issues, including different elevations, the temperature of the truck and the thermometer not being in the canopy.
Can a running wind machine alter the temperature of the probe via interference?
The Altrac temperature probe uses a digital communications protocol to send data from the probe head to the Altrac station. The communicated values are error checked using a CRC algorithm (cyclic redundancy check) to ensure that what the probe communicated to the Altrac station is unaltered.
This means that an Altrac temperature data provided by our temperature probe is the same at the probe head and the Altrac station. Longer wires or electrical interference will not change the temperature.
What if water/ice/dirt/other gets onto the probe head?
If the probe head gets wet, temperature readings can be slow to change and then could very significantly if it becomes windy because of the evaporation of the water from the probe head.
If the probe ices, temperature readings can be slow to change and can vary significantly during the phase changes of water to ice or vice versa.
If the probe head is covered with dirt or buried, the temperature data will be very flat and will not respond like other probes on your property.
Altrac recommends that temperature probes are mounted in the fruiting zone with some sort of cover to protect against water, ice, dirt, chemical sprays, etc.
If you experience significant issues with water or rain, we recommend using solar radiation shield to better protect your probe from the elements.
Technical Questions Answered by Maxim Integrated:
What is the lifetime of calibration for the DS18B20? Can the DS18B20 be recalibrated?
The calibration of the DS18B20 is guaranteed for life; there is no need for recalibration.
Is it possible for the user to calibrate the DS18B20?
No, the DS18B20 is calibrated during fabrication and can not be calibrated by the user.
How does the DS18B20 measure temperature?
The DS18B20 is a silicon band gap temperature sensor. The sensor is based on the principle that the forward voltage drop of a silicon diode is temperature dependent. By measuring the forward voltage drop across the diode, the temperature of the silicon is calculated.
What is the temperature drift over time for the DS18B20?
Temperature drift is not an issue with our local sensors. The devices are trimmed and calibrated in a temperature controlled bath. Once the trim values are set in internal EEPROM, they do not change. Also the internal dual/matched transistors, used in determining the voltage delta and thus the temperature value, do not drift over time. The bias current generated and passed through these matched transistors also has been characterized as stable over time and temperature.
So drift has never been an issue with the DS18B20. This product has been around for 15+ years, and we have never had any reports or data to show that the performance of the DS18B20 is any different years later than it was on day one.
Can Maxim Integrated provide proof of calibration to NIST standards for the DS18B20?
We receive many requests for certificates of calibration for our temperature sensor products. We calibrate and test 100% of our 1-Wire® chips against a NIST traceable standard. However, we do not offer calibration certificates for them. An independent calibration lab calibrates Maxim’s production test instruments. However, Maxim itself is not a certified calibration lab and therefore cannot offer you a formal certificate. Maxim Integrated Products stands behind the statements in our data sheets, and our standard Terms and Conditions of sale protect you against product defects. However, you should not assume that use of our components forms the basis of NIST certification for your own equipment or systems. Such certification requires an unbroken chain of certificates from the thing being certified to the official standard at NIST. Therefore, if your application requires certification of calibration of the temperature sensing capability, you should have your finished product certified by an accredited independent lab. A formal, signed copy of this statement can be downloaded.