Thermometers, thermostats and temperature sensors
Bimetal and capillary thermometers, thermowells, thermostats and temperature sensors: components that show and control tank, oil line and process temperature.
A large share of hydraulic failures start with overheated oil, and often there is not even a component that shows the temperature. On the units we build and overhaul we make the oil temperature visible and, where needed, set up a thermostat that switches the cooler on automatically.
Types
Bimetal thermometers
Work by the different expansion of two different metals; they need no electricity and are used for local reading on tanks and pipelines.
Capillary thermometers
A capillary tube runs between the sensing bulb and the dial, so the temperature can be read on a panel away from the measuring point.
Thermowells
A protective pocket that separates the sensing element from the fluid; the thermometer can be removed without draining the system.
Thermostats
Give a contact at the set temperature: they start the cooler fan, cut off the heater or give an alarm.
Temperature sensors and transmitters
Sensors such as PT100, together with signal converters, pass the temperature to a PLC or a display.
Product images: Pakkens.
How to select
For most hydraulic oils the recommended working temperature is around 40-60 °C; if the temperature stays above this, the oil thins, the seals harden and the oil life shortens. This is why a readable thermometer on the tank is the first component to warn that the temperature is getting out of control.
In thermometer selection the measuring range, the immersion length and the connection are decisive. The sensing element has to reach far enough into the measured fluid; a short immersion length shows the temperature of the body, not of the medium.
If the temperature is to be controlled and not only displayed, a thermostat or a sensor is added: the cooler cuts in at a set temperature and the unit is stopped if it overheats.
Resistance values of a Pt100 temperature sensor
| Temperature | Resistance |
|---|---|
| 0 °C | 100.00 Ω |
| 20 °C | 107.79 Ω |
| 40 °C | 115.54 Ω |
| 60 °C | 123.24 Ω |
| 80 °C | 130.90 Ω |
| 100 °C | 138.51 Ω |
| 150 °C | 157.33 Ω |
Calculation: IEC 60751 equation R = 100 × (1 + A·t + B·t²), A = 3.9083·10⁻³, B = −5.775·10⁻⁷. With this table and a multimeter you can check whether the sensor is faulty.
Frequently asked questions
What temperature should hydraulic oil run at?
For most hydraulic oils the recommended range is around 40-60 °C; the exact value depends on the viscosity grade and the manufacturer of the oil.
Why is a thermowell needed?
The thermometer can be removed and refitted without draining the system or releasing the pressure, and the sensing element is protected from wear by the fluid.
My hydraulic power unit is overheating, what should I do?
The temperature has to be measured first. The cause may be insufficient cooling, or oil returning to tank needlessly over the relief valve; we examine the circuit and find the cause.
Bimetal thermometer or capillary thermometer?
If the thermometer is read at the measuring point, a bimetal type is sufficient. If the measuring point is inaccessible or hot and the value is to be read on a panel, the capillary type is used.
How do I test a Pt100 sensor?
Measure the resistance of the sensor and compare it with the temperature table: it should be 100 Ω at 0 °C and about 107.8 Ω at 20 °C. Infinite resistance indicates an open circuit, resistance close to zero a short circuit.
How do I select the length of a thermowell?
The tip of the sensing element has to reach far enough into the fluid; on a pipeline a length that reaches the middle of the pipe is usually selected. A short thermowell shows the temperature of the body, not of the medium.
Stock, sizes and part numbers
Sizes in stock, manufacturer part numbers and brands are kept up to date in our catalog. Send us your requirement on WhatsApp or by e-mail and we will quote and ship worldwide.
Thermometers, thermostats and temperature sensors in the catalog Online store (Turkish) →