Overview
Temperature values are one of the most common elements measured in any data acquisition system. Temperature measurements are often required in production test systems, energy management systems, environmental monitoring, aviation systems and more. These temperature values are often measured with sensors such as RTD’s (Resistance Temperature Detectors), thermistors, or thermocouples.
Each of these temperature sensors include unique measurement characteristics that must be considered when selecting sensors for a data acquisition system. Characteristics such as measurement range, accuracy, and the electronic circuitry required to measure the sensor are all important considerations. The physical packaging style of the sensor may also be a consideration. For example, will a thermocouple with twisted wires acceptable, or does the application require a screw in metal probe? These characteristics all need to be considered.
When specifying temperature sensors for any system, the first consideration is the temperature measurement range. Each of these sensors contain different measurement ranges. Thermistors offer a small measurement range, typically from sub-zero to a few hundred degrees Celsius. Thermocouples offer a much wider measurement range from sub-zero to over 2000 degrees Celsius.
Sensor accuracy is another important consideration when specifying sensors for a data acquisition system. Each of these sensors offers different accuracy specifications from tenths of one degree up to multiple degrees. Thermistors have the best overall accuracy specifications, generally close to one tenth of one degree Celsius. RTD accuracy specifications are slightly higher than thermistors at a few tenths of one degree Celsius. Thermocouple accuracy on the other hand is generally within a couple of degrees Celsius.
Sensor accuracy can also be impacted by electrical noise. Electrical noise can be generated from sources around the sensor, or along the length of the sensor wiring. Noise can be generated from items such as pumps turning on and off, generators running, and more. While thermocouple measurements are not affected by the length of their wires, they are susceptible to picking up electrical noise. RTD sensors are less immune to electrical noise. But they are susceptible to the length of their wires, commonly referred to as lead resistance. Thermistors with a negative temperature coefficient have such high sensor resistance that they are often immune to both electrical noise and lead resistance.
The electronic circuitry used to measure each of these sensors contains different requirements. Each data acquisition hardware manufacturer designs their own unique circuits and then implements them within their products. Typically, each circuit will contain circuitry to measure millivolt DC signals from the sensor. Thermocouples output a very small millivolt DC signal. Additionally, the thermocouple measurement circuitry will include an electronic cold-junction-compensation reference, as found within all DGH thermocouple measurement products.
Thermistors and RTD’s are resistive devices and do not output millivolt signals. The circuitry required to measure these sensors must include a small electronic current source. The current source signal is passed through the resistive sensor to develop a millivolt signal. That signal is then measured and converted to a temperature value.
The amount of current passing through the sensor can be an important consideration. Some current source values can be as high as one milliamp. This current value should be designed and specified as low as possible to prevent internal self-heating of the temperature sensor. Self-heating can lead to accuracy errors within the system. Each DGH RTD and thermistor products uses a much lower current value to prevent sensor overheating.
Finally, the cost of temperature sensors can be a budget concern. Sensor accuracy requirements will usually lead to increased sensor costs. RTD sensors are usually the highest cost, thermocouples and thermistors are usually much less. Thermistors can be purchased at very economical prices. Sensor packaging styles will also impact the sensor pricing.
While there may be concern for additional sensor parameters in each application, this article attempts to address the primary concerns for measuring temperature with thermocouples, RTD’s and thermistors. Additional information about each temperature sensor, their intended applications, and some DGH measurement solutions are included below.
Thermocouple Sensors
Thermocouple Sensors
Thermocouple sensors are used to measure temperature in a wide range of industrial, commercial, and residential applications. These sensors consist of two wires, each containing a dissimilar metal and they are connected at one end. The dissimilar metals develop a very small voltage between the wires that changes predictably with temperature. This voltage can then be measured and converted into a temperature value.
Thermocouple sensors are available in probe style packaging, or they can be created by twisting or connecting two wires together. They are used in a broad range of applications, including industries such as scientific laboratories, aerospace, automotive, HVAC, energy, oil and gas, residential appliances, pharmaceuticals, and more.
While there are alternatives to thermocouples, they are very popular due to their low cost, simple construction, and ease of installation. Most thermocouples have a wide temperature measuring range, good repeatability, and a quick response time.
D1300 & D1300M Series
• One thermocouple input.
• Thermocouple types: J, K, T, E, R, S, B, C.
• Ranges:
J = -200°C to +760°C B = 0°C to +1820°C
K = -150°C to +1250°C S = 0°C to +1750°C
T = -200°C to +400°C R = 0°C to +1750°C
E = -100°C to +1000°C C = 0°C to +2315°C
• Resolution: 1 Degree.
• Accuracy (from all sources) from 0 to +40°C ambient:
±1.0 °C max (J, K, T, E).
±2.5 °C max (R, S, B, C) (300°C TO FS).
• User selectable scaling °C or °F.
• Automatic cold junction compensation and linearization.
• 2 Digital inputs, Event counter, 3 Digital outputs.
• RS-232 or RS-485 serial outputs available.
• Modbus RTU Serial Output versions available
D5300 & D5300M Series
• Four thermocouple inputs.
• Thermocouple types: J, K, T, E.
• Ranges:
J = -200°C to +760°C
T = -200°C to +400°C
K = -150°C to +1250°C
E = -100°C to +1000°C
• Resolution: 1 Degree.
• Accuracy (from all sources) from 0 to +40°C ambient:
±1.0 °C .
• User selectable °C or °F.
• Automatic cold junction compensation and linearization.
• RS-232 or RS-485 serial outputs available.
• Modbus RTU Serial Output versions available.
D6300 & D6400 Series
• Seven differential thermocouple inputs.
• Thermocouple types: J, K, T, E, R, S, B, and C.
• Ranges:
J = -200−760°C
R = 0−1750°C
K = -150−1250°C
S = 0−1750°C
T = -200−400°C
B = 0−1820°C
E = -100−1000°C
C = 0−2315°C
• Accuracy (from all sources) from 0-40°C ambient:
J, K, T, E = ±1.5°C max.
R, S, B, C = ±3.5°C max (300°C to +F.S.).
• User-Selectable ranges on per channel basis.
• Automatic cold junction compensation.
• Modbus RTU, RS-485 serial output.
D8300 & D8400 Modules
• Seven differential thermocouple inputs.
• Thermocouple types: J, K, T, E, R, S, B, and C.
• Ranges:
J = -200−760°C
R = 0−1750°C
K = -150−1250°C
S = 0−1750°C
T = -200−400°C
B = 0−1820°C
E = -100−1000°C
C = 0−2315°C
• Accuracy (from all sources) from 0-40°C ambient:
J, K, T, E = ±1.5°C max.
R, S, B, C = ±3.5°C max (300°C to +F.S.).
• User-Selectable ranges on per channel basis.
• Automatic cold junction compensation.
• Modbus RTU, USB serial output.
RTD Sensors
RTD Sensors
RTD (Resistance Temperature Detectors) sensors are an alternative to thermocouples. RTD’s are used in applications requiring higher measurement accuracy. However, they often have a smaller measurement range than a thermocouple, with better repeatability and they are also more expensive.
RTD sensor elements often consist of wire wrapped around a heat-resistant ceramic or glass core, but other fabrication techniques are also used. The RTD wire is a pure material, typically platinum (Pt), nickel (Ni), or copper (Cu). The material has an accurate resistance vs temperature relationship which is used to indicate temperature. RTD elements are often placed inside metal housings (probes) and can be fragile.
The resistance of an RTD changes as its ambient temperature changes, increasing in resistance as the temperature increases, and the resistance changes are very repeatable over time. Since an RTD is a resistive device, it cannot produce an electrical output on its own. Electrical circuits are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage. The voltage is then measured and converted to a temperature value.
The amount of current must be low to prevent internal self-heating of the sensor. The amount of current is typically around 1mA. The DGH RTD input products contain a 250-microampere current source to measure the sensor and minimize the risk of self-heating.
RTDs are manufactured to several different resistance vs temperature curves and tolerances. The “DIN” curve is the most common standardized curve that defines the resistance vs temperature characteristics of a Platinum 100-ohm sensor, its tolerance, and the measurable temperature range.
This curve specifies a base resistance of 100 ohms at 0°C, and a temperature coefficient of .00385 Ohm/Ohm/°C. There are three standard tolerance classes for DIN RTDs. These tolerances are defined as follows:
D1400 & D1400M RTD Series
• One RTD Input
• RTD types:
α = .00385, .00392, 100Ω at 0°C,
.00388, 100Ω at 25°C.
• Ranges:
.00385 = -200°C to +850°C.
.00392 = -200°C to +600°C.
.00388 = -100°C to +125°C.
• Resolution: 0.1°.
• Accuracy: ±0.3°C.
• Common mode rejection: 100dB at 50/60Hz.
• Input connections: 2, 3, or 4 wire.
• Excitation current: 0.25mA.
• Lead resistance effect:
3 wire – 2.5°C per Ω of imbalance.
4 wire – negligible.
• Max lead resistance: 50Ω.
• Input protection to 120Vac .
• Automatic linearization and lead compensation.
• User selectable scaling °C or °F.
• 1 Digital output.
• RS-232 or RS-485 serial outputs available.
• Modbus RTU Serial Output versions available.
Thermocouple Sensors
Thermistor Sensors
A thermistor is a semiconductor type of resistor whose resistance changes based on temperature. These resistance changes over temperature are predictable and often larger than the changes that occur with standard resistors.
Thermistors are available with a Positive Temperature Coefficient or Negative Temperature Coefficient. Positive Temperature Coefficient (PTC) thermistors have higher resistance at higher temperatures and Negative Temperature Coefficient (NTC) thermistors have lower resistance at higher temperatures.
Negative Temperature Coefficient thermistors are widely used in temperature monitoring applications, while Positive Temperature Coefficient thermistors are often used as self-resetting overcurrent protectors to limit current through devices and protect sensitive circuitry in over current applications. In fact, DGH some PTC thermistors as over-current protection devices in their products.
The measurement temperature range and accuracy of a thermistor is dependent on the probe type and is typically between −100 °C and 300 °C (−148 °F and 572 °F). Thermistors can achieve accuracies of ±0.1 °C or ±0.2 °C and have excellent long-term stability. The typical operating temperature range of a thermistor is −55 °C to +150 °C. However, some thermistors can operate up to +300 °C. Thermistors differ from RTDs in that the sensor material used in a thermistor is generally a ceramic or polymer, and RTDs are pure metals. The temperature response of each sensor is also different. RTDs are useful in applications requiring wider temperature ranges, and thermistors typically have greater precision within a smaller temperature range.
D1450 & D1450M Series
•One Thermistor Input
• Thermistor types: 2252Ω at 25°C, TD Series.
• Ranges:
2252Ω = -0°C to +100°C.
TD = -40°C to +150°C.
• Resolution:
2252Ω = 0.01°C or F.
TD = 0.1°C or F.
• Accuracy: 2252Ω = ±0.1°C, TD = ±0.2°C.
• User selectable °C or °F.
• 1 Digital input/ Event counter, 2 Digital outputs.
• RS-232 or RS-485 serial outputs available.
• Modbus RTU Serial Output versions available.
D5450 & D5450M Series
• Four Thermistor Inputs
• Thermistor types: 2252Ω at 25°C, TD Series.
• Ranges:
2252Ω = -0°C to +100°C.
TD = -40°C to +150°C.
• Resolution:
2252Ω = 0.01°C or F.
TD = 0.1°C or F.
• Accuracy: 2252Ω = ±0.1°C, TD = ±0.2°C.
• User selectable °C or °F.
• 1 Digital input/ Event counter, 2 Digital outputs.
• RS-232 or RS-485 serial outputs available.
• Modbus RTU Serial Output versions available.
DGH Products
DGH offers high-quality temperature measurement solutions for thermocouples, RTD’s and thermistors. Each product contains precision measurement circuitry to make accurate temperature measurements without the need for external components or circuitry. Each product includes isolated electronic signal conditioning circuitry that is optimized for the specific type of sensor being measured, a high-resolution 15-bit analog to digital (A/D) converter to perform precise temperature measurements, and internal software digital filtering algorithms to convert each measurement to a usable temperature reading.
These products include additional features such as varying amounts of discrete digital inputs and outputs for localized on/off control. The discrete outputs in some products can be configured as on/off alarms based on user-selected temperature values. They are ideal for controlling solid-state relays and annunciators. The discrete inputs may be used for sensing contact closures such as cabinet doors opening and closing, or other detecting other control signals.
Each product contains a removable screw terminal plug for connecting the temperature sensor directly to the device. The removable plugs are also beneficial for making configuration changes or for troubleshooting purposes. Simply remove the plug and connect another plug from a computer into the module, perform the changes and restore the original plug.
These cost-effective temperature measurement products are easy to use, small in size, they support industry standard communications protocols and interfaces, are very accurate, and operate over a wide temperature range. Making them ideal temperature measurement solutions in applications ranging from laboratories to distributed data acquisition systems. DGH temperature products are available in single channel, four channel and seven channel versions. Specifications for each temperature product family are listed in detail below. Further product information is available, and you can call our offices to discuss your application with our technical sales team.