Your country is not included? Please visit our global website Language When positive displacement pumps like diaphragm pumps transfer gases, the conditions on the inlet side are crucial for the pump’s performance. Whenever a gas pump is used, there are certain requirements for its flow rate. As shown in a previous blog post, flow can be presented using volume flow or mass flow. For maximum reliability and precision, the flow rate of KNF gas pumps is measured with the help of mass flow meters and then converted to a volumetric flow. This volumetric flow is specified in accordance with ISO 8778 at 20 °C (293.15 °K, 68 °F) and 1,000 mbar absolute (14.5 psi, 750.06 torr). Real Life Implications Over the year, the mass flow rate measured between the pump and the apparatus fluctuates considerably even though the inlet and outlet pressure of the pump are constant and describe the operation point of a pump. In the wintertime, the mass flow is much higher than during the summer. After checking the equipment and finding that it works as intended, one question arises “Does mass flow rate change with temperature?”. In the following, we will provide the theoretical background necessary to answer this question. Theoretical Basics of Positive Displacement Pumps related to the thermal reference state of the inlet condition with the inlet pressure \(p_{\text{i}}\) and inlet temperature \(T_{\text{i}}\). Where \( \eta \) is the rotational speed, \(V_1\) respective \(V_3\) is the working chamber volume at maximum downstroke position or maximum upstroke position, \( \Pi := \frac{p_{\text{o}}}{p_{\text{i}}} \) is the pressure ratio with the outlet pressure \(p_{\text{o}}\) and the isentropic exponent \( \gamma \). Gas Temperature Affects Mass Flow The conveyed mass flow is therefore calculated as follows: The effective suction speed \( \dot{V}_{\text{S,eff}} \) of a pump is constant for constant pressure ratio \( \pi \), constant fluid represented by the isentropic exponent \( \gamma \) and constant rotational speed \( \eta \). Changing only the inlet temperature, the mass flow will change as well like the equation above indicates. For increasing inlet temperature, the mass flow reduces and vice versa for decreasing inlet temperature. Reducing Uncertainty in Flow and Suction at Changing Inlet Conditions Finally, even though the pump is seemingly working at a constant operation point, meaning the inlet and outlet pressures are constant, changing inlet conditions, such as inlet temperature, will change the amount of gas transferred and therefore the true operating point of the pump. Get in touch with our experts to discuss your mass flow requirements! Contact Us Do you want to find out more about our customized pump solutions and service opportunities? Get in touch with our local sales representatives.